From 72f95d1160e117273505006c0f57981f56a6f317 Mon Sep 17 00:00:00 2001 From: grammarware Date: Thu, 11 Sep 2008 22:17:57 +0000 Subject: [PATCH] streamlines HTML->BGF; connecting second JLS1 grammar to LCI git-svn-id: https://slps.svn.sourceforge.net/svnroot/slps@259 ab42f6e0-554d-0410-b580-99e487e6eeb2 --- shared/tools/html2bgf | 15 +- shared/tools/jls2bgf | 21 - topics/extraction/html2bgf/xpathpre.py | 35 +- topics/java/Makefile | 9 + topics/java/jls1/Makefile | 4 +- topics/java/jls1/collect.kw | 8 + topics/java/jls1/syntax.kw | 2 + topics/java/jls2/Makefile | 3 + topics/java/jls2/collect.kw | 8 + topics/java/jls2/collected.html | 9299 ++++++++++++++++++++++++ topics/java/lci/java.lcf | 30 +- topics/java/lci/xbgf/prepare1.xbgf | 5 +- 12 files changed, 9389 insertions(+), 50 deletions(-) delete mode 100755 shared/tools/jls2bgf create mode 100644 topics/java/jls1/collect.kw create mode 100644 topics/java/jls1/syntax.kw create mode 100644 topics/java/jls2/collect.kw create mode 100644 topics/java/jls2/collected.html diff --git a/shared/tools/html2bgf b/shared/tools/html2bgf index effeaa71..57303f00 100755 --- a/shared/tools/html2bgf +++ b/shared/tools/html2bgf @@ -8,12 +8,21 @@ SLPS=${PWD} cd ${LOCAL1} if [ $# -lt 2 ]; then - echo "This tool extracts a BGF from Java Language Standard" - echo "Usage: $0 [options]" + echo "This tool extracts a BGF from Java Language Specification or a similarly typeset HTML" + echo "Usage:" + echo " html2bgf [] " + echo " or: html2bgf [] -bnf" exit 1 elif [ ! -r $1 ]; then echo "Oops: $1 not found or not readable." exit 1 +elif [ $# -eq 2 ]; then + python ${SLPS}/topics/extraction/html2bgf/html2bgf.py $1 $2 +elif [ "$3" == "-bnf" ]; then + python ${SLPS}/topics/extraction/html2bgf/html2bgf.py $1 $2 $3 else - python ${SLPS}/topics/extraction/html2bgf/html2bgf.py $@ + python ${SLPS}/topics/extraction/html2bgf/xpathpre.py $1 $2 $2.fixed + python ${SLPS}/topics/extraction/html2bgf/html2bgf.py $2.fixed $3 $4 + rm -f $2.fixed fi + diff --git a/shared/tools/jls2bgf b/shared/tools/jls2bgf deleted file mode 100755 index 6ffc6950..00000000 --- a/shared/tools/jls2bgf +++ /dev/null @@ -1,21 +0,0 @@ -#!/bin/sh - -# Get our hands on basedir -LOCAL1=${PWD} -cd `dirname $0` -cd ../.. -SLPS=${PWD} -cd ${LOCAL1} - -if [ $# -lt 2 ]; then - echo "This tool extracts a BGF from Java Language Standard that needs pre-processing" - echo "Usage: $0 [options]" - exit 1 -elif [ ! -r $1 ]; then - echo "Oops: $1 not found or not readable." - exit 1 -else - python ${SLPS}/topics/extraction/html2bgf/xpathpre.py LALR -Difficulties <$1 > $1.fixed - python ${SLPS}/topics/extraction/html2bgf/html2bgf.py $1.fixed $2 $3 - rm -f $1.fixed -fi diff --git a/topics/extraction/html2bgf/xpathpre.py b/topics/extraction/html2bgf/xpathpre.py index fee7d566..aaba8e1a 100755 --- a/topics/extraction/html2bgf/xpathpre.py +++ b/topics/extraction/html2bgf/xpathpre.py @@ -4,7 +4,7 @@ yes = [] no = [] -def checkSection(text,tagN,includeFlag): +def checkSection(text,tagN,includeFlag,p): for chapter in text.split('')[1:]: grammar = includeFlag content = chapter.split('') @@ -16,34 +16,39 @@ def checkSection(text,tagN,includeFlag): grammar = False if grammar and content[1].find('')[1:]: - print chunk.split('')[0].replace('
','').replace(' ',' ') - print '
' + p.write(chunk.split('')[0].replace('
','').replace(' ',' ')) + p.write('
') else: #print 'Going deeper than',content[0].split()[0] if grammar: for chunk in content[1].split('')[0].split('
')[1:]:
-     print chunk.split('
')[0].replace('
','').replace(' ',' ') - print '
' - checkSection(content[1],tagN+1,grammar) + p.write(chunk.split('')[0].replace('
','').replace(' ',' ')) + p.write('
') + checkSection(content[1],tagN+1,grammar,p) -if len(sys.argv)<2: +if len(sys.argv)!=4: print '''This tool simulates a particular XPath query that it can execute upon a badly composed HTML. Usage: - python xpathpre.py keyword [keyword ...] output + python xpathpre.py It will read the input, looking for sections () that contain keywords in the title. Once found, it will output the content of all 
 tags from such sections.
-Keywords can be positive or negative, with positive being default.'''
+Keywords can be positive or negative, with positive being default.
+
 inside 
 is not used.'''
 else:
- for kw in sys.argv[1:]:
-  if kw[0]=='-':
+ for kw in open(sys.argv[1],'r').readlines():
+  kw = kw.strip()
+  if not kw:
+   continue
+  elif kw[0]=='-':
    no.append(kw[1:])
   elif kw[0]=='+':
    yes.append(kw[1:])
   else:
    yes.append(kw)
- print '
'
- checkSection(''.join(sys.stdin.readlines()),1,False)
- print '
'
-
+ out = open(sys.argv[3],'w')
+ out.write('
')
+ checkSection(''.join(open(sys.argv[2],'r').readlines()),1,False,out)
+ out.write('
')
+ out.close()
diff --git a/topics/java/Makefile b/topics/java/Makefile
index cf29593c..b76b273d 100644
--- a/topics/java/Makefile
+++ b/topics/java/Makefile
@@ -23,3 +23,12 @@ rebuild:
 	curl -k http://java.sun.com/docs/books/jls/first_edition/html/10.doc.html >>jls1/collected.html
 	curl -k http://java.sun.com/docs/books/jls/first_edition/html/14.doc.html >>jls1/collected.html
 	curl -k http://java.sun.com/docs/books/jls/first_edition/html/15.doc.html >>jls1/collected.html
+	curl -k http://java.sun.com/docs/books/jls/second_edition/html/typesValues.doc.html >jls2/collected.html
+	curl -k http://java.sun.com/docs/books/jls/second_edition/html/names.doc.html >>jls2/collected.html
+	curl -k http://java.sun.com/docs/books/jls/second_edition/html/packages.doc.html >>jls2/collected.html
+	curl -k http://java.sun.com/docs/books/jls/second_edition/html/classes.doc.html >>jls2/collected.html
+	curl -k http://java.sun.com/docs/books/jls/second_edition/html/interfaces.doc.html >>jls2/collected.html
+	curl -k http://java.sun.com/docs/books/jls/second_edition/html/arrays.doc.html >>jls2/collected.html
+	curl -k http://java.sun.com/docs/books/jls/second_edition/html/statements.doc.html >>jls2/collected.html
+	curl -k http://java.sun.com/docs/books/jls/second_edition/html/expressions.doc.html >>jls2/collected.html
+
diff --git a/topics/java/jls1/Makefile b/topics/java/jls1/Makefile
index d42a2ecb..fded94fc 100644
--- a/topics/java/jls1/Makefile
+++ b/topics/java/jls1/Makefile
@@ -1,7 +1,7 @@
 all:
-	python ../../extraction/html2bgf/xpathpre.py LALR -Difficulties parse.html
+	python ../../extraction/html2bgf/xpathpre.py syntax.kw syntax.html parse.html
 	python ../../extraction/html2bgf/html2bgf.py parse.html jls1.bgf
-	python ../../extraction/html2bgf/xpathpre.py 4 6 7 8 9 10 14 15 parse.html
+	python ../../extraction/html2bgf/xpathpre.py collect.kw collected.html parse.html
 	python ../../extraction/html2bgf/html2bgf.py parse.html jls1c.bgf
 	../../../shared/tools/checkxml bgf jls1.bgf
 	../../../shared/tools/checkxml bgf jls1c.bgf
diff --git a/topics/java/jls1/collect.kw b/topics/java/jls1/collect.kw
new file mode 100644
index 00000000..eab2203c
--- /dev/null
+++ b/topics/java/jls1/collect.kw
@@ -0,0 +1,8 @@
+4
+6
+7
+8
+9
+10
+14
+15
diff --git a/topics/java/jls1/syntax.kw b/topics/java/jls1/syntax.kw
new file mode 100644
index 00000000..b54c0438
--- /dev/null
+++ b/topics/java/jls1/syntax.kw
@@ -0,0 +1,2 @@
+LALR
+-Difficulties
diff --git a/topics/java/jls2/Makefile b/topics/java/jls2/Makefile
index b2540e77..5aed35c7 100644
--- a/topics/java/jls2/Makefile
+++ b/topics/java/jls2/Makefile
@@ -1,6 +1,9 @@
 all:
 	python ../../extraction/html2bgf/html2bgf.py syntax.html jls2.bgf
+	python ../../extraction/html2bgf/xpathpre.py collect.kw collected.html parse.html
+	python ../../extraction/html2bgf/html2bgf.py parse.html jls2c.bgf
 	../../../shared/tools/checkxml bgf jls2.bgf
+	../../../shared/tools/checkxml bgf jls2c.bgf
 
 clean:
 	rm -f *.bgf
diff --git a/topics/java/jls2/collect.kw b/topics/java/jls2/collect.kw
new file mode 100644
index 00000000..c9fe3de1
--- /dev/null
+++ b/topics/java/jls2/collect.kw
@@ -0,0 +1,8 @@
+4
+6.5
+7
+8
+9
+10.6
+14
+15
diff --git a/topics/java/jls2/collected.html b/topics/java/jls2/collected.html
new file mode 100644
index 00000000..8a484f8d
--- /dev/null
+++ b/topics/java/jls2/collected.html
@@ -0,0 +1,9299 @@
+
+
+ Types, Values, and Variables
+
+
+
+
+ 
+
+
+
+
Contents | Prev | Next | IndexJava Language Specification

+Second Edition

+


+ 
+
+

+CHAPTER
+ 4 

+
+
Types, Values, and Variables

+

+
+The Java programming language is a strongly typed language, which means that every variable and every expression has a type that is known at compile time. Types limit the values that a variable (§4.5) can hold or that an expression can produce, limit the operations supported on those values, and determine the meaning of the operations. Strong typing helps detect errors at compile time.

+
+The types of the Java programming language are divided into two categories: primitive types and reference types. The primitive types (§4.2) are the boolean type and the numeric types. The numeric types are the integral types byte, short, int, long, and char, and the floating-point types float and double. The reference types (§4.3) are class types, interface types, and array types. There is also a special null type. An object (§4.3.1) is a dynamically created instance of a class type or a dynamically created array. The values of a reference type are references to objects. All objects, including arrays, support the methods of class Object (§4.3.2). String literals are represented by String objects (§4.3.3).

+
+Names of types are used (§4.4) in declarations, casts, class instance creation expressions, array creation expressions, class literals, and instanceof operator expressions. 

+
+A variable (§4.5) is a storage location. A variable of a primitive type always holds a value of that exact type. A variable of a class type T can hold a null reference or a reference to an instance of class T or of any class that is a subclass of T. A variable of an interface type can hold a null reference or a reference to any instance of any class that implements the interface. If T is a primitive type, then a variable of type "array of T" can hold a null reference or a reference to any array of type "array of T"; if T is a reference type, then a variable of type "array of T" can hold a null reference or a reference to any array of type "array of S" such that type S is assignable (§5.2) to type T. A variable of type Object can hold a null reference or a reference to any object, whether class interface or array.

+
+
4.1    The Kinds of Types and Values

+
+There are two kinds of types in the Java programming language: primitive types (§4.2) and reference types (§4.3). There are, correspondingly, two kinds of data values that can be stored in variables, passed as arguments, returned by methods, and operated on: primitive values (§4.2) and reference values (§4.3).

+
    +Type:
+	PrimitiveType
+	ReferenceType
+

+There is also a special null type, the type of the expression null, which has no name. Because the null type has no name, it is impossible to declare a variable of the null type or to cast to the null type. The null reference is the only possible value of an expression of null type. The null reference can always be cast to any reference type. In practice, the programmer can ignore the null type and just pretend that null is merely a special literal that can be of any reference type.

+
+
4.2    Primitive Types and Values

+
+A primitive type is predefined by the Java programming language and named by its reserved keyword (§3.9):

+
    +PrimitiveType:
+	NumericType
+	boolean
+
+NumericType:
+	IntegralType
+	FloatingPointType
+
+IntegralType: one of
+	byte short int long char
+
+FloatingPointType: one of
+	float double
+

+Primitive values do not share state with other primitive values. A variable whose type is a primitive type always holds a primitive value of that same type. The value of a variable of primitive type can be changed only by assignment operations on that variable.

+
+The numeric types are the integral types and the floating-point types. 

+
+The integral types are byte, short, int, and long, whose values are 8-bit, 16-bit, 32-bit and 64-bit signed two's-complement integers, respectively, and char, whose values are 16-bit unsigned integers representing Unicode characters.

+
+The floating-point types are float, whose values include the 32-bit IEEE 754 floating-point numbers, and double, whose values include the 64-bit IEEE 754 floating-point numbers.

+
+The boolean type has exactly two values: true and false.

+
+
4.2.1    Integral Types and Values

+
+The values of the integral types are integers in the following ranges:

+
    
+
  • For byte, from -128 to 127, inclusive
+
+
  • For short, from -32768 to 32767, inclusive
+
+
  • For int, from -2147483648 to 2147483647, inclusive
+
+
  • For long, from -9223372036854775808 to 9223372036854775807, inclusive
+
+
  • For char, from '\u0000' to '\uffff' inclusive, that is, from 0 to 65535
+

+
4.2.2    Integer Operations

+
+The Java programming language provides a number of operators that act on integral values:

+
    
+
  • The comparison operators, which result in a value of type boolean:
+
      
+
+
    • The numerical comparison operators <, <=, >, and >= (§15.20.1)
+
+
    • The numerical equality operators == and != (§15.21.1)
+
    
+
+
  • The numerical operators, which result in a value of type int or long:
+
+
+
  • The conditional operator ? : (§15.25)
+
+
  • The cast operator, which can convert from an integral value to a value of any specified numeric type (§5.5, §15.16)
+
+
  • The string concatenation operator + (§15.18.1), which, when given a String operand and an integral operand, will convert the integral operand to a String representing its value in decimal form, and then produce a newly created String that is the concatenation of the two strings
+

+Other useful constructors, methods, and constants are predefined in the classes Byte, Short, Integer, Long, and Character.

+
+If an integer operator other than a shift operator has at least one operand of type long, then the operation is carried out using 64-bit precision, and the result of the numerical operator is of type long. If the other operand is not long, it is first widened (§5.1.4) to type long by numeric promotion (§5.6). Otherwise, the operation is carried out using 32-bit precision, and the result of the numerical operator is of type int. If either operand is not an int, it is first widened to type int by numeric promotion.

+
+The built-in integer operators do not indicate overflow or underflow in any way. The only numeric operators that can throw an exception (§11) are the integer divide operator / (§15.17.2) and the integer remainder operator % (§15.17.3), which throw an ArithmeticException if the right-hand operand is zero. 

+
+The example:
+
class Test {
+	public static void main(String[] args) {
+		int i = 1000000;
+		System.out.println(i * i);
+		long l = i;
+		System.out.println(l * l);
+		System.out.println(20296 / (l - i));
+	}
+}
+

+produces the output:

+
-727379968
+1000000000000
+

+and then encounters an ArithmeticException in the division by l - i, because l - i is zero. The first multiplication is performed in 32-bit precision, whereas the second multiplication is a long multiplication. The value -727379968 is the decimal value of the low 32 bits of the mathematical result, 1000000000000, which is a value too large for type int.

+
+Any value of any integral type may be cast to or from any numeric type. There are no casts between integral types and the type boolean.

+
+
4.2.3    Floating-Point Types, Formats, and Values

+
+The floating-point types are float and double, which are conceptually associated with the single-precision 32-bit and double-precision 64-bit format IEEE 754 values and operations as specified in IEEE Standard for Binary Floating-Point Arithmetic, ANSI/IEEE Standard 754-1985 (IEEE, New York).

+
+The IEEE 754 standard includes not only positive and negative numbers that consist of a sign and magnitude, but also positive and negative zeros, positive and negative infinities, and special Not-a-Number values (hereafter abbreviated NaN). A NaN value is used to represent the result of certain invalid operations such as dividing zero by zero. NaN constants of both float and double type are predefined as Float.NaN and Double.NaN.

+
+Every implementation of the Java programming language is required to support two standard sets of floating-point values, called the float value set and the double value set. In addition, an implementation of the Java programming language may support either or both of two extended-exponent floating-point value sets, called the float-extended-exponent value set and the double-extended-exponent value set. These extended-exponent value sets may, under certain circumstances, be used instead of the standard value sets to represent the values of expressions of type float or double (§5.1.8, §15.4).

+
+The finite nonzero values of any floating-point value set can all be expressed in the form , where s is +1 or -1, m is a positive integer less than , and e is an integer between  and , inclusive, and where N and K are parameters that depend on the value set. Some values can be represented in this form in more than one way; for example, supposing that a value v in a value set might be represented in this form using certain values for s, m, and e, then if it happened that m were even and e were less than , one could halve m and increase e by 1 to produce a second representation for the same value v. A representation in this form is called normalized if ; otherwise the representation is said to be denormalized. If a value in a value set cannot be represented in such a way that , then the value is said to be a denormalized value, because it has no normalized representation.

+
+The constraints on the parameters N and K (and on the derived parameters Emin and Emax) for the two required and two optional floating-point value sets are summarized in Table 4.1. 
+

+
+
+

+Floating-point value set parameters
+

+ Parameter
+
+float
+
+float-extended-exponent
+
+ double
+
+ double-extended-exponent
+
+

+ N

+

+ 24

+

+ 24

+

+ 53

+

+ 53

+
+

+ K

+

+ 8

+

+  11

+

+ 11

+

+  15

+
+

+ Emax

+

+ +127

+

+  +1023

+

+ +1023

+

+  +16383

+
+

+ Emin

+

+ -126

+

+  -1022

+

+ -1022

+

+ -16382

+
+
+

+
+
+

+

+

+
+Where one or both extended-exponent value sets are supported by an implementation, then for each supported extended-exponent value set there is a specific implementation-dependent constant K, whose value is constrained by Table 4.1; this value K in turn dictates the values for Emin and Emax.

+
+Each of the four value sets includes not only the finite nonzero values that are ascribed to it above, but also NaN values and the four values positive zero, negative zero, positive infinity, and negative infinity.

+
+Note that the constraints in Table 4.1 are designed so that every element of the float value set is necessarily also an element of the float-extended-exponent value set, the double value set, and the double-extended-exponent value set. Likewise, each element of the double value set is necessarily also an element of the double-extended-exponent value set. Each extended-exponent value set has a larger range of exponent values than the corresponding standard value set, but does not have more precision.

+
+The elements of the float value set are exactly the values that can be represented using the single floating-point format defined in the IEEE 754 standard. The elements of the double value set are exactly the values that can be represented using the double floating-point format defined in the IEEE 754 standard. Note, however, that the elements of the float-extended-exponent and double-extended-exponent value sets defined here do not correspond to the values that can be represented using IEEE 754 single extended and double extended formats, respectively.

+
+The float, float-extended-exponent, double, and double-extended-exponent value sets are not types. It is always correct for an implementation of the Java programming language to use an element of the float value set to represent a value of type float; however, it may be permissible in certain regions of code for an implementation to use an element of the float-extended-exponent value set instead. Similarly, it is always correct for an implementation to use an element of the double value set to represent a value of type double; however, it may be permissible in certain regions of code for an implementation to use an element of the double-extended-exponent value set instead.

+
+Except for NaN, floating-point values are ordered; arranged from smallest to largest, they are negative infinity, negative finite nonzero values, positive and negative zero, positive finite nonzero values, and positive infinity.

+
+IEEE 754 allows multiple distinct NaN values for each of its single and double floating-point formats. While each hardware architecture returns a particular bit pattern for NaN when a new NaN is generated, a programmer can also create NaNs with different bit patterns to encode, for example, retrospective diagnostic information. 

+
+For the most part, the Java platform treats NaN values of a given type as though collapsed into a single canonical value (and hence this specification normally refers to an arbitrary NaN as though to a canonical value). However, version 1.3 the Java platform introduced methods enabling the programmer to distinguish between NaN values: the Float.floatToRawIntBits and Double.doubleToRawLongBits methods. The interested reader is referred to the specifications for the Float and Double classes for more information.

+
+Positive zero and negative zero compare equal; thus the result of the expression 0.0==-0.0 is true and the result of 0.0>-0.0 is false. But other operations can distinguish positive and negative zero; for example, 1.0/0.0 has the value positive infinity, while the value of 1.0/-0.0 is negative infinity.

+
+NaN is unordered, so the numerical comparison operators <, <=, >, and >= return false if either or both operands are NaN (§15.20.1). The equality operator == returns false if either operand is NaN, and the inequality operator != returns true if either operand is NaN (§15.21.1). In particular, x!=x is true if and only if x is NaN, and (x<y) == !(x>=y) will be false if x or y is NaN.

+
+Any value of a floating-point type may be cast to or from any numeric type. There are no casts between floating-point types and the type boolean.

+
+
4.2.4    Floating-Point Operations

+
+The Java programming language provides a number of operators that act on floating-point values:

+
    
+
  • The comparison operators, which result in a value of type boolean:
+
      
+
+
    • The numerical comparison operators <, <=, >, and >= (§15.20.1)
+
+
    • The numerical equality operators == and != (§15.21.1)
+
    
+
+
  • The numerical operators, which result in a value of type float or double:
+
+
+
  • The conditional operator ? : (§15.25)
+
+
  • The cast operator, which can convert from a floating-point value to a value of any specified numeric type (§5.5, §15.16)
+
+
  • The string concatenation operator + (§15.18.1), which, when given a String operand and a floating-point operand, will convert the floating-point operand to a String representing its value in decimal form (without information loss), and then produce a newly created String by concatenating the two strings
+

+Other useful constructors, methods, and constants are predefined in the classes Float, Double, and Math.

+
+If at least one of the operands to a binary operator is of floating-point type, then the operation is a floating-point operation, even if the other is integral.

+
+If at least one of the operands to a numerical operator is of type double, then the operation is carried out using 64-bit floating-point arithmetic, and the result of the numerical operator is a value of type double. (If the other operand is not a double, it is first widened to type double by numeric promotion (§5.6).) Otherwise, the operation is carried out using 32-bit floating-point arithmetic, and the result of the numerical operator is a value of type float. If the other operand is not a float, it is first widened to type float by numeric promotion.

+
+Operators on floating-point numbers behave as specified by IEEE 754 (with the exception of the remainder operator (§15.17.3)). In particular, the Java programming language requires support of IEEE 754 denormalized floating-point numbers and gradual underflow, which make it easier to prove desirable properties of particular numerical algorithms. Floating-point operations do not "flush to zero" if the calculated result is a denormalized number.

+
+The Java programming language requires that floating-point arithmetic behave as if every floating-point operator rounded its floating-point result to the result precision. Inexact results must be rounded to the representable value nearest to the infinitely precise result; if the two nearest representable values are equally near, the one with its least significant bit zero is chosen. This is the IEEE 754 standard's default rounding mode known as round to nearest.

+
+The language uses round toward zero when converting a floating value to an integer (§5.1.3), which acts, in this case, as though the number were truncated, discarding the mantissa bits. Rounding toward zero chooses at its result the format's value closest to and no greater in magnitude than the infinitely precise result.

+
+Floating-point operators produce no exceptions (§11). An operation that overflows produces a signed infinity, an operation that underflows produces a denormalized value or a signed zero, and an operation that has no mathematically definite result produces NaN. All numeric operations with NaN as an operand produce NaN as a result. As has already been described, NaN is unordered, so a numeric comparison operation involving one or two NaNs returns false and any != comparison involving NaN returns true, including x!=x when x is NaN.

+
+The example program:
+
class Test {
+	public static void main(String[] args) {
+		// An example of overflow:
+		double d = 1e308;
+		System.out.print("overflow produces infinity: ");
+		System.out.println(d + "*10==" + d*10);
+		// An example of gradual underflow:
+		d = 1e-305 * Math.PI;
+		System.out.print("gradual underflow: " + d + "\n      ");
+		for (int i = 0; i < 4; i++)
+			System.out.print(" " + (d /= 100000));
+		System.out.println();
+		// An example of NaN:
+		System.out.print("0.0/0.0 is Not-a-Number: ");
+		d = 0.0/0.0;
+		System.out.println(d);
+		// An example of inexact results and rounding:
+		System.out.print("inexact results with float:");
+		for (int i = 0; i < 100; i++) {
+			float z = 1.0f / i;
+			if (z * i != 1.0f)
+				System.out.print(" " + i);
+		}
+		System.out.println();
+		// Another example of inexact results and rounding:
+		System.out.print("inexact results with double:");
+		for (int i = 0; i < 100; i++) {
+			double z = 1.0 / i;
+			if (z * i != 1.0)
+				System.out.print(" " + i);
+		}
+		System.out.println();
+		// An example of cast to integer rounding:
+		System.out.print("cast to int rounds toward 0: ");
+		d = 12345.6;
+		System.out.println((int)d + " " + (int)(-d));
+	}
+}
+

+produces the output:

+
overflow produces infinity: 1.0e+308*10==Infinity
+gradual underflow: 3.141592653589793E-305
+	3.1415926535898E-310 3.141592653E-315 3.142E-320 0.0
+0.0/0.0 is Not-a-Number: NaN
+inexact results with float: 0 41 47 55 61 82 83 94 97
+inexact results with double: 0 49 98
+cast to int rounds toward 0: 12345 -12345
+

+
+This example demonstrates, among other things, that gradual underflow can result in a gradual loss of precision.
+
+The results when i is 0 involve division by zero, so that z becomes positive infinity, and z * 0 is NaN, which is not equal to 1.0.
+

+
4.2.5    The boolean Type and boolean Values

+
+The boolean type represents a logical quantity with two possible values, indicated by the literals true and false (§3.10.3). The boolean operators are:

+
    
+
  • The relational operators == and != (§15.21.2)
+
+
  • The logical-complement operator ! (§15.15.6)
+
+
  • The logical operators &, ^, and | (§15.22.2)
+
+
  • The conditional-and and conditional-or operators && (§15.23) and || (§15.24)
+
+
  • The conditional operator ? : (§15.25)
+
+
  • The string concatenation operator + (§15.18.1), which, when given a String operand and a boolean operand, will convert the boolean operand to a String (either "true" or "false"), and then produce a newly created String that is the concatenation of the two strings
+

+Boolean expressions determine the control flow in several kinds of statements:

+

+A boolean expression also determines which subexpression is evaluated in the conditional ? : operator (§15.25).

+
+Only boolean expressions can be used in control flow statements and as the first operand of the conditional operator ? :. An integer x can be converted to a boolean, following the C language convention that any nonzero value is true, by the expression x!=0. An object reference obj can be converted to a boolean, following  the C language convention that any reference other than null is true, by the expression obj!=null.

+
+A cast of a boolean value to type boolean is allowed (§5.1.1); no other casts on type boolean are allowed. A boolean can be converted to a string by string conversion (§5.4).

+
+
4.3    Reference Types and Values

+
+There are three kinds of reference types: class types (§8), interface types (§9), and array types (§10).

+
    +ReferenceType:
+	ClassOrInterfaceType
+	ArrayType
+
+ClassOrInterfaceType:
+	ClassType
+	InterfaceType
+
+ClassType:
+	TypeName
+
+InterfaceType:
+	TypeName
+
+ArrayType:
+	Type [ ]
+	
+

+Names are described in §6; type names in §6.5 and, specifically, §6.5.5.

+
+The sample code:
+
class Point { int[] metrics; }
+interface Move { void move(int deltax, int deltay); }
+

+declares a class type Point, an interface type Move, and uses an array type int[] (an array of int) to declare the field metrics of the class Point.

+
+
4.3.1    Objects

+
+An object is a class instance or an array.

+
+The reference values (often just references) are pointers to these objects, and a special null reference, which refers to no object.

+
+A class instance is explicitly created by a class instance creation expression (§15.9). An array is explicitly created by an array creation expression (§15.9).

+
+A new class instance is implicitly created when the string concatenation operator + (§15.18.1) is used in an expression, resulting in a new object of type String (§4.3.3). A new array object is implicitly created when an array initializer expression (§10.6) is evaluated; this can occur when a class or interface is initialized (§12.4), when a new instance of a class is created (§15.9), or when a local variable declaration statement is executed (§14.4).

+
+Many of these cases are illustrated in the following example:
+
class Point {
+	int x, y;
+	Point() { System.out.println("default"); }
+	Point(int x, int y) { this.x = x; this.y = y; }
+	// A Point instance is explicitly created at class initialization time:
+	static Point origin = new Point(0,0);
+	// A String can be implicitly created by a + operator:
+	public String toString() {
+		return "(" + x + "," + y + ")";
+	}
+}
+class Test {
+	public static void main(String[] args) {
+		// A Point is explicitly created using newInstance:
+		Point p = null;
+		try {
+			p = (Point)Class.forName("Point").newInstance();
+		} catch (Exception e) {
+			System.out.println(e);
+		}
+		// An array is implicitly created by an array constructor:
+		Point a[] = { new Point(0,0), new Point(1,1) };
+		// Strings are implicitly created by + operators:
+		System.out.println("p: " + p);
+		System.out.println("a: { " + a[0] + ", "
+										   + a[1] + " }");
+		// An array is explicitly created by an array creation expression:
+		String sa[] = new String[2];
+		sa[0] = "he"; sa[1] = "llo";
+		System.out.println(sa[0] + sa[1]);
+	}
+}
+

+which produces the output:

+
default
+p: (0,0)
+a: { (0,0), (1,1) }
+hello
+

+The operators on references to objects are:

+
    
+
  • Field access, using either a qualified name (§6.6) or a field access expression (§15.11)
+
+
  • Method invocation (§15.12)
+
+
  • The cast operator (§5.5, §15.16)
+
+
  • The string concatenation operator + (§15.18.1), which, when given a String operand and a reference, will convert the reference to a String by invoking the toString method of the referenced object (using "null" if either the reference or the result of toString is a null reference), and then will produce a newly created String that is the concatenation of the two strings
+
+
  • The instanceof operator (§15.20.2)
+
+
  • The reference equality operators == and != (§15.21.3)
+
+
  • The conditional operator ? : (§15.25).
+

+There may be many references to the same object. Most objects have state, stored in the fields of objects that are instances of classes or in the variables that are the components of an array object. If two variables contain references to the same object, the state of the object can be modified using one variable's reference to the object, and then the altered state can be observed through the reference in the other variable.

+
+The example program:
+
class Value { int val; }
+class Test {
+	public static void main(String[] args) {
+		int i1 = 3;
+		int i2 = i1;
+		i2 = 4;
+		System.out.print("i1==" + i1);
+		System.out.println(" but i2==" + i2);
+		Value v1 = new Value();
+		v1.val = 5;
+		Value v2 = v1;
+		v2.val = 6;
+		System.out.print("v1.val==" + v1.val);
+		System.out.println(" and v2.val==" + v2.val);
+	}
+}
+

+produces the output:

+
i1==3 but i2==4
+v1.val==6 and v2.val==6
+

+because v1.val and v2.val reference the same instance variable (§4.5.3) in the one Value object created by the only new expression, while i1 and i2 are different variables.

+
+See §10 and §15.10 for examples of the creation and use of arrays.
+
+Each object has an associated lock (§17.13), which is used by synchronized methods (§8.4.3) and the synchronized statement (§14.18) to provide control over concurrent access to state by multiple threads (§17.12).

+
+
4.3.2    The Class Object

+
+The class Object is a superclass (§8.1) of all other classes. A variable of type Object can hold a reference to any object, whether it is an instance of a class or an array (§10). All class and array types inherit the methods of class Object, which are summarized here:

+
package java.lang;
+
+public class Object {
+	public final Class getClass() { . . . }
+	public String toString() { . . . }
+	public boolean equals(Object obj) { . . . }
+	public int hashCode() { . . . }
+	protected Object clone()
+		throws CloneNotSupportedException { . . . }
+	public final void wait()
+		throws IllegalMonitorStateException,
+			InterruptedException { . . . }
+	public final void wait(long millis)
+		throws IllegalMonitorStateException,
+			InterruptedException { . . . }
+	public final void wait(long millis, int nanos) { . . . }
+		throws IllegalMonitorStateException,
+			InterruptedException { . . . }
+	public final void notify() { . . . }
+		throws IllegalMonitorStateException
+	public final void notifyAll() { . . . }
+		throws IllegalMonitorStateException
+	protected void finalize()
+		throws Throwable { . . . }
+}
+

+The members of Object are as follows:

+
    
+
  • The method getClass returns the Class object that represents the class of the object. A Class object exists for each reference type. It can be used, for example, to discover the fully qualified name of a class, its members, its immediate superclass, and any interfaces that it implements. A class method that is declared synchronized (§8.4.3.6) synchronizes on the lock associated with the Class object of the class.
+
+
  • The method toString returns a String representation of the object.
+
+
  • The methods equals and hashCode are very useful in hashtables such as java.util.Hashtable. The method equals defines a notion of object equality, which is based on value, not reference, comparison.
+
+
  • The method clone is used to make a duplicate of an object.
+
+
  • The methods wait, notify, and notifyAll are used in concurrent programming using threads, as described in §17.
+
+
  • The method finalize is run just before an object is destroyed and is described in §12.6.
+

+
4.3.3    The Class String

+
+Instances of class String represent sequences of Unicode characters. A  String object has a constant (unchanging) value. String literals (§3.10.5) are references to instances of class String.

+
+The string concatenation operator + (§15.18.1) implicitly creates a new String object.

+
+
4.3.4    When Reference Types Are the Same

+
+Two reference types are the same compile-time type if they have the same binary name (§13.1), in which case they are sometimes said to be the same class or the same interface.

+
+At run time, several reference types with the same binary name may be loaded simultaneously by different class loaders. These types may or may not represent the same type declaration. Even if two such types do represent the same type declaration, they are considered distinct.

+
+Two reference types are the same run-time type if:

+
    
+
  • They are both class or both interface types, are loaded by the same class loader, and have the same binary name (§13.1), in which case they are sometimes said to be the same run-time class or the same run-time interface.
+
+
  • They are both array types, and their component types are the same run-time type(§10).
+

+
4.4    Where Types Are Used

+
+Types are used when they appear in declarations or in certain expressions.

+
+The following code fragment contains one or more instances of most kinds of usage of a type:
+
import java.util.Random;
+class MiscMath {
+	int divisor;
+	MiscMath(int divisor) {
+		this.divisor = divisor;
+	}
+	float ratio(long l) {
+		try {
+			l /= divisor;
+		} catch (Exception e) {
+			if (e instanceof ArithmeticException)
+				l = Long.MAX_VALUE;
+			else
+				l = 0;
+		}
+		return (float)l;
+	}
+	double gausser() {
+		Random r = new Random();
+		double[] val = new double[2];
+		val[0] = r.nextGaussian();
+		val[1] = r.nextGaussian();
+		return (val[0] + val[1]) / 2;
+	}
+}
+

+In this example, types are used in declarations of the following:

+
    
+
  • Imported types (§7.5); here the type Random, imported from the type java.util.Random of the package java.util, is declared
+
+
  • Fields, which are the class variables and instance variables of classes (§8.3), and constants of interfaces (§9.3); here the field divisor in the class MiscMath  is declared to be of type int
+
+
  • Method parameters (§8.4.1); here the parameter l of the method ratio is declared to be of type long
+
+
  • Method results (§8.4); here the result of the method ratio is declared to be of type float, and the result of the method gausser is declared to be of type double
+
+
  • Constructor parameters (§8.8.1); here the parameter of the constructor for MiscMath is declared to be of type int
+
+
  • Local variables (§14.4, §14.13); the local variables r and val of the method gausser are declared to be of types Random and double[] (array of double)
+
+
  • Exception handler parameters (§14.19); here the exception handler parameter e of the catch clause is declared to be of type Exception
+

+and in expressions of the following kinds:

+
    
+
  • Class instance creations (§15.9); here a local variable r of method gausser is initialized by a class instance creation expression that uses the type Random 
+
+
  • Array creations (§15.10); here the local variable val of method gausser is initialized by an array creation expression that creates an array of double with size 2
+
+
  • Casts (§15.16); here the return statement of the method ratio uses the float type in a cast
+
+
  • The instanceof operator (§15.20.2); here the instanceof operator tests whether e is assignment compatible with the type ArithmeticException
+

+
4.5    Variables

+
+A variable is a storage location and has an associated type, sometimes called its compile-time type, that is either a primitive type (§4.2) or a reference type (§4.3). A variable always contains a value that is assignment compatible (§5.2) with its type. A variable's value is changed by an assignment (§15.26) or by a prefix or postfix ++ (increment) or -- (decrement) operator (§15.14.1, §15.14.2, §15.15.1, §15.15.2).

+
+Compatibility of the value of a variable with its type is guaranteed by the design of the Java programming language. Default values are compatible (§4.5.5) and all assignments to a variable are checked for assignment compatibility (§5.2), usually at compile time, but, in a single case involving arrays, a run-time check is made (§10.10).

+
+
4.5.1    Variables of Primitive Type

+
+A variable of a primitive type always holds a value of that exact primitive type.

+
+
4.5.2    Variables of Reference Type

+
+A variable of reference type can hold either of the following:

+
    
+
  • A null reference
+
+
  • A reference to any object (§4.3) whose class (§4.5.6) is assignment compatible (§5.2) with the type of the variable
+

+
4.5.3    Kinds of Variables

+
+There are seven kinds of variables:

+
    
+
+
  1. A class variable is a field declared using the keyword static within a class declaration (§8.3.1.1), or with or without the keyword static within an interface declaration (§9.3). A class variable is created when its class or interface is prepared (§12.3.2) and is initialized to a default value (§4.5.5). The class variable effectively ceases to exist when its class or interface is unloaded (§12.7).
+
+
  2. An instance variable is a field declared within a class declaration without using the keyword static (§8.3.1.1). If a class T has a field a that is an instance variable, then a new instance variable a is created and initialized to a default value (§4.5.5) as part of each newly created object of class T or of any class that is a subclass of T (§8.1.3). The instance variable effectively ceases to exist when the object of which it is a field is no longer referenced, after any necessary finalization of the object (§12.6) has been completed.
+
+
  3. Array components are unnamed variables that are created and initialized to default values (§4.5.5) whenever a new object that is an array is created (§15.10). The array components effectively cease to exist when the array is no longer referenced. See §10 for a description of arrays.
+
+
  4. Method parameters (§8.4.1) name argument values passed to a method. For every parameter declared in a method declaration, a new parameter variable is created each time that method is invoked (§15.12). The new variable is initialized with the corresponding argument value from the method invocation. The method parameter effectively ceases to exist when the execution of the body of the method is complete.
+
+
  5. Constructor parameters (§8.8.1) name argument values passed to a constructor. For every parameter declared in a constructor declaration, a new parameter variable is created each time a class instance creation expression (§15.9) or explicit constructor invocation (§8.8.5) invokes that constructor. The new variable is initialized with the corresponding argument value from the creation expression or constructor invocation. The constructor parameter effectively ceases to exist when the execution of the body of the constructor is complete.
+
+
  6. An exception-handler parameter is created each time an exception is caught by a catch clause of a try statement (§14.19). The new variable is initialized with the actual object associated with the exception (§11.3, §14.17). The exception-handler parameter effectively ceases to exist when execution of the block associated with the catch clause is complete.
+
+
  7. Local variables are declared by local variable declaration statements (§14.4). Whenever the flow of control enters a block (§14.2) or for statement (§14.13), a new variable is created for each local variable declared in a local variable declaration statement immediately contained within that block or for statement. A local variable declaration statement may contain an expression which initializes the variable. The local variable with an initializing expression is not initialized, however, until the local variable declaration statement that declares it is executed. (The rules of definite assignment (§16) prevent the value of a local variable from being used before it has been initialized or otherwise assigned a value.) The local variable effectively ceases to exist when the execution of the block or for statement is complete.
+

+Were it not for one exceptional situation, a local variable could always be regarded as being created when its local variable declaration statement is executed. The exceptional situation involves the switch statement (§14.10), where it is possible for control to enter a block but bypass execution of a local variable declaration statement. Because of the restrictions imposed by the rules of definite assignment (§16), however, the local variable declared by such a bypassed local variable declaration statement cannot be used before it has been definitely assigned a value by an assignment expression (§15.26). 
+
+The following example contains several different kinds of variables:
+
+class Point {
+	static int numPoints;		// numPoints is a class variable
+	int x, y;			// x and y are instance variables
+	int[] w = new int[10];		// w[0] is an array component
+	int setX(int x) {		// x is a method parameter
+		int oldx = this.x;	// oldx is a local variable
+		this.x = x;
+		return oldx;
+	}
+}
+

+
4.5.4    final Variables

+
+A variable can be declared final. A final variable may only be assigned to once. It is a compile time error if a final variable is assigned to unless it is definitely unassigned (§16) immediately prior to the assignment.

+
+A blank final is a final variable whose declaration lacks an initializer. 

+
+Once a final variable has been assigned, it always contains the same value. If a final variable holds a reference to an object, then the state of the object may be changed by operations on the object, but the variable will always refer to the same object. This applies also to arrays, because arrays are objects; if a final variable holds a reference to an array, then the components of the array may be changed by operations on the array, but the variable will always refer to the same array.

+
+Declaring a variable final can serve as useful documentation that its value will not change and can help avoid programming errors.
+
+
In the example:
+
class Point {
+	int x, y;
+	int useCount;
+	Point(int x, int y) { this.x = x; this.y = y; }
+	final static Point origin = new Point(0, 0);
+}
+

+the class Point declares a final class variable origin. The origin variable holds a reference to an object that is an instance of class Point whose coordinates are (0, 0). The value of the variable Point.origin can never change, so it always refers to the same Point object, the one created by its initializer. However, an operation on this Point object might change its state-for example, modifying its useCount or even, misleadingly, its x or y coordinate.

+
+
4.5.5    Initial Values of Variables

+
+Every variable in a program must have a value before its value is used:

+
    
+
  • Each class variable, instance variable, or array component is initialized with a default value when it is created (§15.9, §15.10):
+
      
+
+
    • For type byte, the default value is zero, that is, the value of (byte)0.
+
+
    • For type short, the default value is zero, that is, the value of (short)0.
+
+
    • For type int, the default value is zero, that is, 0.
+
+
    • For type long, the default value is zero, that is, 0L.
+
+
    • For type float, the default value is positive zero, that is, 0.0f.
+
+
    • For type double, the default value is positive zero, that is, 0.0d.
+
+
    • For type char, the default value is the null character, that is, '\u0000'.
+
+
    • For type boolean, the default value is false.
+
+
    • For all reference types (§4.3), the default value is null.
+
    
+
+
  • Each method parameter (§8.4.1) is initialized to the corresponding argument value provided by the invoker of the method (§15.12).
+
+
  • Each constructor parameter (§8.8.1) is initialized to the corresponding argument value provided by a class instance creation expression (§15.9) or explicit constructor invocation (§8.8.5).
+
+
  • An exception-handler parameter (§14.19) is initialized to the thrown object representing the exception (§11.3, §14.17). 
+
+
  • A local variable (§14.4, §14.13) must be explicitly given a value before it is used, by either initialization (§14.4) or assignment (§15.26), in a way that can be verified by the compiler using the rules for definite assignment (§16).
+

+The example program:

+
class Point {
+	static int npoints;
+	int x, y;
+	Point root;
+}
+class Test {
+	public static void main(String[] args) {
+		System.out.println("npoints=" + Point.npoints);
+		Point p = new Point();
+		System.out.println("p.x=" + p.x + ", p.y=" + p.y);
+		System.out.println("p.root=" + p.root);
+	}
+}
+

+prints:

+
npoints=0
+p.x=0, p.y=0
+p.root=null
+

+illustrating the default initialization of npoints, which occurs when the class Point is prepared (§12.3.2), and the default initialization of x, y, and root, which occurs when a new Point is instantiated. See §12 for a full description of all aspects of loading, linking, and initialization of classes and interfaces, plus a description of the instantiation of classes to make new class instances.

+
+
4.5.6    Types, Classes, and Interfaces

+
+In the Java programming language, every variable and every expression has a type that can be determined at compile time. The type may be a primitive type or a reference type. Reference types include class types and interface types. Reference types are introduced by type declarations, which include class declarations (§8.1) and interface declarations (§9.1). We often use the term type to refer to either a class or an interface.

+
+Every object belongs to some particular class: the class that was mentioned in the creation expression that produced the object, the class whose Class object was used to invoke a reflective method to produce the object, or the String class for objects implicitly created by the string concatenation operator + (§15.18.1). This class is called the class of the object. (Arrays also have a class, as described at the end of this section.) An object is said to be an instance of its class and of all superclasses of its class.

+
+Sometimes a variable or expression is said to have a "run-time type". This refers to the class of the object referred to by the value of the variable or expression at run time, assuming that the value is not null. 

+
+The compile time type of a variable is always declared, and the compile time type of an expression can be deduced at compile time. The compile time type limits the possible values that the variable can hold or the expression can produce at run time. If a run-time value is a reference that is not null, it refers to an object or array that has a class, and that class will necessarily be compatible with the compile-time type.

+
+Even though a variable or expression may have a compile-time type that is an interface type, there are no instances of interfaces. A variable or expression whose type is an interface type can reference any object whose class implements (§8.1.4) that interface.

+
+Here is an example of creating new objects and of the distinction between the type of a variable and the class of an object:
+
public interface Colorable {
+	void setColor(byte r, byte g, byte b);
+}
+class Point { int x, y; }
+class ColoredPoint extends Point implements Colorable {
+	byte r, g, b;
+	public void setColor(byte rv, byte gv, byte bv) {
+		r = rv; g = gv; b = bv;
+	}
+}
+class Test {
+	public static void main(String[] args) {
+		Point p = new Point();
+		ColoredPoint cp = new ColoredPoint();
+		p = cp;
+		Colorable c = cp;
+	}
+}
+

+In this example:

+
    
+
  • The local variable p of the method main of class Test has type Point and is initially assigned a reference to a new instance of class Point.
+
+
  • The local variable cp similarly has as its type ColoredPoint, and is initially assigned a reference to a new instance of class ColoredPoint.
+
+
  • The assignment of the value of cp to the variable p causes p to hold a reference to a ColoredPoint object. This is permitted because ColoredPoint is a subclass of Point, so the class ColoredPoint is assignment compatible (§5.2) with the type Point. A ColoredPoint object includes support for all the methods of a Point. In addition to its particular fields r, g, and b, it has the fields of class Point, namely x and y.
+
+
  • The local variable c has as its type the interface type Colorable, so it can hold a reference to any object whose class implements Colorable; specifically, it can hold a reference to a ColoredPoint.
+
+
  • Note that an expression such as "new Colorable()" is not valid because it is not possible to create an instance of an interface, only of a class.
+

+Every array also has a class; the method getClass, when invoked for an array object, will return a class object (of class Class) that represents the class of the array. 

+
+The classes for arrays have strange names that are not valid identifiers; for example, the class for an array of int components has the name "[I" and so the value of the expression:
+
new int[10].getClass().getName()
+

+is the string "[I"; see the specification of Class.getName for details.

+
+
+

+
+
+
+
Contents | Prev | Next | IndexJava Language Specification

+Second Edition

+Copyright © 2000 Sun Microsystems, Inc.
+All rights reserved
+

+Please send any comments or corrections via our feedback form
+
+
+
+
+ Names
+
+
+
+
+ 
+
+
+
+
Contents | Prev | Next | IndexJava Language Specification

+Second Edition

+


+ 
+
+

+CHAPTER
+ 6 

+
+
Names

+

+
+Names are used to refer to entities declared in a program. A declared entity (§6.1) is a package, class type, interface type, member (class, interface, field, or method) of a reference type, parameter (to a method, constructor, or exception handler), or local variable.

+
+Names in programs are either simple, consisting of a single identifier, or qualified,  consisting of a sequence of identifiers separated by "." tokens (§6.2).

+
+Every declaration that introduces a name has a scope (§6.3), which is the part of the program text within which the declared entity can be referred to by a simple name.

+
+Packages and reference types (that is, class types, interface types, and array types) have members (§6.4). A member can be referred to using a qualified name N.x, where N is a simple or qualified name and x is an identifier. If N names a package, then x is a member of that package, which is either a class or interface type or a subpackage. If N names a reference type or a variable of a reference type, then x names a member of that type, which is either a class, an interface, a field, or a method.

+
+In determining the meaning of a name (§6.5), the context of the occurrence is used to disambiguate among packages, types, variables, and methods with the same name.

+
+Access control (§6.6) can be specified in a class, interface, method, or field declaration to control when access to a member is allowed. Access is a different concept from scope; access specifies the part of the program text within which the declared entity can be referred to by a qualified name, a field access expression (§15.11), or a method invocation expression (§15.12) in which the method is not specified by a simple name. The default access is that a member can be accessed anywhere within the package that contains its declaration; other possibilities are public, protected, and private.

+
+Fully qualified and canonical names (§6.7) and naming conventions (§6.8) are also discussed in this chapter.

+
+The name of a field, parameter, or local variable may be used as an expression (§15.14.1). The name of a method may appear in an expression only as part of a method invocation expression (§15.12). The name of a class or interface type may appear in an expression only as part of a class literal (§15.8.2), a qualified this expression (§15.8.4), a class instance creation expression (§15.9), an array creation expression (§15.10), a cast expression (§15.16), or an instanceof expression (§15.20.2), or as part of a qualified name for a field or method. The name of a package may appear in an expression only as part of a qualified name for a class or interface type.

+
+
6.1    Declarations

+
+A declaration introduces an entity into a program and includes an identifier (§3.8) that can be used in a name to refer to this entity. A declared entity is one of the following:

+
    
+
  • A package, declared in a package declaration (§7.4)
+
+
  • An imported type, declared in a single-type-import declaration (§7.5.1) or a type-import-on-demand declaration (§7.5.2)
+
+
  • A class, declared in a class type declaration (§8.1)
+
+
  • An interface, declared in an interface type declaration (§9.1)
+
+
  • A member of a reference type (§8.2, §9.2, §10.7), one of the following:
+
      
+
+
    • A member class (§8.5, §9.5).
+
+
    • A member interface (§8.5, §9.5).
+
+
    • A field, one of the following:
+
        
+
+
      • A field declared in a class type (§8.3)
+
+
      • A constant field declared in an interface type (§9.3)
+
+
      • The field length, which is implicitly a member of every array type (§10.7)
+
      
+
+
    • A method, one of the following:
+
        
+
+
      • A method (abstract or otherwise) declared in a class type (§8.4)
+
+
      • A method (always abstract) declared in an interface type (§9.4)
+
      
+
    
+
+
  • A parameter, one of the following:
+
      
+
+
    • A parameter of a method or constructor of a class (§8.4.1, §8.8.1)
+
+
    • A parameter of an abstract method of an interface (§9.4)
+
+
    • A parameter of an exception handler declared in a catch clause of a try statement (§14.19)
+
    
+
+
  • A local variable, one of the following:
+
      
+
+
    • A local variable declared in a block (§14.4)
+
+
    • A local variable declared in a for statement (§14.13)
+
    
+

+Constructors (§8.8) are also introduced by declarations, but use the name of the class in which they are declared rather than introducing a new name.

+
+
6.2    Names and Identifiers

+
+A name is used to refer to an entity declared in a program.

+
+There are two forms of names: simple names and qualified names. A simple name is a single identifier. A qualified name consists of a name, a "." token, and an identifier.

+
+In determining the meaning of a name (§6.5), the context in which the name appears is taken into account. The rules of §6.5 distinguish among contexts where a name must denote (refer to) a package (§6.5.3), a type (§6.5.5), a variable or value in an expression (§6.5.6), or a method (§6.5.7).

+
+Not all identifiers in programs are a part of a name. Identifiers are also used in the following situations:

+
    
+
  • In declarations (§6.1), where an identifier may occur to specify the name by which the declared entity will be known
+
+
  • In field access expressions (§15.11), where an identifier occurs after a "." token to indicate a member of an object that is the value of an expression or the keyword super that appears before the "." token
+
+
  • In some method invocation expressions (§15.12), where an identifier may occur after a "." token and before a "(" token to indicate a method to be invoked for an object that is the value of an expression or the keyword super that appears before the "." token
+
+
  • In qualified class instance creation expressions (§15.9), where an identifier occurs immediately to the right of the leftmost new token to indicate a type that must be a member of the compile-time type of the primary expression preceding the "." preceding the leftmost new token.
+
+
  • As labels in labeled statements (§14.7) and in break (§14.14) and continue (§14.15) statements that refer to statement labels.
+

+In the example:

+
class Test {
+	public static void main(String[] args) {
+		Class c = System.out.getClass();
+		System.out.println(c.toString().length() +
+					args[0].length() + args.length);
+	}
+}
+

+the identifiers Test, main, and the first occurrences of args and c are not names; rather, they are used in declarations to specify the names of the declared entities. The names String, Class, System.out.getClass, System.out.println, c.toString, args, and args.length appear in the example. The first occurrence of length is not a name, but rather an identifier appearing in a method invocation expression (§15.12). The second occurrence of length is not a name, but rather an identifier appearing in a method invocation expression (§15.12).

+
+The identifiers used in labeled statements and their associated break and continue statements are completely separate from those used in declarations. Thus, the following code is valid:
+
class TestString {
+	char[] value;
+	int offset, count;
+	int indexOf(TestString str, int fromIndex) {
+		char[] v1 = value, v2 = str.value;
+		int max = offset + (count - str.count);
+		int start = offset + ((fromIndex < 0) ? 0 : fromIndex);
+	i:
+		for (int i = start; i <= max; i++)
+		{
+			int n = str.count, j = i, k = str.offset;
+			while (n-- != 0) {
+				if (v1[j++] != v2[k++])
+					continue i;
+			} 
+			return i - offset;
+		}
+		return -1;
+	}
+}
+

+This code was taken from a version of the class String and its method indexOf, where the label was originally called test. Changing the label to have the same name as the local variable i does not obscure (§6.3.2) the label in the scope of the declaration of i. The identifier max could also have been used as the statement label; the label would not obscure the local variable max within the labeled statement.

+
+
6.3    Scope of a Declaration

+
+The scope of a declaration is the region of the program within which the entity declared by the declaration can be referred to using a simple name (provided it is visible (§6.3.1)). A declaration is said to be in scope at a particular point in a program if and only if the declaration's scope includes that point. 

+
+The scoping rules for various constructs are given in the sections that describe those constructs. For convenience, the rules are repeated here:

+
+ The scope of the declaration of an observable (§7.4.3) top level package is all observable compilation units (§7.3). The declaration of a package that is not observable is never in scope. Subpackage declarations are never in scope.

+
+The scope of a type imported by a single-type-import declaration (§7.5.1) or type-import-on-demand declaration (§7.5.2) is all the class and interface type declarations (§7.6) in the compilation unit in which the import declaration appears.

+
+The scope of a top level type is all type declarations in the package in which the top level type is declared.

+
+The scope of a label declared by a labeled statement is the statement immediately enclosed by the labeled statement.

+
+The scope of a declaration of a member m declared in or inherited by a class type C is the entire body of C, including any nested type declarations.

+
+The scope of the declaration of a member m declared in or inherited by an interface type I is the entire body of I, including any nested type declarations.

+
+The scope of a parameter of a method (§8.4.1) or constructor (§8.8.1) is the entire body of the method or constructor.

+
+The scope of a local variable declaration in a block (§14.4.2) is the rest of the block in which the declaration appears, starting with its own initializer (§14.4) and including any further declarators to the right in the local variable declaration statement. 

+
+The scope of a local class declared in a block is the rest of the immediately enclosing block, including its own class declaration.

+
+The scope of a local variable declared in the ForInit part of a for statement (§14.13) includes all of the following:

+
    
+
  • Its own initializer
+
+
  • Any further declarators to the right in the ForInit part of the for statement
+
+
  • The Expression and ForUpdate parts of the for statement
+
+
  • The contained Statement
+

+The scope of a parameter of an exception handler that is declared in a catch clause of a try statement (§14.19) is the entire block associated with the catch.

+
+These rules imply that declarations of class and interface types need not appear before uses of the types.

+
+In the example:
+
package points;
+class Point {
+	int x, y;
+	PointList list;
+	Point next;
+}
+class PointList {
+	Point first;
+}
+

+the use of PointList in class Point is correct, because the scope of the class declaration PointList includes both class Point and class PointList, as well as any other type declarations in other compilation units of package points.

+
+
6.3.1    Shadowing Declarations

+
+Some declarations may be shadowed in part of their scope by another declaration of the same name, in which case a simple name cannot be used to refer to the declared entity.

+
+A declaration d of a type named n shadows the declarations of any other types named n that are in scope at the point where d occurs throughout the scope of d.

+
+A declaration d of a field, local variable, method parameter, constructor parameter or exception handler parameter named n shadows the declarations of any other fields, local variables, method parameters, constructor parameters or exception handler parameters named n that are in scope at the point where d occurs throughout the scope of d.

+
+A declaration d of a label named n shadows the declarations of any other labels named n that are in scope at the point where d occurs throughout the scope of d.

+
+A declaration d of a method named n shadows the declarations of any other methods named n that are in an enclosing scope at the point where d occurs throughout the scope of d.

+
+A package declaration never shadows any other declaration. 

+
+A single-type-import declaration d in a compilation unit c of package p that imports a type named n shadows the declarations of:

+
    
+
  • any top level type named n declared in another compilation unit of p.
+
+
  • any type named n imported by a type-import-on-demand declaration in c.
+

+throughout c.

+
+A type-import-on-demand declaration never causes any other declaration to be shadowed.

+
+A declaration d is said to be visible at point p in a program if the scope of d includes p, and d is not shadowed by any other declaration at p. When the program point we are discussing is clear from context, we will often simply say that a declaration is visible.

+
+Note that shadowing is distinct from hiding (§8.3, §8.4.6.2, §8.5, §9.3, §9.5). Hiding, in the technical sense defined in this specification, applies only to members which would otherwise be inherited but are not because of a declaration in a subclass. Shadowing is also distinct from obscuring (§6.3.2).
+

+
+Here is an example of shadowing of a field declaration by a local variable declaration:
+
class Test {
+	static int x = 1;
+	public static void main(String[] args) {
+		int x = 0;
+		System.out.print("x=" + x);
+		System.out.println(", Test.x=" + Test.x);
+	}
+}
+

+produces the output:

+
x=0, Test.x=1
+

+This example declares:

+
    
+
  • a class Test
+
+
  • a class (static) variable x that is a member of the class Test
+
+
  • a class method main that is a member of the class Test
+
+
  • a parameter args of the main method
+
+
  • a local variable x of the main method
+
+

+Since the scope of a class variable includes the entire body of the class (§8.2) the class variable x would normally be available throughout the entire body of the method main. In this example, however, the class variable x is shadowed within the body of the method main by the declaration of the local variable x.
+

+
+A local variable has as its scope the rest of the block in which it is declared (§14.4.2); in this case this is the rest of the body of the main method, namely its initializer "0" and the invocations of print and println.
+
+

+This means that:
+
+
    
+
  • The expression "x" in the invocation of print refers to (denotes) the value of the local variable x.
+
+
  • The invocation of println uses a qualified name (§6.6) Test.x, which uses the class type name Test to access the class variable x, because the declaration of Test.x is shadowed at this point and cannot be referred to by its simple name.
+
+

+The following example illustrates the shadowing of one type declaration by another:
+
import java.util.*;
+class Vector {
+	int val[] = { 1 , 2 };
+
+}

+class Test {
+	public static void main(String[] args) {
+		Vector v = new Vector();
+		System.out.println(v.val[0]);
+	}
+}
+

+compiles and prints:

+
1
+

+using the class Vector declared here in preference to class java.util.Vector that might be imported on demand.

+
+
6.3.2    Obscured Declarations

+
+A simple name may occur in contexts where it may potentially be interpreted as the name of a variable, a type or a package. In these situations, the rules of §6.5 specify that a variable will be chosen in preference to a type, and that a type will be chosen in preference to a package. Thus, it is may sometimes be impossible to refer to a visible type or package declaration via its simple name. We say that such a declaration is obscured. 

+
+Obscuring is distinct from shadowing (§6.3.1) and hiding (§8.3, §8.4.6.2, §8.5, §9.3, §9.5). The naming conventions of §6.8 help reduce obscuring.
+

+
+
6.4    Members and Inheritance

+
+Packages and reference types have members. 

+
+This section provides an overview of the members of packages and reference types here, as background for the discussion of qualified names and the determination of the meaning of names. For a complete description of membership, see §7.1, §8.2, §9.2, and §10.7. 
+

+
+
6.4.1    The Members of a Package

+
+The members of a package (§7) are specified in §7.1. For convenience, we repeat that specification here:

+
+The members of a package are subpackages and all the top level (§7.6) class (§8) and top level interface (§9) types declared in all the compilation units (§7.3) of the package.

+
+In general, the subpackages of a package are determined by the host system (§7.2). However, the package java always includes the subpackages lang and io and may include other subpackages. No two distinct members of the same package may have the same simple name (§7.1), but members of different packages may have the same simple name. 

+
+For example, it is possible to declare a package:
+
package vector;
+public class Vector { Object[] vec; }
+

+that has as a member a public class named Vector, even though the package java.util also declares a class named Vector. These two class types are different, reflected by the fact that they have different fully qualified names (§6.7). The fully qualified name of this example Vector is vector.Vector, whereas java.util.Vector is the fully qualified name of the standard Vector class. Because the package vector contains a class named Vector, it cannot also have a subpackage named Vector.

+
+
6.4.2    The Members of a Class Type

+
+The members of a class type (§8.2) are classes (§8.5, §9.5), interfaces (§8.5, §9.5), fields (§8.3, §9.3, §10.7), and methods (§8.4, §9.4). Members are either declared in the type, or inherited because they are accessible members of a superclass or superinterface which are neither private nor hidden nor overridden (§8.4.6).

+
+The members of a class type are all of the following:
+
    
+
  • Members inherited from its direct superclass (§8.1.3), if it has one (the class Object has no direct superclass)
+
+
  • Members inherited from any direct superinterfaces (§8.1.4)
+
+
  • Members declared in the body of the class (§8.1.5)
+

+Constructors (§8.8) are not members.

+
+There is no restriction against a field and a method of a class type having the same simple name. Likewise, there is no restriction against a member class or member interface of a class type having the same simple name as a field or method of that class type.
+
+

+A class may have two or more fields with the same simple name if they are declared in different interfaces and inherited. An attempt to refer to any of the fields by its simple name results in a compile-time error (§6.5.7.2, §8.2).
+
+

+In the example:
+
interface Colors {
+	int WHITE = 0, BLACK = 1;
+}
+interface Separates {
+	int CYAN = 0, MAGENTA = 1, YELLOW = 2, BLACK = 3;
+}
+class Test implements Colors, Separates {
+	public static void main(String[] args) {
+		System.out.println(BLACK); // compile-time error: ambiguous
+	}
+}
+

+the name BLACK in the method main is ambiguous, because class Test has two members named BLACK, one inherited from Colors and one from Separates.

+
+A class type may have two or more methods with the same simple name if the methods have different signatures (§8.4.2), that is, if they have different numbers of parameters or different parameter types in at least one parameter position. Such a method member name is said to be overloaded.
+

+
+A class type may contain a declaration for a method with the same name and the same signature as a method that would otherwise be inherited from a superclass or superinterface. In this case, the method of the superclass or superinterface is not inherited. If the method not inherited is abstract, then the new declaration is said to implement it; if the method not inherited is not abstract, then the new declaration is said to override it.

+
+In the example:
+
class Point {
+	float x, y;
+	void move(int dx, int dy) { x += dx; y += dy; }
+	void move(float dx, float dy) { x += dx; y += dy; }
+	public String toString() { return "("+x+","+y+")"; }
+}
+

+the class Point has two members that are methods with the same name, move. The overloaded move method of class Point chosen for any particular method invocation is determined at compile time by the overloading resolution procedure given in §15.12.

+
+In this example, the members of the class Point are the float instance variables x and y declared in Point, the two declared move methods, the declared toString method, and the members that Point inherits from its implicit direct superclass Object (§4.3.2), such as the method hashCode. Note that Point does not inherit the toString method of class Object because that method is overridden by the declaration of the toString method in class Point.
+

+
+
6.4.3    The Members of an Interface Type

+
+The members of an interface type (§9.2) may be classes (§8.5, §9.5), interfaces (§8.5, §9.5), fields (§8.3, §9.3, §10.7), and methods (§8.4, §9.4).The members of an interface are:

+
    
+
  • Those members declared in the interface.
+
+
  • Those members inherited from direct superinterfaces.
+
+
  • If an interface has no direct superinterfaces, then the interface implicitly declares a public abstract member method m with signature s, return type r, and throws clause t corresponding to each public instance method m with signature s, return type r, and throws clause t declared in Object, unless a method with the same signature, same return type, and a compatible throws clause is explicitly declared by the interface.
+

+An interface may have two or more fields with the same simple name if they are declared in different interfaces and inherited. An attempt to refer to any such field by its simple name results in a compile-time error (§6.5.6.1, §9.2).

+
+In the example:
+
interface Colors {
+	int WHITE = 0, BLACK = 1;
+}
+interface Separates {
+	int CYAN = 0, MAGENTA = 1, YELLOW = 2, BLACK = 3;
+}
+interface ColorsAndSeparates extends Colors, Separates {
+	int DEFAULT = BLACK;								 	// compile-time error: ambiguous
+}
+

+the members of the interface ColorsAndSeparates include those members inherited from Colors and those inherited from Separates, namely WHITE, BLACK (first of two), CYAN, MAGENTA, YELLOW, and BLACK (second of two). The member name BLACK is ambiguous in the interface ColorsAndSeparates.

+
+
6.4.4    The Members of an Array Type

+
+The members of an array type are specified in §10.7. For convenience, we repeat that specification here.

+
+The members of an array type are all of the following:

+
    
+
  • The public final field length, which contains the number of components of the array (length may be positive or zero)
+
+
  • The public method clone, which overrides the method of the same name in class Object and throws no checked exceptions
+
+
  • All the members inherited from class Object; the only method of Object that is not inherited is its clone method
+

+

+
+The example:

+
class Test {
+	public static void main(String[] args) {
+		int[] ia = new int[3];
+		int[] ib = new int[6];
+		System.out.println(ia.getClass() == ib.getClass());
+		System.out.println("ia has length=" + ia.length);
+	}
+}
+

+produces the output:

+
true
+ia has length=3
+

+This example uses the method getClass inherited from class Object and the field length. The result of the comparison of the Class objects in the first println demonstrates that all arrays whose components are of type int are instances of the same array type, which is int[].

+
+
6.5    Determining the Meaning of a Name

+
+The meaning of a name depends on the context in which it is used. The determination of the meaning of a name requires three steps. First, context causes a name syntactically to fall into one of six categories: PackageName, TypeName, ExpressionName, MethodName, PackageOrTypeName, or AmbiguousName. Second, a name that is initially classified by its context as an AmbiguousName or as a Package-OrTypeName is then reclassified to be a PackageName, TypeName, or ExpressionName. Third, the resulting category then dictates the final determination of the meaning of the name (or a compilation error if the name has no meaning).

+
    
+PackageName:
+	Identifier
+	PackageName . Identifier
+
+TypeName:
+	Identifier
+	PackageOrTypeName . Identifier
+
+ExpressionName:
+	Identifier
+	AmbiguousName . Identifier
+
+MethodName:
+	Identifier
+	AmbiguousName . Identifier
+
+PackageOrTypeName:
+	Identifier
+	PackageOrTypeName . Identifier
+
+AmbiguousName:
+	Identifier
+	AmbiguousName . Identifier
+	

+
+The use of context helps to minimize name conflicts between entities of different kinds. Such conflicts will be rare if the naming conventions described in §6.8 are followed. Nevertheless, conflicts may arise unintentionally as types developed by different programmers or different organizations evolve. For example, types, methods, and fields may have the same name. It is always possible to distinguish between a method and a field with the same name, since the context of a use always tells whether a method is intended.
+
+

6.5.1 Syntactic Classification of a Name According to Context

+ +A name is syntactically classified as a PackageName in these contexts:

+

    +
  • In a package declaration (§7.4) + +
  • To the left of the "." in a qualified PackageName +
+A name is syntactically classified as a TypeName in these contexts:

+

    +
  • In a single-type-import declaration (§7.5.1) + +
  • In an extends clause in a class declaration (§8.1.3) + +
  • In an implements clause in a class declaration (§8.1.4) + +
  • In an extends clause in an interface declaration (§9.1.2) + +
  • As a Type (or the part of a Type that remains after all brackets are deleted) in any of the following contexts: +
      + +
    • In a field declaration (§8.3, §9.3) + +
    • As the result type of a method (§8.4, §9.4) + +
    • As the type of a formal parameter of a method or constructor (§8.4.1, §8.8.1, §9.4) + +
    • As the type of an exception that can be thrown by a method or constructor (§8.4.4, §8.8.4, §9.4) + +
    • As the type of a local variable (§14.4) + +
    • As the type of an exception parameter in a catch clause of a try statement (§14.19) + +
    • As the type in a class literal (§15.8.2) + +
    • As the qualifying type of a qualified this expression (§15.8.4). + +
    • As the class type which is to be instantiated in an unqualified class instance creation expression (§15.9) + +
    • As the direct superclass or direct superinterface of an anonymous class (§15.9.5) which is to be instantiated in an unqualified class instance creation expression (§15.9) + +
    • As the element type of an array to be created in an array creation expression (§15.10) + +
    • As the qualifying type of field access using the keyword super (§15.11.2) + +
    • As the qualifying type of a method invocation using the keyword super (§15.12) + +
    • As the type mentioned in the cast operator of a cast expression (§15.16) + +
    • As the type that follows the instanceof relational operator (§15.20.2) +
    +
+A name is syntactically classified as an ExpressionName in these contexts:

+

    +
  • As the qualifying expression in a qualified superclass constructor invocation (§8.8.5.1) + +
  • As the qualifying expression in a qualified class instance creation expression (§15.9) + +
  • As the array reference expression in an array access expression (§15.13) + +
  • As a PostfixExpression (§15.14) + +
  • As the left-hand operand of an assignment operator (§15.26) +
+A name is syntactically classified as a MethodName in this context:

+

    +
  • Before the "(" in a method invocation expression (§15.12) +
+A name is syntactically classified as a PackageOrTypeName in these contexts:

+

    +
  • To the left of the "." in a qualified TypeName + +
  • In a type-import-on-demand declaration (§7.5.2) +
+A name is syntactically classified as an AmbiguousName in these contexts:

+

    +
  • To the left of the "." in a qualified ExpressionName + +
  • To the left of the "." in a qualified MethodName + +
  • To the left of the "." in a qualified AmbiguousName +
+

6.5.2 Reclassification of Contextually Ambiguous Names

+ +An AmbiguousName is then reclassified as follows:

+

    +
  • If the AmbiguousName is a simple name, consisting of a single Identifier: +
      + +
    • If the Identifier appears within the scope (§6.3) of a local variable declaration (§14.4) or parameter declaration (§8.4.1, §8.8.1, §14.19) or field declaration (§8.3) with that name, then the AmbiguousName is reclassified as an ExpressionName. + +
    • Otherwise, if the Identifier appears within the scope (§6.3) of a local class declaration (§14.3) or member type declaration (§8.5, §9.5) with that name, then the AmbiguousName is reclassified as a TypeName. + +
    • Otherwise, if a type of that name is declared in the compilation unit (§7.3) containing the Identifier, either by a single-type-import declaration (§7.5.1) or by a top level class (§8) or interface type declaration (§9), then the AmbiguousName is reclassified as a TypeName. + +
    • Otherwise, if a type of that name is declared in another compilation unit (§7.3) of the package (§7.1) of the compilation unit containing the Identifier, then the AmbiguousName is reclassified as a TypeName. + +
    • Otherwise, if a type of that name is declared by exactly one type-import-on-demand declaration (§7.5.2) of the compilation unit containing the Identifier, then the AmbiguousName is reclassified as a TypeName. + +
    • Otherwise, if a type of that name is declared by more than one type-import-on-demand declaration of the compilation unit containing the Identifier, then a compile-time error results. + +
    • Otherwise, the AmbiguousName is reclassified as a PackageName. A later step determines whether or not a package of that name actually exists. +
    + +
  • If the AmbiguousName is a qualified name, consisting of a name, a ".", and an Identifier, then the name to the left of the "." is first reclassified, for it is itself an AmbiguousName. There is then a choice: +
      + +
    • If the name to the left of the "." is reclassified as a PackageName, then if there is a package whose name is the name to the left of the "." and that package contains a declaration of a type whose name is the same as the Identifier, then this AmbiguousName is reclassified as a TypeName. Otherwise, this AmbiguousName is reclassified as a PackageName. A later step determines whether or not a package of that name actually exists. + +
    • If the name to the left of the "." is reclassified as a TypeName, then if the Identifier is the name of a method or field of the class or interface denoted by TypeName, this AmbiguousName is reclassified as an ExpressionName. Otherwise, if the Identifier is the name of a member type of the class or interface denoted by TypeName, this AmbiguousName is reclassified as a TypeName. Otherwise, a compile-time error results. + +
    • If the name to the left of the "." is reclassified as an ExpressionName, then let T be the type of the expression denoted by ExpressionName. If the Identifier is the name of a method or field of the class or interface denoted by T, this AmbiguousName is reclassified as an ExpressionName. Otherwise, if the Identifier is the name of a member type (§8.5, §9.5) of the class or interface denoted by T, then this AmbiguousName is reclassified as a TypeName. Otherwise, a compile-time error results. + +
    +

    +As an example, consider the following contrived "library code": +

package org.rpgpoet;
+import java.util.Random;
+interface Music { Random[] wizards = new Random[4]; }
+
+and then consider this example code in another package:

+

package bazola;
+class Gabriel {
+	static int n = org.rpgpoet.Music.wizards.length;
+}
+
+First of all, the name org.rpgpoet.Music.wizards.length is classified as an ExpressionName because it functions as a PostfixExpression. Therefore, each of the names:

+ +

org.rpgpoet.Music.wizards
+org.rpgpoet.Music
+org.rpgpoet
+org 
+
+is initially classified as an AmbiguousName. These are then reclassified:

+

    +
  • The simple name org is reclassified as a PackageName (since there is no variable or type named org in scope). + +
  • Next, assuming that there is no class or interface named rpgpoet in any compilation unit of package org (and we know that there is no such class or interface because package org has a subpackage named rpgpoet), the qualified name org.rpgpoet is reclassified as a PackageName. + +
  • Next, because package org.rpgpoet has an interface type named Music, the qualified name org.rpgpoet.Music is reclassified as a TypeName. + +
  • Finally, because the name org.rpgpoet.Music is a TypeName, the qualified name org.rpgpoet.Music.wizards is reclassified as an ExpressionName. +
+

6.5.3 Meaning of Package Names

+ +The meaning of a name classified as a PackageName is determined as follows.

+ +

6.5.3.1 Simple Package Names

+ +If a package name consists of a single Identifier, then this identifier denotes a top level package named by that identifier. If no top level package of that name is in scope (§7.4.4), then a compile-time error occurs.

+ +

6.5.3.2 Qualified Package Names

+ +If a package name is of the form Q.Id, then Q must also be a package name. The package name Q.Id names a package that is the member named Id within the package named by Q. If Q does not name an observable package (§7.4.3), or Id is not the simple name an observable subpackage of that package, then a compile-time error occurs.

+ +

6.5.4 Meaning of PackageOrTypeNames

+ +

6.5.4.1 Simple PackageOrTypeNames

+ +If the PackageOrTypeName, Q, occurs in the scope of a type named Q, then the PackageOrTypeName is reclassified as a TypeName.

+ +Otherwise, the PackageOrTypeName is reclassified as a PackageName. The meaning of the PackageOrTypeName is the meaning of the reclassified name.

+ +

6.5.4.2 Qualified PackageOrTypeNames

+ +Given a qualified PackageOrTypeName of the form Q.Id, if the type or package denoted by Q has a member type named Id, then the qualified PackageOrTypeName name is reclassified as a TypeName.

+ +Otherwise, it is reclassified as a PackageName. The meaning of the qualified PackageOrTypeName is the meaning of the reclassified name.

+ +

6.5.5 Meaning of Type Names

+ +The meaning of a name classified as a TypeName is determined as follows.

+ +

6.5.5.1 Simple Type Names

+ +If a type name consists of a single Identifier, then the identifier must occur in the scope of a declaration of a type with this name, or a compile-time error occurs.

+ +It is possible that the identifier occurs within the scope of more than one type with that name, in which case the type denoted by the name is determined as follows:

+

    +
  • If the simple type name occurs within the scope of a visible local class declaration (§14.3) with that name, then the simple type name denotes that local class type. + +
  • Otherwise, if the simple type name occurs within the scope of exactly one visible member type (§8.5, §9.5), then the simple type name denotes that member type. + +
  • Otherwise, if the simple type name occurs within the scope of more than one visible member type, then the name is ambiguous as a type name; a compile-time error occurs. + +
  • Otherwise, if a type with that name is declared in the current compilation unit (§7.3), either by a single-type-import declaration (§7.5.1) or by a declaration of a class or interface type (§7.6), then the simple type name denotes that type. + +
  • Otherwise, if a type with that name is declared in another compilation unit (§7.3) of the package (§7.1) containing the identifier, then the identifier denotes that type. + +
  • Otherwise, if a type of that name is declared by exactly one type-import-on-demand declaration (§7.5.2) of the compilation unit containing the identifier, then the simple type name denotes that type. + +
  • Otherwise, if a type of that name is declared by more than one type-import-on-demand declaration of the compilation unit, then the name is ambiguous as a type name; a compile-time error occurs. + +
  • Otherwise, the name is undefined as a type name; a compile-time error occurs. +
+This order for considering type declarations is designed to choose the most explicit of two or more applicable type declarations.

+ +

6.5.5.2 Qualified Type Names

+ +If a type name is of the form Q.Id, then Q must be either a type name or a package name. If Id names exactly one type that is a member of the type or package denoted by Q, then the qualified type name denotes that type. If Id does not name a member type (§8.5, §9.5) within Q, or the member type named Id within Q is not accessible (§6.6), or Id names more than one member type within Q, then a compile-time error occurs.

+ +The example: +

package wnj.test;
+class Test {
+	public static void main(String[] args) {
+		java.util.Date date =
+			new java.util.Date(System.currentTimeMillis());
+		System.out.println(date.toLocaleString());
+	}
+}
+
+produced the following output the first time it was run:

+

Sun Jan 21 22:56:29 1996
+
+In this example the name java.util.Date must denote a type, so we first use the procedure recursively to determine if java.util is an accessible type or a package, which it is, and then look to see if the type Date is accessible in this package.

+ +

6.5.6 Meaning of Expression Names

+ +The meaning of a name classified as an ExpressionName is determined as follows.

+ +

6.5.6.1 Simple Expression Names

+ +If an expression name consists of a single Identifier, then:

+

    +
  • If the Identifier appears within the scope (§6.3) of a visible local variable declaration (§14.4) or visible parameter declaration (§8.4.1, §8.8.1, §14.19) with that name, then the expression name denotes a variable, that is, that local variable or parameter. There is necessarily at most one such local variable or parameter. The type of the expression name is the declared type of the local variable or parameter. + +
  • Otherwise, if the Identifier appears within a class declaration (§8): +
      + +
    • If the Identifier appears within the scope (§6.3) of a visible field declaration with that name, then there must be a lexically enclosing type declaration of which that field is a member. Let T be the innermost such declaration. If there is not exactly one member of T that is a field with that name, then a compile-time error results. + +
    • Otherwise, if the single member field with that name is declared final (§8.3.1.2), then the expression name denotes the value of the field. The type of the expression name is the declared type of the field. If the Identifier appears in a context that requires a variable and not a value, then a compile-time error occurs. + +
    • Otherwise, the expression name denotes a variable, the single member field with that name. The type of the expression name is the field's declared type. +
    +
+
    If the field is an instance variable (§8.3.1.1), the expression name must appear within the declaration of an instance method (§8.4), constructor (§8.8), or instance variable initializer (§8.3.2.2). If it appears within a static method (§8.4.3.2), static initializer (§8.7), or initializer for a static variable (§8.3.1.1, §12.4.2), then a compile-time error occurs. +
    +
  • Otherwise, the identifier appears within an interface declaration (§9): +
      + +
    • If the Identifier appears within the scope (§6.3) of a visible field declaration with that name, then there must be an enclosing type declaration T of which that field is a member. If there is not exactly one member of T that is a field with that name, then a compile-time error results. + +
    • Otherwise, the expression name denotes the value of the single member field of that name. The type of the expression name is the declared type of the field. If the Identifier appears in a context that requires a variable and not a value, then a compile-time error occurs. +
    +
+In the example:

+

class Test {
+	static int v;
+	static final int f = 3;
+	public static void main(String[] args) {
+		int i;
+		i = 1;
+		v = 2;
+		f = 33;										// compile-time error
+		System.out.println(i + " " + v + " " + f);
+	}
+}
+
+the names used as the left-hand-sides in the assignments to i, v, and f denote the local variable i, the field v, and the value of f (not the variable f, because f is a final variable). The example therefore produces an error at compile time because the last assignment does not have a variable as its left-hand side. If the erroneous assignment is removed, the modified code can be compiled and it will produce the output:

+

1 2 3
+
+

6.5.6.2 Qualified Expression Names

+ +If an expression name is of the form Q.Id, then Q has already been classified as a package name, a type name, or an expression name:

+

    +
  • If Q is a package name, then a compile-time error occurs. + +
  • If Q is a type name that names a class type (§8), then: +
      + +
    • If there is not exactly one accessible (§6.6) member of the class type that is a field named Id, then a compile-time error occurs. + +
    • Otherwise, if the single accessible member field is not a class variable (that is, it is not declared static), then a compile-time error occurs. + +
    • Otherwise, if the class variable is declared final, then Q.Id denotes the value of the class variable. The type of the expression Q.Id is the declared type of the class variable. If Q.Id appears in a context that requires a variable and not a value, then a compile-time error occurs. + +
    • Otherwise, Q.Id denotes the class variable. The type of the expression Q.Id is the declared type of the class variable. +
    + +
  • If Q is a type name that names an interface type (§9), then: +
      + +
    • If there is not exactly one accessible (§6.6) member of the interface type that is a field named Id, then a compile-time error occurs. + +
    • Otherwise, Q.Id denotes the value of the field. The type of the expression Q.Id is the declared type of the field. If Q.Id appears in a context that requires a variable and not a value, then a compile-time error occurs. +
    + +
  • If Q is an expression name, let T be the type of the expression Q: +
      + +
    • If T is not a reference type, a compile-time error occurs. + +
    • If there is not exactly one accessible (§6.6) member of the type T that is a field named Id, then a compile-time error occurs. + +
    • Otherwise, if this field is any of the following: +
        + +
      • A field of an interface type + +
      • A final field of a class type (which may be either a class variable or an instance variable) + +
      • The final field length of an array type +
        then Q.Id denotes the value of the field. The type of the expression Q.Id is the declared type of the field. If Q.Id appears in a context that requires a variable and not a value, then a compile-time error occurs. +
        +
      + +
    • Otherwise, Q.Id denotes a variable, the field Id of class T, which may be either a class variable or an instance variable. The type of the expression Q.Id is the declared type of the field + +

      The example: +

    +
class Point {
+	int x, y;
+	static int nPoints;
+}
+class Test {
+	public static void main(String[] args) {
+		int i = 0;
+		i.x++;			// compile-time error
+		Point p = new Point();
+		p.nPoints();	// compile-time error
+	}
+}
+
+encounters two compile-time errors, because the int variable i has no members, and because nPoints is not a method of class Point.

+ +

6.5.7 Meaning of Method Names

+ +A MethodName can appear only in a method invocation expression (§15.12). The meaning of a name classified as a MethodName is determined as follows.

+ +

6.5.7.1 Simple Method Names

+ +If a method name consists of a single Identifier, then Identifier is the method name to be used for method invocation. The Identifier must name at least one method of a class or interface within whose declaration the Identifier appears. See §15.12 for further discussion of the interpretation of simple method names in method invocation expressions.

+ +

6.5.7.2 Qualified Method Names

+ +If a method name is of the form Q.Id, then Q has already been classified as a package name, a type name, or an expression name. If Q is a package name, then a compile-time error occurs. Otherwise, Id is the method name to be used for method invocation. If Q is a type name, then Id must name at least one static method of the type Q. If Q is an expression name, then let T be the type of the expression Q; Id must name at least one method of the type T. See §15.12 for further discussion of the interpretation of qualified method names in method invocation expressions.

+ +

6.6 Access Control

+ +The Java programming language provides mechanisms for access control, to prevent the users of a package or class from depending on unnecessary details of the implementation of that package or class. If access is permitted, then the accessed entity is said to be accessible.

+ +Note that accessibility is a static property that can be determined at compile time; it depends only on types and declaration modifiers. Qualified names are a means of access to members of packages and reference types; related means of access include field access expressions (§15.11) and method invocation expressions (§15.12). All three are syntactically similar in that a "." token appears, preceded by some indication of a package, type, or expression having a type and followed by an Identifier that names a member of the package or type. These are collectively known as constructs for qualified access.

+ +Access control applies to qualified access and to the invocation of constructors by class instance creation expressions (§15.9) and explicit constructor invocations (§8.8.5). Accessibility also affects inheritance of class members (§8.2), including hiding and method overriding (§8.4.6.1).

+ +

6.6.1 Determining Accessibility

+
    +
  • A package is always accessible. + +
  • If a class or interface type is declared public, then it may be accessed by any code, provided that the compilation unit (§7.3) in which it is declared is observable. If a top level class or interface type is not declared public, then it may be accessed only from within the package in which it is declared. + +
  • An array type is accessible if and only if its element type is accessible. + +
  • A member (class, interface, field, or method) of a reference (class, interface, or array) type or a constructor of a class type is accessible only if the type is accessible and the member or constructor is declared to permit access: +
      + +
    • If the member or constructor is declared public, then access is permitted. All members of interfaces are implicitly public. + +
    • Otherwise, if the member or constructor is declared protected, then access is permitted only when one of the following is true: +
          + +
        • Access to the member or constructor occurs from within the package containing the class in which the protected member or constructor is declared. + +
        • Access is correct as described in §6.6.2. +
        +
      +
    • Otherwise, if the member or constructor is declared private, then access is permitted if and only if it occurs within the body of the top level class (§7.6) that encloses the declaration of the member. + +
    • Otherwise, we say there is default access, which is permitted only when the access occurs from within the package in which the type is declared. +
    +
+

6.6.2 Details on protected Access

+ +A protected member or constructor of an object may be accessed from outside the package in which it is declared only by code that is responsible for the implementation of that object.

+ +

6.6.2.1 Access to a protected Member

+ +Let C be the class in which a protected member m is declared. Access is permitted only within the body of a subclass S of C. In addition, if Id denotes an instance field or instance method, then:

+

    +
  • If the access is by a qualified name Q.Id, where Q is an ExpressionName, then the access is permitted if and only if the type of the expression Q is S or a subclass of S. + +
  • If the access is by a field access expression E.Id, where E is a Primary expression, or by a method invocation expression E.Id(. . .), where E is a Primary expression, then the access is permitted if and only if the type of E is S or a subclass of S. +
+

6.6.2.2 Qualified Access to a protected Constructor

+ +Let C be the class in which a protected constructor is declared and let S be the innermost class in whose declaration the use of the protected constructor occurs. Then:

+

    +
  • If the access is by a superclass constructor invocation super(. . .) or by a qualified superclass constructor invocation of the form E.super(. . .), where E is a Primary expression, then the access is permitted. + +
  • If the access is by an anonymous class instance creation expression of the form new C(. . .){...} or by a qualified class instance creation expression of the form E.new C(. . .){...}, where E is a Primary expression, then the access is permitted. + +
  • Otherwise, if the access is by a simple class instance creation expression of the form new C(. . .) or by a qualified class instance creation expression of the form E.new C(. . .), where E is a Primary expression, then the access is not permitted. A protected constructor can be accessed by a class instance creation expression (that does not declare an anonymous class) only from within the package in which it is defined. +
+

6.6.3 An Example of Access Control

+ +For examples of access control, consider the two compilation units:

+

package points;
+class PointVec { Point[] vec; }
+
+and:

+

package points;
+public class Point {
+	protected int x, y;
+	public void move(int dx, int dy) { x += dx; y += dy; }
+	public int getX() { return x; }
+	public int getY() { return y; }
+}
+
+which declare two class types in the package points:

+

    +
  • The class type PointVec is not public and not part of the public interface of the package points, but rather can be used only by other classes in the package. + +
  • The class type Point is declared public and is available to other packages. It is part of the public interface of the package points. + +
  • The methods move, getX, and getY of the class Point are declared public and so are available to any code that uses an object of type Point. + +
  • The fields x and y are declared protected and are accessible outside the package points only in subclasses of class Point, and only when they are fields of objects that are being implemented by the code that is accessing them. +
+See §6.6.7 for an example of how the protected access modifier limits access.

+ +

6.6.4 Example: Access to public and Non-public Classes

+ +If a class lacks the public modifier, access to the class declaration is limited to the package in which it is declared (§6.6). In the example:

+

package points;
+public class Point {
+	public int x, y;
+	public void move(int dx, int dy) { x += dx; y += dy; }
+}
+class PointList {
+	Point next, prev;
+}
+
+two classes are declared in the compilation unit. The class Point is available outside the package points, while the class PointList is available for access only within the package.

+ +Thus a compilation unit in another package can access points.Point, either by using its fully qualified name: +

package pointsUser;
+class Test {
+	public static void main(String[] args) {
+		points.Point p = new points.Point();
+		System.out.println(p.x + " " + p.y);
+	}
+}
+
+or by using a single-type-import declaration (§7.5.1) that mentions the fully qualified name, so that the simple name may be used thereafter:

+

package pointsUser;
+import points.Point;
+class Test {
+	public static void main(String[] args) {
+		Point p = new Point();
+		System.out.println(p.x + " " + p.y);
+	}
+}
+
+However, this compilation unit cannot use or import points.PointList, which is not declared public and is therefore inaccessible outside package points.

+ +

6.6.5 Example: Default-Access Fields, Methods, and Constructors

+ +If none of the access modifiers public, protected, or private are specified, a class member or constructor is accessible throughout the package that contains the declaration of the class in which the class member is declared, but the class member or constructor is not accessible in any other package.

+ +If a public class has a method or constructor with default access, then this method or constructor is not accessible to or inherited by a subclass declared outside this package. + +

+For example, if we have: +

package points;
+public class Point {
+	public int x, y;
+	void move(int dx, int dy) { x += dx; y += dy; }
+	public void moveAlso(int dx, int dy) { move(dx, dy); }
+}
+
+then a subclass in another package may declare an unrelated move method, with the same signature (§8.3.2) and return type. Because the original move method is not accessible from package morepoints, super may not be used:

+

package morepoints;
+public class PlusPoint extends points.Point {
+	public void move(int dx, int dy) {
+		super.move(dx, dy);								// compile-time error
+		moveAlso(dx, dy);
+	}
+}
+
+Because move of Point is not overridden by move in PlusPoint, the method moveAlso in Point never calls the method move in PlusPoint.

+ +Thus if you delete the super.move call from PlusPoint and execute the test program: +

import points.Point;
+import morepoints.PlusPoint;
+class Test {
+    public static void main(String[] args) {
+        PlusPoint pp = new PlusPoint();
+        pp.move(1, 1);
+	}
+}
+
+it terminates normally. If move of Point were overridden by move in PlusPoint, then this program would recurse infinitely, until a StackoverflowError occurred.

+ +

6.6.6 Example: public Fields, Methods, and Constructors

+ +A public class member or constructor is accessible throughout the package where it is declared and from any other package, provided the package in which it is declared is observable (§7.4.3). For example, in the compilation unit:

+

package points;
+public class Point {
+	int x, y;
+	public void move(int dx, int dy) {
+		x += dx; y += dy;
+		moves++;
+	}
+	public static int moves = 0;
+}
+
+the public class Point has as public members the move method and the moves field. These public members are accessible to any other package that has access to package points. The fields x and y are not public and therefore are accessible only from within the package points.

+ +

6.6.7 Example: protected Fields, Methods, and Constructors

+ +Consider this example, where the points package declares:

+

package points;
+public class Point {
+	protected int x, y;
+	void warp(threePoint.Point3d a) {
+		if (a.z > 0)		// compile-time error: cannot access a.z
+			a.delta(this);
+	}
+}
+
+and the threePoint package declares:

+

package threePoint;
+import points.Point;
+public class Point3d extends Point {
+	protected int z;
+	public void delta(Point p) {
+		p.x += this.x;		// compile-time error: cannot access p.x
+		p.y += this.y;		// compile-time error: cannot access p.y
+	}
+	public void delta3d(Point3d q) {
+		q.x += this.x;
+		q.y += this.y;
+		q.z += this.z;
+	}
+}
+
+which defines a class Point3d. A compile-time error occurs in the method delta here: it cannot access the protected members x and y of its parameter p, because while Point3d (the class in which the references to fields x and y occur) is a subclass of Point (the class in which x and y are declared), it is not involved in the implementation of a Point (the type of the parameter p). The method delta3d can access the protected members of its parameter q, because the class Point3d is a subclass of Point and is involved in the implementation of a Point3d.

+ +The method delta could try to cast (§5.5, §15.16) its parameter to be a Point3d, but this cast would fail, causing an exception, if the class of p at run time were not Point3d. + +

A compile-time error also occurs in the method warp: it cannot access the protected member z of its parameter a, because while the class Point (the class in which the reference to field z occurs) is involved in the implementation of a Point3d (the type of the parameter a), it is not a subclass of Point3d (the class in which z is declared). + +

6.6.8 Example: private Fields, Methods, and Constructors

+ +A private class member or constructor is accessible only within the class body in which the member is declared and is not inherited by subclasses. In the example:

+

class Point {
+	Point() { setMasterID(); }
+	int x, y;
+	private int ID;
+	private static int masterID = 0;
+	private void setMasterID() { ID = masterID++; }
+}
+
+the private members ID, masterID, and setMasterID may be used only within the body of class Point. They may not be accessed by qualified names, field access expressions, or method invocation expressions outside the body of the declaration of Point.

+ +See §8.8.8 for an example that uses a private constructor. + +

6.7 Fully Qualified Names and Canonical Names

+ +Every package, top level class, top level interface, and primitive type has a fully qualified name. An array type has a fully qualified name if and only if its element type has a fully qualified name.

+

    +
  • The fully qualified name of a primitive type is the keyword for that primitive type, namely boolean, char, byte, short, int, long, float, or double. + +
  • The fully qualified name of a named package that is not a subpackage of a named package is its simple name. + +
  • The fully qualified name of a named package that is a subpackage of another named package consists of the fully qualified name of the containing package, followed by ".", followed by the simple (member) name of the subpackage. + +
  • The fully qualified name of a top level class or top level interface that is declared in an unnamed package is the simple name of the class or interface. + +
  • The fully qualified name of a top level class or top level interface that is declared in a named package consists of the fully qualified name of the package, followed by ".", followed by the simple name of the class or interface. + +
  • A member class or member interface M of another class C has a fully qualified name if and only if C has a fully qualified name. In that case, the fully qualified name of M consists of the fully qualified name of C, followed by ".", followed by the simple name of M. + +
  • The fully qualified name of an array type consists of the fully qualified name of the component type of the array type followed by "[]". +
+Examples:

+

    +
  • The fully qualified name of the type long is "long". + +
  • The fully qualified name of the package java.lang is "java.lang" because it is subpackage lang of package java. + +
  • The fully qualified name of the class Object, which is defined in the package java.lang, is "java.lang.Object". + +
  • The fully qualified name of the interface Enumeration, which is defined in the package java.util, is "java.util.Enumeration". + +
  • The fully qualified name of the type "array of double" is "double[]". + +
  • The fully qualified name of the type "array of array of array of array of String" is "java.lang.String[][][][]". +
+In the example:

+

package points;
+class Point { int x, y; }
+class PointVec {
+	Point[] vec;
+}
+
+the fully qualified name of the type Point is "points.Point"; the fully qualified name of the type PointVec is "points.PointVec"; and the fully qualified name of the type of the field vec of class PointVec is "points.Point[]".

+ +Every package, top level class, top level interface, and primitive type has a canonical name. An array type has a canonical name if and only if its element type has a canonical name. A member class or member interface M declared in another class C has a canonical name if and only if C has a canonical name. In that case, the canonical name of M consists of the canonical name of C, followed by ".", followed by the simple name of M. For every package, top level class, top level interface and primitive type, the canonical name is the same as the fully qualified name. The canonical name of an array type is defined only when the component type of the array has a canonical name. In that case, the canonical name of the array type consists of the canonical name of the component type of the array type followed by "[]".

+ +The difference between a fully qualified name and a canonical name can be seen in examples such as: +

package p;
+class O1 { class I{}}
+class O2 extends O1{};
+
+
+In this example both p.O1.I and p.O2.I are fully qualified names that denote the same class, but only p.O1.I is its canonical name.

+ +

6.8 Naming Conventions

+ +The class libraries of the Java platform attempt to use, whenever possible, names chosen according to the conventions presented here. These conventions help to make code more readable and avoid certain kinds of name conflicts.

+ +We recommend these conventions for use in all programs written in the Java programming language. However, these conventions should not be followed slavishly if long-held conventional usage dictates otherwise. So, for example, the sin and cos methods of the class java.lang.Math have mathematically conventional names, even though these method names flout the convention suggested here because they are short and are not verbs. + +

6.8.1 Package Names

+ +Names of packages that are to be made widely available should be formed as described in §7.7. Such names are always qualified names whose first identifier consists of two or three lowercase letters that name an Internet domain, such as com, edu, gov, mil, net, org, or a two-letter ISO country code such as uk or jp. Here are examples of hypothetical unique names that might be formed under this convention:

+

com.JavaSoft.jag.Oak
+org.npr.pledge.driver
+uk.ac.city.rugby.game
+
+
+Names of packages intended only for local use should have a first identifier that begins with a lowercase letter, but that first identifier specifically should not be the identifier java; package names that start with the identifier java are reserved by Sun for naming Java platform packages. + +

+When package names occur in expressions: +

    +
  • If a package name is obscured by a field declaration, then import declarations (§7.5) can usually be used to make available the type names declared in that package. + +
  • If a package name is obscured by a declaration of a parameter or local variable, then the name of the parameter or local variable can be changed without affecting other code. + +

    +The first component of a package name is normally not easily mistaken for a type name, as a type name normally begins with a single uppercase letter. (The Java programming language does not actually rely on case distinctions to determine whether a name is a package name or a type name.) +

+

6.8.2 Class and Interface Type Names

+ +Names of class types should be descriptive nouns or noun phrases, not overly long, in mixed case with the first letter of each word capitalized. For example:

+

ClassLoader
+SecurityManager
+Thread
+Dictionary
+BufferedInputStream
+
+ +Likewise, names of interface types should be short and descriptive, not overly long, in mixed case with the first letter of each word capitalized. The name may be a descriptive noun or noun phrase, which is appropriate when an interface is used as if it were an abstract superclass, such as interfaces java.io.DataInput and java.io.DataOutput; or it may be an adjective describing a behavior, as for the interfaces Runnable and Cloneable. + +

+Obscuring involving class and interface type names is rare. Names of fields, parameters, and local variables normally do not obscure type names because they conventionally begin with a lowercase letter whereas type names conventionally begin with an uppercase letter. + +

6.8.3 Method Names

+ +Method names should be verbs or verb phrases, in mixed case, with the first letter lowercase and the first letter of any subsequent words capitalized. Here are some additional specific conventions for method names:

+

    +
  • Methods to get and set an attribute that might be thought of as a variable V should be named getV and setV. An example is the methods getPriority and setPriority of class Thread. + +
  • A method that returns the length of something should be named length, as in class String. + +
  • A method that tests a boolean condition V about an object should be named isV. An example is the method isInterrupted of class Thread. + +
  • A method that converts its object to a particular format F should be named toF. Examples are the method toString of class Object and the methods toLocaleString and toGMTString of class java.util.Date. +
+Whenever possible and appropriate, basing the names of methods in a new class on names in an existing class that is similar, especially a class from the Java Application Programming Interface classes, will make it easier to use.

+ +Method names cannot obscure or be obscured by other names (§6.5.7). + +

6.8.4 Field Names

+ +Names of fields that are not final should be in mixed case with a lowercase first letter and the first letters of subsequent words capitalized. Note that well-designed classes have very few public or protected fields, except for fields that are constants (final static fields) (§6.8.5).

+ +Fields should have names that are nouns, noun phrases, or abbreviations for nouns. Examples of this convention are the fields buf, pos, and count of the class java.io.ByteArrayInputStream and the field bytesTransferred of the class java.io.InterruptedIOException. + +

Obscuring involving field names is rare. +

    +
  • If a field name obscures a package name, then an import declaration (§7.5) can usually be used to make available the type names declared in that package. + +
  • If a field name obscures a type name, then a fully qualified name for the type can be used unless the type name denotes a local class (§14.3). + +
  • Field names cannot obscure method names. + +
  • If a field name is shadowed by a declaration of a parameter or local variable, then the name of the parameter or local variable can be changed without affecting other code. +
+

6.8.5 Constant Names

+ +The names of constants in interface types should be, and final variables of class types may conventionally be, a sequence of one or more words, acronyms, or abbreviations, all uppercase, with components separated by underscore "_" characters. Constant names should be descriptive and not unnecessarily abbreviated. Conventionally they may be any appropriate part of speech. Examples of names for constants include MIN_VALUE, MAX_VALUE, MIN_RADIX, and MAX_RADIX of the class Character.

+ +A group of constants that represent alternative values of a set, or, less frequently, masking bits in an integer value, are sometimes usefully specified with a common acronym as a name prefix, as in: +

interface ProcessStates {
+	int PS_RUNNING = 0;
+	int PS_SUSPENDED = 1;
+}
+
+Obscuring involving constant names is rare:

+

    +
  • Constant names normally have no lowercase letters, so they will not normally obscure names of packages or types, nor will they normally shadow fields, whose names typically contain at least one lowercase letter. + +
  • Constant names cannot obscure method names, because they are distinguished syntactically. +
+

6.8.6 Local Variable and Parameter Names

+ +Local variable and parameter names should be short, yet meaningful. They are often short sequences of lowercase letters that are not words. For example:

+

    +
  • Acronyms, that is the first letter of a series of words, as in cp for a variable holding a reference to a ColoredPoint + +
  • Abbreviations, as in buf holding a pointer to a buffer of some kind + +
  • Mnemonic terms, organized in some way to aid memory and understanding, typically by using a set of local variables with conventional names patterned after the names of parameters to widely used classes. For example: +
      + +
    • in and out, whenever some kind of input and output are involved, patterned after the fields of System + +
    • off and len, whenever an offset and length are involved, patterned after the parameters to the read and write methods of the interfaces DataInput and DataOutput of java.io + +
    +
+

One-character local variable or parameter names should be avoided, except for temporary and looping variables, or where a variable holds an undistinguished value of a type. Conventional one-character names are: + +

    +
  • b for a byte + +
  • c for a char + +
  • d for a double + +
  • e for an Exception + +
  • f for a float + +
  • i, j, and k for integers + +
  • l for a long + +
  • o for an Object + +
  • s for a String + +
  • v for an arbitrary value of some type + +
+Local variable or parameter names that consist of only two or three lowercase letters should not conflict with the initial country codes and domain names that are the first component of unique package names (§7.7). + +

+ + +

+ + + +
Contents | Prev | Next | IndexJava Language Specification
+Second Edition
+Copyright © 2000 Sun Microsystems, Inc. +All rights reserved +
+Please send any comments or corrections via our feedback form + + + + + Packages + + + + + + + + +
Contents | Prev | Next | IndexJava Language Specification
+Second Edition
+

+ + +

+CHAPTER + 7

+ +

Packages

+

+ +Programs are organized as sets of packages. Each package has its own set of names for types, which helps to prevent name conflicts. A top level type is accessible (§6.6) outside the package that declares it only if the type is declared public.

+ +The naming structure for packages is hierarchical (§7.1). The members of a package are class and interface types (§7.6), which are declared in compilation units of the package, and subpackages, which may contain compilation units and subpackages of their own.

+ +A package can be stored in a file system (§7.2.1) or in a database (§7.2.2). Packages that are stored in a file system have certain constraints on the organization of their compilation units to allow a simple implementation to find classes easily.

+ +A package consists of a number of compilation units (§7.3). A compilation unit automatically has access to all types declared in its package and also automatically imports all of the public types declared in the predefined package java.lang.

+ +For small programs and casual development, a package can be unnamed (§7.4.2) or have a simple name, but if code is to be widely distributed, unique package names should be chosen (§7.7). This can prevent the conflicts that would otherwise occur if two development groups happened to pick the same package name and these packages were later to be used in a single program. + +

7.1 Package Members

+ +The members of a package are subpackages and all the top level (§7.6) class (§8) and top level interface (§9) types declared in all the compilation units (§7.3) of the package.

+ +For example, in the Java Application Programming Interface:

+

    +
  • The package java has subpackages awt, applet, io, lang, net, and util, but no compilation units. + +
  • The package java.awt has a subpackage named image, as well as a number of compilation units containing declarations of class and interface types. +
+If the fully qualified name (§6.7) of a package is P, and Q is a subpackage of P, then P.Q is the fully qualified name of the subpackage.

+ +A package may not contain two members of the same name, or a compile-time error results.

+ +Here are some examples: +

    +
  • Because the package java.awt has a subpackage image, it cannot (and does not) contain a declaration of a class or interface type named image. + +
  • If there is a package named mouse and a member type Button in that package (which then might be referred to as mouse.Button), then there cannot be any package with the fully qualified name mouse.Button or mouse.Button.Click. + +
  • If com.sun.java.jag is the fully qualified name of a type, then there cannot be any package whose fully qualified name is either com.sun.java.jag or com.sun.java.jag.scrabble. + +
+The hierarchical naming structure for packages is intended to be convenient for organizing related packages in a conventional manner, but has no significance in itself other than the prohibition against a package having a subpackage with the same simple name as a top level type (§7.6) declared in that package. There is no special access relationship between a package named oliver and another package named oliver.twist, or between packages named evelyn.wood and evelyn.waugh. For example, the code in a package named oliver.twist has no better access to the types declared within package oliver than code in any other package. + +

7.2 Host Support for Packages

+ +Each host determines how packages, compilation units, and subpackages are created and stored, and which compilation units are observable (§7.3) in a particular compilation.

+ +The observability of compilation units in turn determines which packages are observable, and which packages are in scope. + +

The packages may be stored in a local file system in simple implementations of the Java platform. Other implementations may use a distributed file system or some form of database to store source and/or binary code. + +

7.2.1 Storing Packages in a File System

+ +As an extremely simple example, all the packages and source and binary code on a system might be stored in a single directory and its subdirectories. Each immediate subdirectory of this directory would represent a top level package, that is, one whose fully qualified name consists of a single simple name. The directory might contain the following immediate subdirectories:

+

com
+gls
+jag
+java
+wnj
+
+where directory java would contain the Java Application Programming Interface packages; the directories jag, gls, and wnj might contain packages that three of the authors of this specification created for their personal use and to share with each other within this small group; and the directory com would contain packages procured from companies that used the conventions described in §7.7 to generate unique names for their packages.

+ +Continuing the example, the directory java would contain, among others, the following subdirectories: +

applet	
+awt
+io
+lang
+net
+util
+
+corresponding to the packages java.applet, java.awt, java.io, java.lang, java.net, and java.util that are defined as part of the Java Application Programming Interface.

+ +Still continuing the example, if we were to look inside the directory util, we might see the following files: +

+BitSet.java				Observable.java
+BitSet.class				Observable.class
+Date.java				Observer.java
+Date.class				Observer.class
+...
+
+where each of the .java files contains the source for a compilation unit (§7.3) that contains the definition of a class or interface whose binary compiled form is contained in the corresponding .class file.

+ +Under this simple organization of packages, an implementation of the Java platform would transform a package name into a pathname by concatenating the components of the package name, placing a file name separator (directory indicator) between adjacent components. + +

For example, if this simple organization were used on a UNIX system, where the file name separator is /, the package name: +

jag.scrabble.board
+
+would be transformed into the directory name:

+

jag/scrabble/board
+
+and:

+

com.sun.sunsoft.DOE
+
+would be transformed to the directory name:

+

com/sun/sunsoft/DOE
+
+
+A package name component or class name might contain a character that cannot correctly appear in a host file system's ordinary directory name, such as a Unicode character on a system that allows only ASCII characters in file names. As a convention, the character can be escaped by using, say, the @ character followed by four hexadecimal digits giving the numeric value of the character, as in the \uxxxx escape (§3.3), so that the package name: +
+children.activities.crafts.papierM\u00e2ch\u00e9
+
+which can also be written using full Unicode as:

+

children.activities.crafts.papierMâché
+
+might be mapped to the directory name:

+

children/activities/crafts/papierM@00e2ch@00e9
+
+If the @ character is not a valid character in a file name for some given host file system, then some other character that is not valid in an identifier could be used instead.

+ +

7.2.2 Storing Packages in a Database

+ +A host system may store packages and their compilation units and subpackages in a database.

+ +Such a database must not impose the optional restrictions (§7.6) on compilation units in file-based implementations. For example, a system that uses a database to store packages may not enforce a maximum of one public class or interface per compilation unit. + +

+Systems that use a database must, however, provide an option to convert a program to a form that obeys the restrictions, for purposes of export to file-based implementations.

+ +

7.3 Compilation Units

+ +CompilationUnit is the goal symbol (§2.1) for the syntactic grammar (§2.3) of Java programs. It is defined by the following productions:

+

    +CompilationUnit:
+	PackageDeclarationopt ImportDeclarationsopt TypeDeclarationsopt
+
+ImportDeclarations:
+	ImportDeclaration
+	ImportDeclarations ImportDeclaration
+
+TypeDeclarations:
+	TypeDeclaration
+	TypeDeclarations TypeDeclaration
+
+Types declared in different compilation units can depend on each other, circularly. A Java compiler must arrange to compile all such types at the same time.

+ +A compilation unit consists of three parts, each of which is optional:

+

    +
  • A package declaration (§7.4), giving the fully qualified name (§6.7) of the package to which the compilation unit belongs. A compilation unit that has no package declaration is part of an unnamed package (§7.4.2). + +
  • import declarations (§7.5) that allow types from other packages to be referred to using their simple names + +
  • Top level type declarations (§7.6) of class and interface types +
+Which compilation units are observable is determined by the host system. However, all the compilation units of the package java and its subpackages lang and io must always be observable. The observability of a compilation unit influences the observability of its package (§7.4.3).

+ +Every compilation unit automatically and implicitly imports every public type name declared by the predefined package java.lang, so that the names of all those types are available as simple names, as described in §7.5.3.

+ +

7.4 Package Declarations

+ +A package declaration appears within a compilation unit to indicate the package to which the compilation unit belongs.

+ +

7.4.1 Named Packages

+ +A package declaration in a compilation unit specifies the name (§6.2) of the package to which the compilation unit belongs.

+

    +PackageDeclaration:
+	package PackageName ;
+
+The package name mentioned in a package declaration must be the fully qualified name (§6.7) of the package.

+ +

7.4.2 Unnamed Packages

+ +A compilation unit that has no package declaration is part of an unnamed package.

+ +Note that an unnamed package cannot have subpackages, since the syntax of a package declaration always includes a reference to a named top level package. +

+ +As an example, the compilation unit:

+

class FirstCall {
+	public static void main(String[] args) {
+		System.out.println("Mr. Watson, come here. "
+						+ "I want you.");
+	}
+}
+
+defines a very simple compilation unit as part of an unnamed package.

+ +An implementation of the Java platform must support at least one unnamed package; it may support more than one unnamed package but is not required to do so. Which compilation units are in each unnamed package is determined by the host system.

+ +In implementations of the Java platform that use a hierarchical file system for storing packages, one typical strategy is to associate an unnamed package with each directory; only one unnamed package is observable at a time, namely the one that is associated with the "current working directory." The precise meaning of "current working directory" depends on the host system. + +

+Unnamed packages are provided by the Java platform principally for convenience when developing small or temporary applications or when just beginning development. + +

7.4.3 Observability of a Package

+ +A package is observable if and only if either:

+

    +
  • A compilation unit containing a declaration of the package is observable. + +
  • A subpackage of the package is observable. + +
+One can conclude from the rule above and from the requirements on observable compilation units, that the packages java, java.lang, and java.io are always observable. + +

7.4.4 Scope of a Package Declaration

+ +The scope of the declaration of an observable (§7.4.3) top level package is all observable compilation units (§7.3). The declaration of a package that is not observable is never in scope. Subpackage declarations are never in scope.

+ +It follows that the package java is always in scope (§6.3). +

+ +Package declarations never shadow other declarations.

+ +

7.5 Import Declarations

+ +An import declaration allows a named type to be referred to by a simple name (§6.2) that consists of a single identifier. Without the use of an appropriate import declaration, the only way to refer to a type declared in another package is to use a fully qualified name (§6.7).

+

    +ImportDeclaration:
+	SingleTypeImportDeclaration
+	TypeImportOnDemandDeclaration
+
+A single-type-import declaration (§7.5.1) imports a single named type, by mentioning its canonical name. A type-import-on-demand declaration (§7.5.2) imports all the accessible types of a named type or package as needed.

+ +The scope of a type imported by a single-type-import declaration (§7.5.1) or type-import-on-demand declaration (§7.5.2) is all the class and interface type declarations (§7.6) in the compilation unit in which the import declaration appears.

+ +An import declaration makes types available by their simple names only within the compilation unit that actually contains the import declaration. The scope of the entities(s) it introduces specifically does not include the package statement, other import declarations in the current compilation unit, or other compilation units in the same package. See §7.5.4 for an illustrative example.

+ +

7.5.1 Single-Type-Import Declaration

+ +A single-type-import declaration imports a single type by giving its canonical name, making it available under a simple name in the class and interface declarations of the compilation unit in which the single-type import declaration appears.

+

    +SingleTypeImportDeclaration:
+	import TypeName ;
+
+The TypeName must be the canonical name of a class or interface type; a compile-time error occurs if the named type does not exist. The named type must be accessible (§6.6) or a compile-time error occurs.

+ +A single-type-import declaration d in a compilation unit c of package p that imports a type named n shadows the declarations of:

+

    +
  • any top level type named n declared in another compilation unit of p. + +
  • any type named n imported by a type-import-on-demand declaration in c. +
+throughout c.

+ +The example: +

import java.util.Vector;
+
+causes the simple name Vector to be available within the class and interface declarations in a compilation unit. Thus, the simple name Vector refers to the type Vector in the package java.util in all places where it is not shadowed (§6.3.1) or obscured (§6.3.2) by a declaration of a field, parameter, local variable, or nested type declaration with the same name.

+ +If two single-type-import declarations in the same compilation unit attempt to import types with the same simple name, then a compile-time error occurs, unless the two types are the same type, in which case the duplicate declaration is ignored. If another top level type with the same simple name is otherwise declared in the current compilation unit except by a type-import-on-demand declaration (§7.5.2), then a compile-time error occurs.

+ +So the sample program: +

import java.util.Vector;
+class Vector { Object[] vec; }
+
+causes a compile-time error because of the duplicate declaration of Vector, as does:

+

import java.util.Vector;
+import myVector.Vector;
+
+where myVector is a package containing the compilation unit:

+

package myVector;
+public class Vector { Object[] vec; }
+
+The compiler keeps track of types by their binary names (§13.1).

+ +Note that an import statement cannot import a subpackage, only a type. For example, it does not work to try to import java.util and then use the name util.Random to refer to the type java.util.Random: +

+import java.util;		// incorrect: compile-time error
+class Test { util.Random generator; }
+
+

7.5.2 Type-Import-on-Demand Declaration

+ +A type-import-on-demand declaration allows all accessible (§6.6) types declared in the type or package named by a canonical name to be imported as needed.

+

    +TypeImportOnDemandDeclaration:
+	import PackageOrTypeName . * ;
+
+It is a compile-time error for a type-import-on-demand declaration to name a type or package that is not accessible. Two or more type-import-on-demand declarations in the same compilation unit may name the same type or package; the effect is as if there were exactly one such declaration. It is not a compile-time error to name the current package or java.lang in a type-import-on-demand declaration. The type-import-on-demand declaration is ignored in such cases

+ +A type-import-on-demand declaration never causes any other declaration to be shadowed.

+ +The example:

+

import java.util.*;
+
+causes the simple names of all public types declared in the package java.util to be available within the class and interface declarations of the compilation unit. Thus, the simple name Vector refers to the type Vector in the package java.util in all places in the compilation unit where that type declaration is not shadowed (§6.3.1) or obscured (§6.3.2). The declaration might be shadowed by a single-type-import declaration of a type whose simple name is Vector; by a type named Vector and declared in the package to which the compilation unit belongs; or any nested classes or interfaces. The declaration might be obscured by a declaration of a field, parameter, or local variable named Vector (It would be unusual for any of these conditions to occur.)

+ +

7.5.3 Automatic Imports

+ +Each compilation unit automatically imports all of the public type names declared in the predefined package java.lang, as if the declaration:

+

import java.lang.*;
+
+appeared at the beginning of each compilation unit, immediately following any package statement.

+ +

7.5.4 A Strange Example

+ +Package names and type names are usually different under the naming conventions described in §6.8. Nevertheless, in a contrived example where there is an unconventionally-named package Vector, which declares a public class named Mosquito:

+

package Vector;
+public class Mosquito { int capacity; }
+
+and then the compilation unit:

+

package strange.example;
+import java.util.Vector;
+import Vector.Mosquito;
+class Test {
+	public static void main(String[] args) {
+		System.out.println(new Vector().getClass());
+		System.out.println(new Mosquito().getClass());
+	}
+}
+
+the single-type-import declaration (§7.5.1) importing class Vector from package java.util does not prevent the package name Vector from appearing and being correctly recognized in subsequent import declarations. The example compiles and produces the output:

+

class java.util.Vector
+class Vector.Mosquito
+
+

7.6 Top Level Type Declarations

+ +A top level type declaration declares a top level class type (§8) or a top level interface type (§9):

+

    +TypeDeclaration:
+	ClassDeclaration
+	InterfaceDeclaration
+	;
+
+By default, the top level types declared in a package are accessible only within the compilation units of that package, but a type may be declared to be public to grant access to the type from code in other packages (§6.6, §8.1.1, §9.1.1).

+ +The scope of a top level type is all type declarations in the package in which the top level type is declared.

+ +If a top level type named T is declared in a compilation unit of a package whose fully qualified name is P, then the fully qualified name of the type is P.T. If the type is declared in an unnamed package (§7.4.2), then the type has the fully qualified name T.

+ +Thus in the example:

+

package wnj.points;
+class Point { int x, y; }
+
+the fully qualified name of class Point is wnj.points.Point.

+ +An implementation of the Java platform must keep track of types within packages by their binary names (§13.1). Multiple ways of naming a type must be expanded to binary names to make sure that such names are understood as referring to the same type.

+ +For example, if a compilation unit contains the single-type-import declaration (§7.5.1): +

import java.util.Vector;
+
+then within that compilation unit the simple name Vector and the fully qualified name java.util.Vector refer to the same type.

+ +When packages are stored in a file system (§7.2.1), the host system may choose to enforce the restriction that it is a compile-time error if a type is not found in a file under a name composed of the type name plus an extension (such as .java or .jav) if either of the following is true:

+

    +
  • The type is referred to by code in other compilation units of the package in which the type is declared. + +
  • The type is declared public (and therefore is potentially accessible from code in other packages). +
+This restriction implies that there must be at most one such type per compilation unit. This restriction makes it easy for a compiler for the Java programming language or an implementation of the Java virtual machine to find a named class within a package; for example, the source code for a public type wet.sprocket.Toad would be found in a file Toad.java in the directory wet/sprocket, and the corresponding object code would be found in the file Toad.class in the same directory.

+ +When packages are stored in a database (§7.2.2), the host system must not impose such restrictions.

+ +In practice, many programmers choose to put each class or interface type in its own compilation unit, whether or not it is public or is referred to by code in other compilation units. + +A compile-time error occurs if the name of a top level type appears as the name of any other top level class or interface type declared in the same package (§7.6).

+ +A compile-time error occurs if the name of a top level type is also declared as a type by a single-type-import declaration (§7.5.1) in the compilation unit (§7.3) containing the type declaration.

+ +In the example: +

class Point { int x, y; }
+
+the class Point is declared in a compilation unit with no package statement, and thus Point is its fully qualified name, whereas in the example:

+

package vista;
+class Point { int x, y; }
+
+the fully qualified name of the class Point is vista.Point. (The package name vista is suitable for local or personal use; if the package were intended to be widely distributed, it would be better to give it a unique package name (§7.7).)

+ +In the example: +

package test;
+import java.util.Vector;
+class Point {
+	int x, y;
+}
+interface Point {			// compile-time error #1
+	int getR();
+	int getTheta();
+}
+class Vector { Point[] pts; }		// compile-time error #2
+
+the first compile-time error is caused by the duplicate declaration of the name Point as both a class and an interface in the same package. A second error detected at compile time is the attempt to declare the name Vector both by a class type declaration and by a single-type-import declaration.

+ +Note, however, that it is not an error for the name of a class to also to name a type that otherwise might be imported by a type-import-on-demand declaration (§7.5.2) in the compilation unit (§7.3) containing the class declaration. In the example: +

package test;
+import java.util.*;
+class Vector { Point[] pts; }		// not a compile-time error
+
+the declaration of the class Vector is permitted even though there is also a class java.util.Vector. Within this compilation unit, the simple name Vector refers to the class test.Vector, not to java.util.Vector (which can still be referred to by code within the compilation unit, but only by its fully qualified name).

+ +As another example, the compilation unit: +

package points;
+class Point {
+	int x, y;			// coordinates
+	PointColor color;		// color of this point
+	Point next;			// next point with this color
+	static int nPoints;
+}
+class PointColor {
+	Point first;			// first point with this color
+	PointColor(int color) {
+		this.color = color;
+	}
+	private int color;		// color components
+}
+
+defines two classes that use each other in the declarations of their class members. Because the class types Point and PointColor have all the type declarations in package points, including all those in the current compilation unit, as their scope, this example compiles correctly-that is, forward reference is not a problem.

+ +It is a compile-time error if a top level type declaration contains any one of the following access modifiers: protected, private or static.

+ +

7.7 Unique Package Names

+ +Developers should take steps to avoid the possibility of two published packages having the same name by choosing unique package names for packages that are widely distributed. This allows packages to be easily and automatically installed and catalogued. This section specifies a suggested convention for generating such unique package names. Implementations of the Java platform are encouraged to provide automatic support for converting a set of packages from local and casual package names to the unique name format described here.

+ +If unique package names are not used, then package name conflicts may arise far from the point of creation of either of the conflicting packages. This may create a situation that is difficult or impossible for the user or programmer to resolve. The class ClassLoader can be used to isolate packages with the same name from each other in those cases where the packages will have constrained interactions, but not in a way that is transparent to a naïve program. + +

You form a unique package name by first having (or belonging to an organization that has) an Internet domain name, such as sun.com. You then reverse this name, component by component, to obtain, in this example, com.sun, and use this as a prefix for your package names, using a convention developed within your organization to further administer package names. + +

In some cases, the internet domain name may not be a valid package name. Here are some suggested conventions for dealing with these situations: +

    +
  • If the domain name contains a hyphen, or any other special character not allowed in an identifier (§3.8), convert it into an underscore. + +
  • If any of the resulting package name components are keywords (§3.9) then append underscore to them. + +
  • If any of the resulting package name components start with a digit, or any other character that is not allowed as an initial character of an identifier, have an underscore prefixed to the component. + +
+Such a convention might specify that certain directory name components be division, department, project, machine, or login names. Some possible examples: +
com.sun.sunsoft.DOE
+com.sun.java.jag.scrabble
+com.apple.quicktime.v2
+edu.cmu.cs.bovik.cheese
+gov.whitehouse.socks.mousefinder
+
+The first component of a unique package name is always written in all-lowercase ASCII letters and should be one of the top level domain names, currently com, edu, gov, mil, net, org, or one of the English two-letter codes identifying countries as specified in ISO Standard 3166, 1981. For more information, refer to the documents stored at ftp://rs.internic.net/rfc, for example, rfc920.txt and rfc1032.txt.

+ +The name of a package is not meant to imply where the package is stored within the Internet; for example, a package named edu.cmu.cs.bovik.cheese is not necessarily obtainable from Internet address cmu.edu or from cs.cmu.edu or from bovik.cs.cmu.edu. The suggested convention for generating unique package names is merely a way to piggyback a package naming convention on top of an existing, widely known unique name registry instead of having to create a separate registry for package names. + +

+ + +

+ + + +
Contents | Prev | Next | IndexJava Language Specification
+Second Edition
+Copyright © 2000 Sun Microsystems, Inc. +All rights reserved +
+Please send any comments or corrections via our feedback form + + + + + Classes + + + + + + + + +
Contents | Prev | Next | IndexJava Language Specification
+Second Edition
+

+ + +

+CHAPTER + 8

+ +

Classes

+

+ +Class declarations define new reference types and describe how they are implemented (§8.1).

+ +A nested class is any class whose declaration occurs within the body of another class or interface. A top level class is a class that is not a nested class.

+ +This chapter discusses the common semantics of all classes-top level (§7.6) and nested (including member classes (§8.5, §9.5), local classes (§14.3) and anonymous classes (§15.9.5)). Details that are specific to particular kinds of classes are discussed in the sections dedicated to these constructs.

+ +A named class may be declared abstract (§8.1.1.1) and must be declared abstract if it is incompletely implemented; such a class cannot be instantiated, but can be extended by subclasses. A class may be declared final (§8.1.1.2), in which case it cannot have subclasses. If a class is declared public, then it can be referred to from other packages.

+ +Each class except Object is an extension of (that is, a subclass of) a single existing class (§8.1.3) and may implement interfaces (§8.1.4). + +

+The body of a class declares members (fields and methods and nested classes and interfaces), instance and static initializers, and constructors (§8.1.5). The scope (§6.3) of a member (§8.2) is the entire declaration of the class to which the member belongs. Field, method, member class, member interface, and constructor declarations may include the access modifiers (§6.6) public, protected, or private. The members of a class include both declared and inherited members (§8.2). Newly declared fields can hide fields declared in a superclass or superinterface. Newly declared class members and interface members can hide class or interface members declared in a superclass or superinterface. Newly declared methods can hide, implement, or override methods declared in a superclass or superinterface.

+ +Field declarations (§8.3) describe class variables, which are incarnated once, and instance variables, which are freshly incarnated for each instance of the class. A field may be declared final (§8.3.1.2), in which case it can be assigned to only once. Any field declaration may include an initializer.

+ +Member class declarations (§8.5) describe nested classes that are members of the surrounding class. Member classes may be static, in which case they have no access to the instance variables of the surrounding class; or they may be inner classes (§8.1.2).

+ +Member interface declarations (§8.5) describe nested interfaces that are members of the surrounding class.

+ +Method declarations (§8.4) describe code that may be invoked by method invocation expressions (§15.12). A class method is invoked relative to the class type; an instance method is invoked with respect to some particular object that is an instance of the class type. A method whose declaration does not indicate how it is implemented must be declared abstract. A method may be declared final (§8.4.3.3), in which case it cannot be hidden or overridden. A method may be implemented by platform-dependent native code (§8.4.3.4). A synchronized method (§8.4.3.6) automatically locks an object before executing its body and automatically unlocks the object on return, as if by use of a synchronized statement (§14.18), thus allowing its activities to be synchronized with those of other threads (§17).

+ +Method names may be overloaded (§8.4.7).

+ +Instance initializers (§8.6) are blocks of executable code that may be used to help initialize an instance when it is created (§15.9).

+ +Static initializers (§8.7) are blocks of executable code that may be used to help initialize a class when it is first loaded (§12.4).

+ +Constructors (§8.8) are similar to methods, but cannot be invoked directly by a method call; they are used to initialize new class instances. Like methods, they may be overloaded (§8.8.6).

+ +

8.1 Class Declaration

+ +A class declaration specifies a new named reference type:

+

    +ClassDeclaration:
+	ClassModifiersopt class Identifier Superopt Interfacesopt ClassBody
+
+The Identifier in a class declaration specifies the name of the class. A compile-time error occurs if a class has the same simple name as any of its enclosing classes or interfaces.

+ +

8.1.1 Class Modifiers

+ +A class declaration may include class modifiers.

+

    +ClassModifiers:
+	ClassModifier
+	ClassModifiers ClassModifier
+
+ClassModifier: one of
+	public protected private
+	abstract static final strictfp
+
+Not all modifiers are applicable to all kinds of class declarations. The access modifier public pertains only to top level classes (§7.6) and to member classes (§8.5, §9.5), and is discussed in §6.6, §8.5 and §9.5. The access modifiers protected and private pertain only to member classes within a directly enclosing class declaration (§8.5) and are discussed in §8.5.1. The access modifier static pertains only to member classes (§8.5, §9.5). A compile-time error occurs if the same modifier appears more than once in a class declaration.

+ +If two or more class modifiers appear in a class declaration, then it is customary, though not required, that they appear in the order consistent with that shown above in the production for ClassModifier.

+ +

8.1.1.1 abstract Classes

+ +An abstract class is a class that is incomplete, or to be considered incomplete. Only abstract classes may have abstract methods (§8.4.3.1, §9.4), that is, methods that are declared but not yet implemented. If a class that is not abstract contains an abstract method, then a compile-time error occurs. A class C has abstract methods if any of the following is true:

+

    +
  • C explicitly contains a declaration of an abstract method (§8.4.3). + +
  • Any of C's superclasses declares an abstract method that has not been implemented (§8.4.6.1) in C or any of its superclasses. + +
  • A direct superinterface (§8.1.4) of C declares or inherits a method (which is therefore necessarily abstract) and C neither declares nor inherits a method that implements it. + +

    +

+In the example: +
abstract class Point {
+	int x = 1, y = 1;
+	void move(int dx, int dy) {
+		x += dx;
+		y += dy;
+		alert();
+	}
+	abstract void alert();
+}
+abstract class ColoredPoint extends Point {
+	int color;
+}
+class SimplePoint extends Point {
+	void alert() { }
+}
+
+a class Point is declared that must be declared abstract, because it contains a declaration of an abstract method named alert. The subclass of Point named ColoredPoint inherits the abstract method alert, so it must also be declared abstract. On the other hand, the subclass of Point named SimplePoint provides an implementation of alert, so it need not be abstract.

+ +A compile-time error occurs if an attempt is made to create an instance of an abstract class using a class instance creation expression (§15.9).

+ +Thus, continuing the example just shown, the statement: +

	Point p = new Point();
+
+would result in a compile-time error; the class Point cannot be instantiated because it is abstract. However, a Point variable could correctly be initialized with a reference to any subclass of Point, and the class SimplePoint is not abstract, so the statement:

+

	Point p = new SimplePoint();
+
+would be correct.

+ +A subclass of an abstract class that is not itself abstract may be instantiated, resulting in the execution of a constructor for the abstract class and, therefore, the execution of the field initializers for instance variables of that class. Thus, in the example just given, instantiation of a SimplePoint causes the default constructor and field initializers for x and y of Point to be executed. + +It is a compile-time error to declare an abstract class type such that it is not possible to create a subclass that implements all of its abstract methods. This situation can occur if the class would have as members two abstract methods that have the same method signature (§8.4.2) but different return types.

+ +As an example, the declarations: +

interface Colorable { void setColor(int color); }
+abstract class Colored implements Colorable {
+	abstract int setColor(int color);
+}
+
+result in a compile-time error: it would be impossible for any subclass of class Colored to provide an implementation of a method named setColor, taking one argument of type int, that can satisfy both abstract method specifications, because the one in interface Colorable requires the same method to return no value, while the one in class Colored requires the same method to return a value of type int (§8.4).

+ +A class type should be declared abstract only if the intent is that subclasses can be created to complete the implementation. If the intent is simply to prevent instantiation of a class, the proper way to express this is to declare a constructor (§8.8.8) of no arguments, make it private, never invoke it, and declare no other constructors. A class of this form usually contains class methods and variables. The class Math is an example of a class that cannot be instantiated; its declaration looks like this: +

public final class Math {
+	private Math() { }		// never instantiate this class
+	. . . declarations of class variables and methods . . .
+
+}
+
+

8.1.1.2 final Classes

+ +A class can be declared final if its definition is complete and no subclasses are desired or required. A compile-time error occurs if the name of a final class appears in the extends clause (§8.1.3) of another class declaration; this implies that a final class cannot have any subclasses. A compile-time error occurs if a class is declared both final and abstract, because the implementation of such a class could never be completed (§8.1.1.1).

+ +Because a final class never has any subclasses, the methods of a final class are never overridden (§8.4.6.1). + +

8.1.1.3 strictfp Classes

+ +The effect of the strictfp modifier is to make all float or double expressions within the class declaration be explicitly FP-strict (§15.4). This implies that all methods declared in the class, and all nested types declared in the class, are implicitly strictfp.

+ +Note also that all float or double expressions within all variable initializers, instance initializers, static initializers and constructors of the class will also be explicitly FP-strict. + +

8.1.2 Inner Classes and Enclosing Instances

+ +An inner class is a nested class that is not explicitly or implicitly declared static. Inner classes may not declare static initializers (§8.7) or member interfaces. Inner classes may not declare static members, unless they are compile-time constant fields (§15.28).

+ +To illustrate these rules, consider the example below: +

class HasStatic{
+	static int j = 100;
+}
+class Outer{
+	class Inner extends HasStatic{
+		static final x = 3;		// ok - compile-time constant
+		static int y = 4; 		// compile-time error, an inner class
+	}
+	static class NestedButNotInner{
+		static int z = 5; 		// ok, not an inner class
+	}
+	interface NeverInner{}		// interfaces are never inner
+}
+
+Inner classes may inherit static members that are not compile-time constants even though they may not declare them. Nested classes that are not inner classes may declare static members freely, in accordance with the usual rules of the Java programming language. Member interfaces (§8.5) are always implicitly static so they are never considered to be inner classes.

+ +A statement or expression occurs in a static context if and only if the innermost method, constructor, instance initializer, static initializer, field initializer, or explicit constructor statement enclosing the statement or expression is a static method, a static initializer, the variable initializer of a static variable, or an explicit constructor invocation statement (§8.8.5).

+ +An inner class C is a direct inner class of a class O if O is the immediately lexically enclosing class of C and the declaration of C does not occur in a static context. A class C is an inner class of class O if it is either a direct inner class of O or an inner class of an inner class of O.

+ +A class O is the zeroth lexically enclosing class of itself. A class O is the nth lexically enclosing class of a class C if it is the immediately enclosing class of the n - 1st lexically enclosing class of C.

+ +An instance i of a direct inner class C of a class O is associated with an instance of O, known as the immediately enclosing instance of i. The immediately enclosing instance of an object, if any, is determined when the object is created (§15.9.2).

+ +An object o is the zeroth lexically enclosing instance of itself. An object o is the nth lexically enclosing instance of an instance i if it is the immediately enclosing instance of the n - 1st lexically enclosing instance of i.

+ +When an inner class refers to an instance variable that is a member of a lexically enclosing class, the variable of the corresponding lexically enclosing instance is used. A blank final (§4.5.4) field of a lexically enclosing class may not be assigned within an inner class.

+ +An instance of an inner class I whose declaration occurs in a static context has no lexically enclosing instances. However, if I is immediately declared within a static method or static initializer then I does have an enclosing block, which is the innermost block statement lexically enclosing the declaration of I.

+ +Furthermore, for every superclass S of C which is itself a direct inner class of a class SO, there is an instance of SO associated with i, known as the immediately enclosing instance of i with respect to S. The immediately enclosing instance of an object with respect to its class' direct superclass, if any, is determined when the superclass constructor is invoked via an explicit constructor invocation statement.

+ +Any local variable, formal method parameter or exception handler parameter used but not declared in an inner class must be declared final, and must be definitely assigned (§16) before the body of the inner class.

+ +Inner classes include local (§14.3), anonymous (§15.9.5) and non-static member classes (§8.5). Here are some examples: +

class Outer {
+	int i = 100;
+	static void classMethod() {
+		final int l = 200;
+		class LocalInStaticContext{
+			int k = i; // compile-time error
+			int m = l; // ok
+		}
+	}
+	void foo() {
+		class Local { // a local class
+			int j = i;
+		}
+	}
+}
+
+ +The declaration of class LocalInStaticContext occurs in a static context-within the static method classMethod. Instance variables of class Outer are not available within the body of a static method. In particular, instance variables of Outer are not available inside the body of LocalInStaticContext. However, local variables from the surrounding method may be referred to without error (provided they are marked final). + +

Inner classes whose declarations do not occur in a static context may freely refer to the instance variables of their enclosing class. An instance variable is always defined with respect to an instance. In the case of instance variables of an enclosing class, the instance variable must be defined with respect to an enclosing instance of that class. So, for example, the class Local above has an enclosing instance of class Outer. As a further example: +

+class WithDeepNesting{
+	boolean toBe;
+	WithDeepNesting(boolean b) { toBe = b;}
+	class Nested {
+		boolean theQuestion;
+		class DeeplyNested {
+			DeeplyNested(){
+				theQuestion = toBe || !toBe;
+			}
+		}
+	}
+}
+
+Here, every instance of WithDeepNesting.Nested.DeeplyNested has an enclosing instance of class WithDeepNesting.Nested (its immediately enclosing instance) and an enclosing instance of class WithDeepNesting (its 2nd lexically enclosing instance).

+ +

8.1.3 Superclasses and Subclasses

+ +The optional extends clause in a class declaration specifies the direct superclass of the current class. A class is said to be a direct subclass of the class it extends. The direct superclass is the class from whose implementation the implementation of the current class is derived. The extends clause must not appear in the definition of the class Object, because it is the primordial class and has no direct superclass. If the class declaration for any other class has no extends clause, then the class has the class Object as its implicit direct superclass.

+

    +Super:
+	extends ClassType
+
+The following is repeated from §4.3 to make the presentation here clearer:

+

    +ClassType:
+	TypeName
+
+The ClassType must name an accessible (§6.6) class type, or a compile-time error occurs. If the specified ClassType names a class that is final (§8.1.1.2), then a compile-time error occurs; final classes are not allowed to have subclasses.

+ +In the example: +

class Point { int x, y; }
+final class ColoredPoint extends Point { int color; }
+class Colored3DPoint extends ColoredPoint { int z; } // error
+
+the relationships are as follows:

+

    +
  • The class Point is a direct subclass of Object. + +
  • The class Object is the direct superclass of the class Point. + +
  • The class ColoredPoint is a direct subclass of class Point. + +
  • The class Point is the direct superclass of class ColoredPoint. +
+The declaration of class Colored3dPoint causes a compile-time error because it attempts to extend the final class ColoredPoint.

+ +The subclass relationship is the transitive closure of the direct subclass relationship. A class A is a subclass of class C if either of the following is true:

+

    +
  • A is the direct subclass of C. + +
  • There exists a class B such that A is a subclass of B, and B is a subclass of C, applying this definition recursively. +
+Class C is said to be a superclass of class A whenever A is a subclass of C.

+ +In the example: +

class Point { int x, y; }
+class ColoredPoint extends Point { int color; }
+final class Colored3dPoint extends ColoredPoint { int z; }
+
+the relationships are as follows:

+

    +
  • The class Point is a superclass of class ColoredPoint. + +
  • The class Point is a superclass of class Colored3dPoint. + +
  • The class ColoredPoint is a subclass of class Point. + +
  • The class ColoredPoint is a superclass of class Colored3dPoint. + +
  • The class Colored3dPoint is a subclass of class ColoredPoint. + +
  • The class Colored3dPoint is a subclass of class Point. +
+A class C directly depends on a type T if T is mentioned in the extends or implements clause of C either as a superclass or superinterface, or as a qualifier within a superclass or superinterface name. A class C depends on a reference type T if any of the following conditions hold:

+

    +
  • C directly depends on T. + +
  • C directly depends on an interface I that depends (§9.1.2) on T. + +
  • C directly depends on a class D that depends on T (using this definition recursively). +
+It is a compile-time error if a class depends on itself.

+ +For example: +

class Point extends ColoredPoint { int x, y; }
+class ColoredPoint extends Point { int color; }
+
+causes a compile-time error.

+ +If circularly declared classes are detected at run time, as classes are loaded (§12.2), then a ClassCircularityError is thrown.

+ +

8.1.4 Superinterfaces

+ +The optional implements clause in a class declaration lists the names of interfaces that are direct superinterfaces of the class being declared:

+

    +Interfaces:
+	implements InterfaceTypeList
+
+InterfaceTypeList:
+	InterfaceType
+	InterfaceTypeList , InterfaceType
+
+The following is repeated from §4.3 to make the presentation here clearer:

+

    +InterfaceType:
+	TypeName
+
+Each InterfaceType must name an accessible (§6.6) interface type, or a compile-time error occurs.

+ +A compile-time error occurs if the same interface is mentioned two or more times in a single implements clause.

+ +This is true even if the interface is named in different ways; for example, the code: +

class Redundant implements java.lang.Cloneable, Cloneable {
+	int x;
+}
+
+results in a compile-time error because the names java.lang.Cloneable and Cloneable refer to the same interface.

+ +An interface type I is a superinterface of class type C if any of the following is true:

+

    +
  • I is a direct superinterface of C. + +
  • C has some direct superinterface J for which I is a superinterface, using the definition of "superinterface of an interface" given in §9.1.2. + +
  • I is a superinterface of the direct superclass of C. +
+A class is said to implement all its superinterfaces.

+ +In the example: +

public interface Colorable {
+	void setColor(int color);
+	int getColor();
+}
+public interface Paintable extends Colorable {
+	int MATTE = 0, GLOSSY = 1;
+	void setFinish(int finish);
+	int getFinish();
+}
+class Point { int x, y; }
+class ColoredPoint extends Point implements Colorable {
+	int color;
+	public void setColor(int color) { this.color = color; }
+	public int getColor() { return color; }
+}
+class PaintedPoint extends ColoredPoint implements Paintable 
+{
+	int finish;
+	public void setFinish(int finish) {
+		this.finish = finish;
+	}
+	public int getFinish() { return finish; }
+}
+
+the relationships are as follows:

+

    +
  • The interface Paintable is a superinterface of class PaintedPoint. + +
  • The interface Colorable is a superinterface of class ColoredPoint and of class PaintedPoint. + +
  • The interface Paintable is a subinterface of the interface Colorable, and Colorable is a superinterface of Paintable, as defined in §9.1.2. +
+A class can have a superinterface in more than one way. In this example, the class PaintedPoint has Colorable as a superinterface both because it is a superinterface of ColoredPoint and because it is a superinterface of Paintable.

+ +Unless the class being declared is abstract, the declarations of all the method members of each direct superinterface must be implemented either by a declaration in this class or by an existing method declaration inherited from the direct superclass, because a class that is not abstract is not permitted to have abstract methods (§8.1.1.1).

+ +Thus, the example: +

interface Colorable {
+	void setColor(int color);
+	int getColor();
+}
+class Point { int x, y; };
+class ColoredPoint extends Point implements Colorable {
+	int color;
+}
+
+causes a compile-time error, because ColoredPoint is not an abstract class but it fails to provide an implementation of methods setColor and getColor of the interface Colorable.

+ +It is permitted for a single method declaration in a class to implement methods of more than one superinterface. For example, in the code: +

interface Fish { int getNumberOfScales(); }
+interface Piano { int getNumberOfScales(); }
+class Tuna implements Fish, Piano {
+	// You can tune a piano, but can you tuna fish?
+	int getNumberOfScales() { return 91; }
+}
+
+the method getNumberOfScales in class Tuna has a name, signature, and return type that matches the method declared in interface Fish and also matches the method declared in interface Piano; it is considered to implement both.

+ +On the other hand, in a situation such as this: +

interface Fish { int getNumberOfScales(); }
+interface StringBass { double getNumberOfScales(); }
+class Bass implements Fish, StringBass {
+	// This declaration cannot be correct, no matter what type is used.
+	public ??? getNumberOfScales() { return 91; }
+}
+
+It is impossible to declare a method named getNumberOfScales with the same signature and return type as those of both the methods declared in interface Fish and in interface StringBass, because a class can have only one method with a given signature (§8.4). Therefore, it is impossible for a single class to implement both interface Fish and interface StringBass (§8.4.6).

+ +

8.1.5 Class Body and Member Declarations

+ +A class body may contain declarations of members of the class, that is, fields (§8.3), classes (§8.5), interfaces (§8.5) and methods (§8.4). A class body may also contain instance initializers (§8.6), static initializers (§8.7), and declarations of constructors (§8.8) for the class.

+

    +ClassBody:
+	{ ClassBodyDeclarationsopt }
+
+ClassBodyDeclarations:
+	ClassBodyDeclaration
+	ClassBodyDeclarations ClassBodyDeclaration
+
+ClassBodyDeclaration:
+	ClassMemberDeclaration
+	InstanceInitializer
+	StaticInitializer
+	ConstructorDeclaration
+
+ClassMemberDeclaration:
+	FieldDeclaration
+	MethodDeclaration
+	ClassDeclaration						
+	InterfaceDeclaration
+	;			 
+
+The scope of a declaration of a member m declared in or inherited by a class type C is the entire body of C, including any nested type declarations.

+ +If C itself is a nested class, there may be definitions of the same kind (variable, method, or type) for m in enclosing scopes. (The scopes may be blocks, classes, or packages.) In all such cases, the member m declared or inherited in C shadows (§6.3.1) the other definitions of m.

+ +

8.2 Class Members

+ +The members of a class type are all of the following:

+

    +
  • Members inherited from its direct superclass (§8.1.3), except in class Object, which has no direct superclass + +
  • Members inherited from any direct superinterfaces (§8.1.4) + +
  • Members declared in the body of the class (§8.1.5) +
+Members of a class that are declared private are not inherited by subclasses of that class. Only members of a class that are declared protected or public are inherited by subclasses declared in a package other than the one in which the class is declared.

+ +Constructors, static initializers, and instance initializers are not members and therefore are not inherited.

+ +The example: +

class Point {
+	int x, y;
+	private Point() { reset(); }
+	Point(int x, int y) { this.x = x; this.y = y; }
+	private void reset() { this.x = 0; this.y = 0; }
+}
+class ColoredPoint extends Point {
+	int color;
+	void clear() { reset(); }		// error
+}
+class Test {
+	public static void main(String[] args) {
+		ColoredPoint c = new ColoredPoint(0, 0);	// error
+		c.reset();				// error
+	}
+}
+
+causes four compile-time errors:

+

    +
  • An error occurs because ColoredPoint has no constructor declared with two integer parameters, as requested by the use in main. This illustrates the fact that ColoredPoint does not inherit the constructors of its superclass Point. + +
  • Another error occurs because ColoredPoint declares no constructors, and therefore a default constructor for it is automatically created (§8.8.7), and this default constructor is equivalent to: +
	ColoredPoint() { super(); }
+
+ +which invokes the constructor, with no arguments, for the direct superclass of the class ColoredPoint. The error is that the constructor for Point that takes no arguments is private, and therefore is not accessible outside the class Point, even through a superclass constructor invocation (§8.8.5). + +Two more errors occur because the method reset of class Point is private, and therefore is not inherited by class ColoredPoint. The method invocations in method clear of class ColoredPoint and in method main of class Test are therefore not correct.

+ +

8.2.1 Examples of Inheritance

+ +This section illustrates inheritance of class members through several examples.

+ +

8.2.1.1 Example: Inheritance with Default Access

+ +Consider the example where the points package declares two compilation units:

+

package points;
+public class Point {
+	int x, y;
+	public void move(int dx, int dy) { x += dx; y += dy; }
+}
+
+and:

+

package points;
+public class Point3d extends Point {
+	int z;
+	public void move(int dx, int dy, int dz) {
+		x += dx; y += dy; z += dz;
+	}
+}
+
+and a third compilation unit, in another package, is:

+

import points.Point3d;
+class Point4d extends Point3d {
+	int w;
+	public void move(int dx, int dy, int dz, int dw) {
+		x += dx; y += dy; z += dz; w += dw; // compile-time errors
+	}
+}
+
+Here both classes in the points package compile. The class Point3d inherits the fields x and y of class Point, because it is in the same package as Point. The class Point4d, which is in a different package, does not inherit the fields x and y of class Point or the field z of class Point3d, and so fails to compile.

+ +A better way to write the third compilation unit would be: +

import points.Point3d;
+class Point4d extends Point3d {
+	int w;
+	public void move(int dx, int dy, int dz, int dw) {
+		super.move(dx, dy, dz); w += dw;
+	}
+}
+
+using the move method of the superclass Point3d to process dx, dy, and dz. If Point4d is written in this way it will compile without errors.

+ +

8.2.1.2 Inheritance with public and protected

+ +Given the class Point:

+

package points;
+public class Point {
+	public int x, y;
+	protected int useCount = 0;
+	static protected int totalUseCount = 0;
+	public void move(int dx, int dy) {
+		x += dx; y += dy; useCount++; totalUseCount++;
+	}
+}
+
+the public and protected fields x, y, useCount and totalUseCount are inherited in all subclasses of Point.

+ +Therefore, this test program, in another package, can be compiled successfully: +

class Test extends points.Point {
+	public void moveBack(int dx, int dy) {
+		x -= dx; y -= dy; useCount++; totalUseCount++;
+	}
+}
+
+

8.2.1.3 Inheritance with private

+ +In the example:

+

class Point {
+	int x, y;
+	void move(int dx, int dy) {
+		x += dx; y += dy; totalMoves++;
+	}
+	private static int totalMoves;
+	void printMoves() { System.out.println(totalMoves); }
+}
+class Point3d extends Point {
+	int z;
+	void move(int dx, int dy, int dz) {
+		super.move(dx, dy); z += dz; totalMoves++;
+	}
+}
+
+the class variable totalMoves can be used only within the class Point; it is not inherited by the subclass Point3d. A compile-time error occurs because method move of class Point3d tries to increment totalMoves.

+ +

8.2.1.4 Accessing Members of Inaccessible Classes

+ +Even though a class might not be declared public, instances of the class might be available at run time to code outside the package in which it is declared if it has a public superclass or superinterface. An instance of the class can be assigned to a variable of such a public type. An invocation of a public method of the object referred to by such a variable may invoke a method of the class if it implements or overrides a method of the public superclass or superinterface. (In this situation, the method is necessarily declared public, even though it is declared in a class that is not public.)

+ +Consider the compilation unit: +

package points;
+public class Point {
+	public int x, y;
+	public void move(int dx, int dy) {
+		x += dx; y += dy;
+	}
+}
+
+and another compilation unit of another package:

+

package morePoints;
+class Point3d extends points.Point {
+	public int z;
+	public void move(int dx, int dy, int dz) {
+		super.move(dx, dy); z += dz;
+	}
+	public void move(int dx, int dy) {
+		move(dx, dy, 0);
+	}
+}
+public class OnePoint {
+	public static points.Point getOne() { 
+		return new Point3d(); 
+	}
+}
+
+An invocation morePoints.OnePoint.getOne() in yet a third package would return a Point3d that can be used as a Point, even though the type Point3d is not available outside the package morePoints. The two argument version of method move could then be invoked for that object, which is permissible because method move of Point3d is public (as it must be, for any method that overrides a public method must itself be public, precisely so that situations such as this will work out correctly). The fields x and y of that object could also be accessed from such a third package.

+ +While the field z of class Point3d is public, it is not possible to access this field from code outside the package morePoints, given only a reference to an instance of class Point3d in a variable p of type Point. This is because the expression p.z is not correct, as p has type Point and class Point has no field named z; also, the expression ((Point3d)p).z is not correct, because the class type Point3d cannot be referred to outside package morePoints. + +

The declaration of the field z as public is not useless, however. If there were to be, in package morePoints, a public subclass Point4d of the class Point3d: +

package morePoints;
+public class Point4d extends Point3d {
+	public int w;
+	public void move(int dx, int dy, int dz, int dw) {
+		super.move(dx, dy, dz); w += dw;
+	}
+}
+
+then class Point4d would inherit the field z, which, being public, could then be accessed by code in packages other than morePoints, through variables and expressions of the public type Point4d.

+ +

8.3 Field Declarations

+ +The variables of a class type are introduced by field declarations:

+

    +FieldDeclaration:
+	FieldModifiersopt Type VariableDeclarators ;
+
+VariableDeclarators:
+	VariableDeclarator
+	VariableDeclarators , VariableDeclarator
+
+VariableDeclarator:
+	VariableDeclaratorId
+	VariableDeclaratorId = VariableInitializer
+
+VariableDeclaratorId:
+	Identifier
+	VariableDeclaratorId [ ]
+
+VariableInitializer:
+	Expression
+	ArrayInitializer
+	
+
+The FieldModifiers are described in §8.3.1. The Identifier in a FieldDeclarator may be used in a name to refer to the field. Fields are members; the scope (§6.3) of a field declaration is specified in §8.1.5. More than one field may be declared in a single field declaration by using more than one declarator; the FieldModifiers and Type apply to all the declarators in the declaration. Variable declarations involving array types are discussed in §10.2.

+ +It is a compile-time error for the body of a class declaration to declare two fields with the same name. Methods, types, and fields may have the same name, since they are used in different contexts and are disambiguated by different lookup procedures (§6.5).

+ +If the class declares a field with a certain name, then the declaration of that field is said to hide any and all accessible declarations of fields with the same name in superclasses, and superinterfaces of the class. The field declaration also shadows (§6.3.1) declarations of any accessible fields in enclosing classes or interfaces, and any local variables, formal method parameters, and exception handler parameters with the same name in any enclosing blocks.

+ +If a field declaration hides the declaration of another field, the two fields need not have the same type.

+ +A class inherits from its direct superclass and direct superinterfaces all the non-private fields of the superclass and superinterfaces that are both accessible to code in the class and not hidden by a declaration in the class.

+ +Note that a private field of a superclass might be accessible to a subclass (for example, if both classes are members of the same class). Nevertheless, a private field is never inherited by a subclass. + +

It is possible for a class to inherit more than one field with the same name (§8.3.3.3). Such a situation does not in itself cause a compile-time error. However, any attempt within the body of the class to refer to any such field by its simple name will result in a compile-time error, because such a reference is ambiguous. + +

There might be several paths by which the same field declaration might be inherited from an interface. In such a situation, the field is considered to be inherited only once, and it may be referred to by its simple name without ambiguity. + +

A hidden field can be accessed by using a qualified name (if it is static) or by using a field access expression (§15.11) that contains the keyword super or a cast to a superclass type. See §15.11.2 for discussion and an example. + +

A value stored in a field of type float is always an element of the float value set (§4.2.3); similarly, a value stored in a field of type double is always an element of the double value set. It is not permitted for a field of type float to contain an element of the float-extended-exponent value set that is not also an element of the float value set, nor for a field of type double to contain an element of the double-extended-exponent value set that is not also an element of the double value set. + +

8.3.1 Field Modifiers

+
    +FieldModifiers:
+	FieldModifier
+	FieldModifiers FieldModifier
+
+FieldModifier: one of
+	public protected private
+	static final transient volatile
+
+The access modifiers public, protected, and private are discussed in §6.6. A compile-time error occurs if the same modifier appears more than once in a field declaration, or if a field declaration has more than one of the access modifiers public, protected, and private.

+ +If two or more (distinct) field modifiers appear in a field declaration, it is customary, though not required, that they appear in the order consistent with that shown above in the production for FieldModifier. + +

8.3.1.1 static Fields

+ +If a field is declared static, there exists exactly one incarnation of the field, no matter how many instances (possibly zero) of the class may eventually be created. A static field, sometimes called a class variable, is incarnated when the class is initialized (§12.4).

+ +A field that is not declared static (sometimes called a non-static field) is called an instance variable. Whenever a new instance of a class is created, a new variable associated with that instance is created for every instance variable declared in that class or any of its superclasses. The example program:

+

class Point {
+	int x, y, useCount;
+	Point(int x, int y) { this.x = x; this.y = y; }
+	final static Point origin = new Point(0, 0);
+}
+class Test {
+	public static void main(String[] args) {
+		Point p = new Point(1,1);
+		Point q = new Point(2,2);
+		p.x = 3; p.y = 3; p.useCount++; p.origin.useCount++;
+		System.out.println("(" + q.x + "," + q.y + ")");
+		System.out.println(q.useCount);
+		System.out.println(q.origin == Point.origin);
+		System.out.println(q.origin.useCount);
+	}
+}
+
+prints:

+

(2,2)
+0
+true
+1
+
+showing that changing the fields x, y, and useCount of p does not affect the fields of q, because these fields are instance variables in distinct objects. In this example, the class variable origin of the class Point is referenced both using the class name as a qualifier, in Point.origin, and using variables of the class type in field access expressions (§15.11), as in p.origin and q.origin. These two ways of accessing the origin class variable access the same object, evidenced by the fact that the value of the reference equality expression (§15.21.3):

+

q.origin==Point.origin
+
+is true. Further evidence is that the incrementation:

+

p.origin.useCount++;
+
+causes the value of q.origin.useCount to be 1; this is so because p.origin and q.origin refer to the same variable.

+ +

8.3.1.2 final Fields

+ +A field can be declared final (§4.5.4). Both class and instance variables (static and non-static fields) may be declared final.

+ +It is a compile-time error if a blank final (§4.5.4) class variable is not definitely assigned (§16.7) by a static initializer (§8.7) of the class in which it is declared.

+ +A blank final instance variable must be definitely assigned (§16.8) at the end of every constructor (§8.8) of the class in which it is declared; otherwise a compile-time error occurs.

+ +

8.3.1.3 transient Fields

+ +Variables may be marked transient to indicate that they are not part of the persistent state of an object.

+ +If an instance of the class Point: +

class Point {
+	int x, y;
+	transient float rho, theta;
+}
+
+were saved to persistent storage by a system service, then only the fields x and y would be saved. This specification does not specify details of such services; see the specification of java.io.Serializable for an example of such a service.

+ +

8.3.1.4 volatile Fields

+ +As described in §17, the Java programming language allows threads that access shared variables to keep private working copies of the variables; this allows a more efficient implementation of multiple threads. These working copies need be reconciled with the master copies in the shared main memory only at prescribed synchronization points, namely when objects are locked or unlocked. As a rule, to ensure that shared variables are consistently and reliably updated, a thread should ensure that it has exclusive use of such variables by obtaining a lock that, conventionally, enforces mutual exclusion for those shared variables.

+ +The Java programming language provides a second mechanism, volatile fields, that is more convenient for some purposes. + +

+A field may be declared volatile, in which case a thread must reconcile its working copy of the field with the master copy every time it accesses the variable. Moreover, operations on the master copies of one or more volatile variables on behalf of a thread are performed by the main memory in exactly the order that the thread requested.

+ +If, in the following example, one thread repeatedly calls the method one (but no more than Integer.MAX_VALUE times in all), and another thread repeatedly calls the method two: +

class Test {
+	static int i = 0, j = 0;
+	static void one() { i++; j++; }
+	static void two() {
+		System.out.println("i=" + i + " j=" + j);
+	}
+}
+
+then method two could occasionally print a value for j that is greater than the value of i, because the example includes no synchronization and, under the rules explained in §17, the shared values of i and j might be updated out of order.

+ +One way to prevent this out-or-order behavior would be to declare methods one and two to be synchronized (§8.4.3.6): +

class Test {
+	static int i = 0, j = 0;
+	static synchronized void one() { i++; j++; }
+	static synchronized void two() {
+		System.out.println("i=" + i + " j=" + j);
+	}
+}
+
+This prevents method one and method two from being executed concurrently, and furthermore guarantees that the shared values of i and j are both updated before method one returns. Therefore method two never observes a value for j greater than that for i; indeed, it always observes the same value for i and j.

+ +Another approach would be to declare i and j to be volatile: +

class Test {
+	static volatile int i = 0, j = 0;
+	static void one() { i++; j++; }
+	static void two() {
+		System.out.println("i=" + i + " j=" + j);
+	}
+}
+
+ +This allows method one and method two to be executed concurrently, but guarantees that accesses to the shared values for i and j occur exactly as many times, and in exactly the same order, as they appear to occur during execution of the program text by each thread. Therefore, the shared value for j is never greater than that for i, because each update to i must be reflected in the shared value for i before the update to j occurs. It is possible, however, that any given invocation of method two might observe a value for j that is much greater than the value observed for i, because method one might be executed many times between the moment when method two fetches the value of i and the moment when method two fetches the value of j. + +See §17 for more discussion and examples.

+ +A compile-time error occurs if a final variable is also declared volatile.

+ +

8.3.2 Initialization of Fields

+ +If a field declarator contains a variable initializer, then it has the semantics of an assignment (§15.26) to the declared variable, and:

+

    +
  • If the declarator is for a class variable (that is, a static field), then the variable initializer is evaluated and the assignment performed exactly once, when the class is initialized (§12.4). + +
  • If the declarator is for an instance variable (that is, a field that is not static), then the variable initializer is evaluated and the assignment performed each time an instance of the class is created (§12.5). + +The example: +
class Point {
+	int x = 1, y = 5;
+}
+class Test {
+	public static void main(String[] args) {
+		Point p = new Point();
+		System.out.println(p.x + ", " + p.y);
+	}
+}
+
+produces the output:

+

1, 5
+
+because the assignments to x and y occur whenever a new Point is created.

+ +Variable initializers are also used in local variable declaration statements (§14.4), where the initializer is evaluated and the assignment performed each time the local variable declaration statement is executed.

+ +It is a compile-time error if the evaluation of a variable initializer for a static field or for an instance variable of a named class (or of an interface) can complete abruptly with a checked exception (§11.2).

+ +

8.3.2.1 Initializers for Class Variables

+ +If a reference by simple name to any instance variable occurs in an initialization expression for a class variable, then a compile-time error occurs.

+ +If the keyword this (§15.8.3) or the keyword super (§15.11.2, §15.12) occurs in an initialization expression for a class variable, then a compile-time error occurs.

+ +One subtlety here is that, at run time, static variables that are final and that are initialized with compile-time constant values are initialized first. This also applies to such fields in interfaces (§9.3.1). These variables are "constants" that will never be observed to have their default initial values (§4.5.5), even by devious programs. See §12.4.2 and §13.4.8 for more discussion. + +Use of class variables whose declarations appear textually after the use is sometimes restricted, even though these class variables are in scope. See §8.3.2.3 for the precise rules governing forward reference to class variables.

+ +

8.3.2.2 Initializers for Instance Variables

+ +Initialization expressions for instance variables may use the simple name of any static variable declared in or inherited by the class, even one whose declaration occurs textually later.

+ +Thus the example: +

class Test {
+	float f = j;
+	static int j = 1;
+}
+
+compiles without error; it initializes j to 1 when class Test is initialized, and initializes f to the current value of j every time an instance of class Test is created.

+ +Initialization expressions for instance variables are permitted to refer to the current object this (§15.8.3) and to use the keyword super (§15.11.2, §15.12).

+ +Use of instance variables whose declarations appear textually after the use is sometimes restricted, even though these instance variables are in scope. See §8.3.2.3 for the precise rules governing forward reference to instance variables.

+ +

8.3.2.3 Restrictions on the use of Fields during Initialization

+ +The declaration of a member needs to appear before it is used only if the member is an instance (respectively static) field of a class or interface C and all of the following conditions hold:

+

    +
  • The usage occurs in an instance (respectively static) variable initializer of C or in an instance (respectively static) initializer of C. + +
  • The usage is not on the left hand side of an assignment. + +
  • C is the innermost class or interface enclosing the usage. +
+A compile-time error occurs if any of the three requirements above are not met.

+ +This means that a compile-time error results from the test program: +

	class Test {
+		int i = j;	// compile-time error: incorrect forward reference
+		int j = 1;
+	}
+
+whereas the following example compiles without error:

+

	class Test {
+		Test() { k = 2; }
+		int j = 1;
+		int i = j;
+		int k;
+	}
+
+even though the constructor (§8.8) for Test refers to the field k that is declared three lines later.

+ +These restrictions are designed to catch, at compile time, circular or otherwise malformed initializations. Thus, both: +

class Z {
+	static int i = j + 2; 
+	static int j = 4;
+}
+
+and:

+

class Z {
+	static { i = j + 2; }
+	static int i, j;
+	static { j = 4; }
+}
+
+result in compile-time errors. Accesses by methods are not checked in this way, so:

+

class Z {
+	static int peek() { return j; }
+	static int i = peek();
+	static int j = 1;
+}
+class Test {
+	public static void main(String[] args) {
+		System.out.println(Z.i);
+	}
+}
+
+produces the output:

+

0
+
+because the variable initializer for i uses the class method peek to access the value of the variable j before j has been initialized by its variable initializer, at which point it still has its default value (§4.5.5).

+ +A more elaborate example is: +

class UseBeforeDeclaration {
+	static {
+		x = 100; // ok - assignment
+		int y = x + 1; // error - read before declaration
+		int v = x = 3; // ok - x at left hand side of assignment
+		int z = UseBeforeDeclaration.x * 2;
+	// ok - not accessed via simple name
+		Object o = new Object(){ 
+			void foo(){x++;} // ok - occurs in a different class
+			{x++;} // ok - occurs in a different class
+    		};
+  }
+	{
+		j = 200; // ok - assignment
+		j = j + 1; // error - right hand side reads before declaration
+		int k = j = j + 1; 
+		int n = j = 300; // ok - j at left hand side of assignment
+		int h = j++; // error - read before declaration
+		int l = this.j * 3; // ok - not accessed via simple name
+		Object o = new Object(){ 
+			void foo(){j++;} // ok - occurs in a different class
+			{ j = j + 1;} // ok - occurs in a different class
+		};
+	}
+	int w = x= 3; // ok - x at left hand side of assignment
+	int p = x; // ok - instance initializers may access static fields
+	static int u = (new Object(){int bar(){return x;}}).bar();
+	// ok - occurs in a different class
+	static int x;
+	int m = j = 4; // ok - j at left hand side of assignment
+	int o = (new Object(){int bar(){return j;}}).bar(); 
+	// ok - occurs in a different class
+	int j;
+}
+
+

8.3.3 Examples of Field Declarations

+ +The following examples illustrate some (possibly subtle) points about field declarations.

+ +

8.3.3.1 Example: Hiding of Class Variables

+ +The example:

+

class Point {
+	static int x = 2;
+}
+class Test extends Point {
+	static double x = 4.7;
+	public static void main(String[] args) {
+		new Test().printX();
+	}
+	void printX() {
+		System.out.println(x + " " + super.x);
+	}
+}
+
+produces the output:

+

4.7 2
+
+because the declaration of x in class Test hides the definition of x in class Point, so class Test does not inherit the field x from its superclass Point. Within the declaration of class Test, the simple name x refers to the field declared within class Test. Code in class Test may refer to the field x of class Point as super.x (or, because x is static, as Point.x). If the declaration of Test.x is deleted:

+

class Point {
+	static int x = 2;
+}
+class Test extends Point {
+	public static void main(String[] args) {
+		new Test().printX();
+	}
+	void printX() {
+		System.out.println(x + " " + super.x);
+	}
+}
+
+then the field x of class Point is no longer hidden within class Test; instead, the simple name x now refers to the field Point.x. Code in class Test may still refer to that same field as super.x. Therefore, the output from this variant program is:

+

2 2
+
+

8.3.3.2 Example: Hiding of Instance Variables

+ +This example is similar to that in the previous section, but uses instance variables rather than static variables. The code:

+

class Point {
+	int x = 2;
+}
+class Test extends Point {
+	double x = 4.7;
+	void printBoth() {
+		System.out.println(x + " " + super.x);
+	}
+	public static void main(String[] args) {
+		Test sample = new Test();
+		sample.printBoth();
+		System.out.println(sample.x + " " + 
+								((Point)sample).x);
+	}
+}
+
+produces the output:

+

4.7 2
+4.7 2
+
+because the declaration of x in class Test hides the definition of x in class Point, so class Test does not inherit the field x from its superclass Point. It must be noted, however, that while the field x of class Point is not inherited by class Test, it is nevertheless implemented by instances of class Test. In other words, every instance of class Test contains two fields, one of type int and one of type float. Both fields bear the name x, but within the declaration of class Test, the simple name x always refers to the field declared within class Test. Code in instance methods of class Test may refer to the instance variable x of class Point as super.x.

+ +Code that uses a field access expression to access field x will access the field named x in the class indicated by the type of reference expression. Thus, the expression sample.x accesses a float value, the instance variable declared in class Test, because the type of the variable sample is Test, but the expression ((Point)sample).x accesses an int value, the instance variable declared in class Point, because of the cast to type Point. + +

If the declaration of x is deleted from class Test, as in the program: +

class Point {
+	static int x = 2;
+}
+class Test extends Point {
+	void printBoth() {
+		System.out.println(x + " " + super.x);
+	}
+	public static void main(String[] args) {
+		Test sample = new Test();
+		sample.printBoth();
+		System.out.println(sample.x + " " +
+												((Point)sample).x);
+	}
+}
+
+then the field x of class Point is no longer hidden within class Test. Within instance methods in the declaration of class Test, the simple name x now refers to the field declared within class Point. Code in class Test may still refer to that same field as super.x. The expression sample.x still refers to the field x within type Test, but that field is now an inherited field, and so refers to the field x declared in class Point. The output from this variant program is:

+

2 2
+2 2
+
+

8.3.3.3 Example: Multiply Inherited Fields

+ +A class may inherit two or more fields with the same name, either from two interfaces or from its superclass and an interface. A compile-time error occurs on any attempt to refer to any ambiguously inherited field by its simple name. A qualified name or a field access expression that contains the keyword super (§15.11.2) may be used to access such fields unambiguously. In the example:

+

interface Frob { float v = 2.0f; }
+class SuperTest { int v = 3; }
+class Test extends SuperTest implements Frob {
+	public static void main(String[] args) {
+		new Test().printV();
+	}
+	void printV() { System.out.println(v); }
+}
+
+the class Test inherits two fields named v, one from its superclass SuperTest and one from its superinterface Frob. This in itself is permitted, but a compile-time error occurs because of the use of the simple name v in method printV: it cannot be determined which v is intended.

+ +The following variation uses the field access expression super.v to refer to the field named v declared in class SuperTest and uses the qualified name Frob.v to refer to the field named v declared in interface Frob: +

interface Frob { float v = 2.0f; }
+class SuperTest { int v = 3; }
+class Test extends SuperTest implements Frob {
+	public static void main(String[] args) {
+		new Test().printV();
+	}
+	void printV() {
+		System.out.println((super.v + Frob.v)/2);
+	}
+}
+
+It compiles and prints:

+

2.5
+
+ +Even if two distinct inherited fields have the same type, the same value, and are both final, any reference to either field by simple name is considered ambiguous and results in a compile-time error. In the example: +
+interface Color { int RED=0, GREEN=1, BLUE=2; }
+interface TrafficLight { int RED=0, YELLOW=1, GREEN=2; }
+class Test implements Color, TrafficLight {
+	public static void main(String[] args) {
+		System.out.println(GREEN);			// compile-time error
+		System.out.println(RED);		// compile-time error
+	}
+}
+
+it is not astonishing that the reference to GREEN should be considered ambiguous, because class Test inherits two different declarations for GREEN with different values. The point of this example is that the reference to RED is also considered ambiguous, because two distinct declarations are inherited. The fact that the two fields named RED happen to have the same type and the same unchanging value does not affect this judgment.

+ +

8.3.3.4 Example: Re-inheritance of Fields

+ +If the same field declaration is inherited from an interface by multiple paths, the field is considered to be inherited only once. It may be referred to by its simple name without ambiguity. For example, in the code:

+

public interface Colorable {
+	int RED = 0xff0000, GREEN = 0x00ff00, BLUE = 0x0000ff;
+}
+public interface Paintable extends Colorable {
+	int MATTE = 0, GLOSSY = 1;
+}
+class Point { int x, y; }
+class ColoredPoint extends Point implements Colorable {
+	. . .
+}
+class PaintedPoint extends ColoredPoint implements Paintable 
+{
+	. . .       RED       . . .
+}
+
+the fields RED, GREEN, and BLUE are inherited by the class PaintedPoint both through its direct superclass ColoredPoint and through its direct superinterface Paintable. The simple names RED, GREEN, and BLUE may nevertheless be used without ambiguity within the class PaintedPoint to refer to the fields declared in interface Colorable.

+ +

8.4 Method Declarations

+ +A method declares executable code that can be invoked, passing a fixed number of values as arguments.

+

    +MethodDeclaration:
+	MethodHeader MethodBody
+
+MethodHeader:
+	MethodModifiersopt ResultType MethodDeclarator Throwsopt
+
+ResultType:
+	Type
+	void
+
+MethodDeclarator:
+	Identifer ( FormalParameterListopt )
+
+The MethodModifiers are described in §8.4.3, the Throws clause in §8.4.4, and the MethodBody in §8.4.5. A method declaration either specifies the type of value that the method returns or uses the keyword void to indicate that the method does not return a value.

+ +The Identifier in a MethodDeclarator may be used in a name to refer to the method. A class can declare a method with the same name as the class or a field, member class or member interface of the class.

+ +For compatibility with older versions of the Java platform, a declaration form for a method that returns an array is allowed to place (some or all of) the empty bracket pairs that form the declaration of the array type after the parameter list. This is supported by the obsolescent production:

+

    +MethodDeclarator:
+	MethodDeclarator [ ]
+
+but should not be used in new code.

+ +It is a compile-time error for the body of a class to have as members two methods with the same signature (§8.4.2) (name, number of parameters, and types of any parameters). Methods and fields may have the same name, since they are used in different contexts and are disambiguated by the different lookup procedures (§6.5).

+ +

8.4.1 Formal Parameters

+ +The formal parameters of a method or constructor, if any, are specified by a list of comma-separated parameter specifiers. Each parameter specifier consists of a type (optionally preceded by the final modifier) and an identifier (optionally followed by brackets) that specifies the name of the parameter:

+

    +FormalParameterList:
+	FormalParameter
+	FormalParameterList , FormalParameter
+
+FormalParameter:
+	finalopt Type VariableDeclaratorId
+
+The following is repeated from §8.3 to make the presentation here clearer:

+

    +VariableDeclaratorId:
+	Identifier
+	VariableDeclaratorId [ ]
+
+If a method or constructor has no parameters, only an empty pair of parentheses appears in the declaration of the method or constructor.

+ +If two formal parameters of the same method or constructor are declared to have the same name (that is, their declarations mention the same Identifier), then a compile-time error occurs.

+ +It is a compile-time error if a method or constructor parameter that is declared final is assigned to within the body of the method or constructor.

+ +When the method or constructor is invoked (§15.12), the values of the actual argument expressions initialize newly created parameter variables, each of the declared Type, before execution of the body of the method or constructor. The Identifier that appears in the DeclaratorId may be used as a simple name in the body of the method or constructor to refer to the formal parameter.

+ +The scope of a parameter of a method (§8.4.1) or constructor (§8.8.1) is the entire body of the method or constructor.

+ +These parameter names may not be redeclared as local variables of the method, or as exception parameters of catch clauses in a try statement of the method or constructor. However, a parameter of a method or constructor may be shadowed anywhere inside a class declaration nested within that method or constructor. Such a nested class declaration could declare either a local class (§14.3) or an anonymous class (§15.9).

+ +Formal parameters are referred to only using simple names, never by using qualified names (§6.6).

+ +A method or constructor parameter of type float always contains an element of the float value set (§4.2.3); similarly, a method or constructor parameter of type double always contains an element of the double value set. It is not permitted for a method or constructor parameter of type float to contain an element of the float-extended-exponent value set that is not also an element of the float value set, nor for a method parameter of type double to contain an element of the double-extended-exponent value set that is not also an element of the double value set.

+ +Where an actual argument expression corresponding to a parameter variable is not FP-strict (§15.4), evaluation of that actual argument expression is permitted to use intermediate values drawn from the appropriate extended-exponent value sets. Prior to being stored in the parameter variable the result of such an expression is mapped to the nearest value in the corresponding standard value set by method invocation conversion (§5.3).

+ +

8.4.2 Method Signature

+ +The signature of a method consists of the name of the method and the number and types of formal parameters to the method. A class may not declare two methods with the same signature, or a compile-time error occurs.

+ +The example: +

class Point implements Move {
+	int x, y;
+	abstract void move(int dx, int dy);
+	void move(int dx, int dy) { x += dx; y += dy; }
+}
+
+causes a compile-time error because it declares two move methods with the same signature. This is an error even though one of the declarations is abstract.

+ +

8.4.3 Method Modifiers

+
    +MethodModifiers:
+	MethodModifier
+	MethodModifiers MethodModifier
+
+MethodModifier: one of
+	public protected private abstract static
+	final synchronized native strictfp
+
+The access modifiers public, protected, and private are discussed in §6.6. A compile-time error occurs if the same modifier appears more than once in a method declaration, or if a method declaration has more than one of the access modifiers public, protected, and private. A compile-time error occurs if a method declaration that contains the keyword abstract also contains any one of the keywords private, static, final, native, strictfp, or synchronized. A compile-time error occurs if a method declaration that contains the keyword native also contains strictfp.

+ +If two or more method modifiers appear in a method declaration, it is customary, though not required, that they appear in the order consistent with that shown above in the production for MethodModifier. + +

8.4.3.1 abstract Methods

+ +An abstract method declaration introduces the method as a member, providing its signature (name and number and type of parameters), return type, and throws clause (if any), but does not provide an implementation. The declaration of an abstract method m must appear directly within an abstract class (call it A); otherwise a compile-time error results. Every subclass of A that is not abstract must provide an implementation for m, or a compile-time error occurs as specified in §8.1.1.1.

+ +It is a compile-time error for a private method to be declared abstract.

+ +It would be impossible for a subclass to implement a private abstract method, because private methods are not inherited by subclasses; therefore such a method could never be used. + +

+It is a compile-time error for a static method to be declared abstract.

+ +It is a compile-time error for a final method to be declared abstract.

+ +An abstract class can override an abstract method by providing another abstract method declaration.

+ +This can provide a place to put a documentation comment, or to declare that the set of checked exceptions (§11.2) that can be thrown by that method, when it is implemented by its subclasses, is to be more limited. For example, consider this code: +

class BufferEmpty extends Exception {
+	BufferEmpty() { super(); }
+	BufferEmpty(String s) { super(s); }
+}
+class BufferError extends Exception {
+	BufferError() { super(); }
+	BufferError(String s) { super(s); }
+}
+public interface Buffer {
+	char get() throws BufferEmpty, BufferError;
+}
+public abstract class InfiniteBuffer implements Buffer {
+	abstract char get() throws BufferError;
+}
+
+ +The overriding declaration of method get in class InfiniteBuffer states that method get in any subclass of InfiniteBuffer never throws a BufferEmpty exception, putatively because it generates the data in the buffer, and thus can never run out of data. + +An instance method that is not abstract can be overridden by an abstract method.

+ +For example, we can declare an abstract class Point that requires its subclasses to implement toString if they are to be complete, instantiable classes: +

abstract class Point {
+	int x, y;
+	public abstract String toString();
+}
+
+This abstract declaration of toString overrides the non-abstract toString method of class Object. (Class Object is the implicit direct superclass of class Point.) Adding the code:

+

class ColoredPoint extends Point {
+	int color;
+	public String toString() {
+		return super.toString() + ": color " + color; // error
+	}
+}
+
+results in a compile-time error because the invocation super.toString() refers to method toString in class Point, which is abstract and therefore cannot be invoked. Method toString of class Object can be made available to class ColoredPoint only if class Point explicitly makes it available through some other method, as in:

+

abstract class Point {
+	int x, y;
+	public abstract String toString();
+	protected String objString() { return super.toString(); }
+}
+class ColoredPoint extends Point {
+	int color;
+	public String toString() {
+		return objString() + ": color " + color;	// correct
+	}
+}
+
+

8.4.3.2 static Methods

+ +A method that is declared static is called a class method. A class method is always invoked without reference to a particular object. An attempt to reference the current object using the keyword this or the keyword super in the body of a class method results in a compile-time error. It is a compile-time error for a static method to be declared abstract.

+ +A method that is not declared static is called an instance method, and sometimes called a non-static method. An instance method is always invoked with respect to an object, which becomes the current object to which the keywords this and super refer during execution of the method body.

+ +

8.4.3.3 final Methods

+ +A method can be declared final to prevent subclasses from overriding or hiding it. It is a compile-time error to attempt to override or hide a final method.

+ +A private method and all methods declared in a final class (§8.1.1.2) are implicitly final, because it is impossible to override them. It is permitted but not required for the declarations of such methods to redundantly include the final keyword.

+ +It is a compile-time error for a final method to be declared abstract.

+ +At run time, a machine-code generator or optimizer can "inline" the body of a final method, replacing an invocation of the method with the code in its body. The inlining process must preserve the semantics of the method invocation. In particular, if the target of an instance method invocation is null, then a NullPointerException must be thrown even if the method is inlined. The compiler must ensure that the exception will be thrown at the correct point, so that the actual arguments to the method will be seen to have been evaluated in the correct order prior to the method invocation.

+ +Consider the example: +

final class Point {
+	int x, y;
+	void move(int dx, int dy) { x += dx; y += dy; }
+}
+class Test {
+	public static void main(String[] args) {
+		Point[] p = new Point[100];
+		for (int i = 0; i < p.length; i++) {
+			p[i] = new Point();
+			p[i].move(i, p.length-1-i);
+		}
+	}
+}
+
+Here, inlining the method move of class Point in method main would transform the for loop to the form:

+

		for (int i = 0; i < p.length; i++) {
+			p[i] = new Point();
+			Point pi = p[i];
+			int j = p.length-1-i;
+			pi.x += i;
+			pi.y += j;
+		}
+
+The loop might then be subject to further optimizations.

+ +Such inlining cannot be done at compile time unless it can be guaranteed that Test and Point will always be recompiled together, so that whenever Point-and specifically its move method-changes, the code for Test.main will also be updated. + +

8.4.3.4 native Methods

+ +A method that is native is implemented in platform-dependent code, typically written in another programming language such as C, C++, FORTRAN, or assembly language. The body of a native method is given as a semicolon only, indicating that the implementation is omitted, instead of a block.

+ +A compile-time error occurs if a native method is declared abstract.

+ +For example, the class RandomAccessFile of the package java.io might declare the following native methods: +

package java.io;
+public class RandomAccessFile
+	implements DataOutput, DataInput
+{	. . .
+	public native void open(String name, boolean writeable)
+		throws IOException;
+	public native int readBytes(byte[] b, int off, int len)
+		throws IOException;
+	public native void writeBytes(byte[] b, int off, int len)
+		throws IOException;
+	public native long getFilePointer() throws IOException;
+	public native void seek(long pos) throws IOException;
+	public native long length() throws IOException;
+	public native void close() throws IOException;
+}
+
+

8.4.3.5 strictfp Methods

+ +The effect of the strictfp modifier is to make all float or double expressions within the method body be explicitly FP-strict (§15.4).

+ +

8.4.3.6 synchronized Methods

+ +A synchronized method acquires a lock (§17.1) before it executes. For a class (static) method, the lock associated with the Class object for the method's class is used. For an instance method, the lock associated with this (the object for which the method was invoked) is used.

+ +These are the same locks that can be used by the synchronized statement (§14.18); thus, the code: +

class Test {
+	int count;
+	synchronized void bump() { count++; }
+	static int classCount;
+	static synchronized void classBump() {
+		classCount++;
+	}
+}
+
+has exactly the same effect as:

+

class BumpTest {
+	int count;
+	void bump() {
+		synchronized (this) {
+			count++;
+		}
+	}
+	static int classCount;
+	static void classBump() {
+		try {
+			synchronized (Class.forName("BumpTest")) {
+				classCount++;
+			}
+		} catch (ClassNotFoundException e) {
+				...
+		}
+	}
+}
+
+The more elaborate example:

+

public class Box {
+	private Object boxContents;
+	public synchronized Object get() {
+		Object contents = boxContents;
+		boxContents = null;
+		return contents;
+	}
+	public synchronized boolean put(Object contents) {
+		if (boxContents != null)
+			return false;
+		boxContents = contents;
+		return true;
+	}
+}
+
+defines a class which is designed for concurrent use. Each instance of the class Box has an instance variable contents that can hold a reference to any object. You can put an object in a Box by invoking put, which returns false if the box is already full. You can get something out of a Box by invoking get, which returns a null reference if the box is empty.

+ +If put and get were not synchronized, and two threads were executing methods for the same instance of Box at the same time, then the code could misbehave. It might, for example, lose track of an object because two invocations to put occurred at the same time. +

+ +See §17 for more discussion of threads and locks. + +

8.4.4 Method Throws

+ +A throws clause is used to declare any checked exceptions (§11.2) that can result from the execution of a method or constructor:

+

    +Throws:
+	throws ClassTypeList
+
+ClassTypeList:
+	ClassType
+	ClassTypeList , ClassType
+
+A compile-time error occurs if any ClassType mentioned in a throws clause is not the class Throwable or a subclass of Throwable. It is permitted but not required to mention other (unchecked) exceptions in a throws clause.

+ +For each checked exception that can result from execution of the body of a method or constructor, a compile-time error occurs unless that exception type or a superclass of that exception type is mentioned in a throws clause in the declaration of the method or constructor.

+ +The requirement to declare checked exceptions allows the compiler to ensure that code for handling such error conditions has been included. Methods or constructors that fail to handle exceptional conditions thrown as checked exceptions will normally result in a compile-time error because of the lack of a proper exception type in a throws clause. The Java programming language thus encourages a programming style where rare and otherwise truly exceptional conditions are documented in this way. + +

+The predefined exceptions that are not checked in this way are those for which declaring every possible occurrence would be unimaginably inconvenient:

+

    +
  • Exceptions that are represented by the subclasses of class Error, for example OutOfMemoryError, are thrown due to a failure in or of the virtual machine. Many of these are the result of linkage failures and can occur at unpredictable points in the execution of a program. Sophisticated programs may yet wish to catch and attempt to recover from some of these conditions. + +
  • The exceptions that are represented by the subclasses of the class RuntimeException, for example NullPointerException, result from run-time integrity checks and are thrown either directly from the program or in library routines. It is beyond the scope of the Java programming language, and perhaps beyond the state of the art, to include sufficient information in the program to reduce to a manageable number the places where these can be proven not to occur. +
+A method that overrides or hides another method (§8.4.6), including methods that implement abstract methods defined in interfaces, may not be declared to throw more checked exceptions than the overridden or hidden method.

+ +More precisely, suppose that B is a class or interface, and A is a superclass or superinterface of B, and a method declaration n in B overrides or hides a method declaration m in A. If n has a throws clause that mentions any checked exception types, then m must have a throws clause, and for every checked exception type listed in the throws clause of n, that same exception class or one of its superclasses must occur in the throws clause of m; otherwise, a compile-time error occurs.

+ +See §11 for more information about exceptions and a large example.

+ +

8.4.5 Method Body

+ +A method body is either a block of code that implements the method or simply a semicolon, indicating the lack of an implementation. The body of a method must be a semicolon if and only if the method is either abstract (§8.4.3.1) or native (§8.4.3.4).

+

    +MethodBody:
+	Block 
+	;
+
+A compile-time error occurs if a method declaration is either abstract or native and has a block for its body. A compile-time error occurs if a method declaration is neither abstract nor native and has a semicolon for its body.

+ +If an implementation is to be provided for a method but the implementation requires no executable code, the method body should be written as a block that contains no statements: "{ }". + +

+If a method is declared void, then its body must not contain any return statement (§14.16) that has an Expression.

+ +If a method is declared to have a return type, then every return statement (§14.16) in its body must have an Expression. A compile-time error occurs if the body of the method can complete normally (§14.1).

+ +In other words, a method with a return type must return only by using a return statement that provides a value return; it is not allowed to "drop off the end of its body." +

+ +Note that it is possible for a method to have a declared return type and yet contain no return statements. Here is one example: +

class DizzyDean {
+	int pitch() { throw new RuntimeException("90 mph?!"); }
+}
+
+

8.4.6 Inheritance, Overriding, and Hiding

+ +A class inherits from its direct superclass and direct superinterfaces all the non-private methods (whether abstract or not) of the superclass and superinterfaces that are accessible to code in the class and are neither overridden (§8.4.6.1) nor hidden (§8.4.6.2) by a declaration in the class.

+ +

8.4.6.1 Overriding (by Instance Methods)

+ +An instance method m1 declared in a class C overrides another method with the same signature, m2, declared in class A if both +
    + +
  1. C is a subclass of A. + +
  2. Either +
      + +
    • m2 is non-private and accessible from C, or + +
    • m1 overrides a method m3, m3 distinct from m1, m3 distinct from m2, such that m3 overrides m2. +
    +
+ +Moreover, if m1 is not abstract, then m1 is said to implement any and all declarations of abstract methods that it overrides.

+ +A compile-time error occurs if an instance method overrides a static method.

+ +In this respect, overriding of methods differs from hiding of fields (§8.3), for it is permissible for an instance variable to hide a static variable. + +

+An overridden method can be accessed by using a method invocation expression (§15.12) that contains the keyword super. Note that a qualified name or a cast to a superclass type is not effective in attempting to access an overridden method; in this respect, overriding of methods differs from hiding of fields. See §15.12.4.9 for discussion and examples of this point. + +

+The presence or absence of the strictfp modifier has absolutely no effect on the rules for overriding methods and implementing abstract methods. For example, it is permitted for a method that is not FP-strict to override an FP-strict method and it is permitted for an FP-strict method to override a method that is not FP-strict.

+ +

8.4.6.2 Hiding (by Class Methods)

+ +If a class declares a static method, then the declaration of that method is said to hide any and all methods with the same signature in the superclasses and superinterfaces of the class that would otherwise be accessible to code in the class. A compile-time error occurs if a static method hides an instance method.

+ +In this respect, hiding of methods differs from hiding of fields (§8.3), for it is permissible for a static variable to hide an instance variable. Hiding is also distinct from shadowing (§6.3.1) and obscuring (§6.3.2). + +

+A hidden method can be accessed by using a qualified name or by using a method invocation expression (§15.12) that contains the keyword super or a cast to a superclass type. In this respect, hiding of methods is similar to hiding of fields. + +

8.4.6.3 Requirements in Overriding and Hiding

+ +If a method declaration overrides or hides the declaration of another method, then a compile-time error occurs if they have different return types or if one has a return type and the other is void. Moreover, a method declaration must not have a throws clause that conflicts (§8.4.4) with that of any method that it overrides or hides; otherwise, a compile-time error occurs.

+ +In these respects, overriding of methods differs from hiding of fields (§8.3), for it is permissible for a field to hide a field of another type. + +

+The access modifier (§6.6) of an overriding or hiding method must provide at least as much access as the overridden or hidden method, or a compile-time error occurs. In more detail:

+

    +
  • If the overridden or hidden method is public, then the overriding or hiding method must be public; otherwise, a compile-time error occurs. + +
  • If the overridden or hidden method is protected, then the overriding or hiding method must be protected or public; otherwise, a compile-time error occurs. + +
  • If the overridden or hidden method has default (package) access, then the overriding or hiding method must not be private; otherwise, a compile-time error occurs. + +
+Note that a private method cannot be hidden or overridden in the technical sense of those terms. This means that a subclass can declare a method with the same signature as a private method in one of its superclasses, and there is no requirement that the return type or throws clause of such a method bear any relationship to those of the private method in the superclass. + +

8.4.6.4 Inheriting Methods with the Same Signature

+ +It is possible for a class to inherit more than one method with the same signature. Such a situation does not in itself cause a compile-time error. There are then two possible cases:

+

    +
  • If one of the inherited methods is not abstract, then there are two subcases: +
      + +
    • If the method that is not abstract is static, a compile-time error occurs. + +
    • Otherwise, the method that is not abstract is considered to override, and therefore to implement, all the other methods on behalf of the class that inherits it. A compile-time error occurs if, comparing the method that is not abstract with each of the other of the inherited methods, for any such pair, either they have different return types or one has a return type and the other is void. Moreover, a compile-time error occurs if the inherited method that is not abstract has a throws clause that conflicts (§8.4.4) with that of any other of the inherited methods. +
    + +
  • If all the inherited methods are abstract, then the class is necessarily an abstract class and is considered to inherit all the abstract methods. A compile-time error occurs if, for any two such inherited methods, either they have different return types or one has a return type and the other is void. (The throws clauses do not cause errors in this case.) +
+It is not possible for two or more inherited methods with the same signature not to be abstract, because methods that are not abstract are inherited only from the direct superclass, not from superinterfaces.

+ +There might be several paths by which the same method declaration might be inherited from an interface. This fact causes no difficulty and never, of itself, results in a compile-time error.

+ +

8.4.7 Overloading

+ +If two methods of a class (whether both declared in the same class, or both inherited by a class, or one declared and one inherited) have the same name but different signatures, then the method name is said to be overloaded. This fact causes no difficulty and never of itself results in a compile-time error. There is no required relationship between the return types or between the throws clauses of two methods with the same name but different signatures.

+ +Methods are overridden on a signature-by-signature basis.

+ +If, for example, a class declares two public methods with the same name, and a subclass overrides one of them, the subclass still inherits the other method. In this respect, the Java programming language differs from C++. + +

+When a method is invoked (§15.12), the number of actual arguments and the compile-time types of the arguments are used, at compile time, to determine the signature of the method that will be invoked (§15.12.2). If the method that is to be invoked is an instance method, the actual method to be invoked will be determined at run time, using dynamic method lookup (§15.12.4). + +

8.4.8 Examples of Method Declarations

+ +The following examples illustrate some (possibly subtle) points about method declarations.

+ +

8.4.8.1 Example: Overriding

+ +In the example:

+

class Point {
+	int x = 0, y = 0;
+	void move(int dx, int dy) { x += dx; y += dy; }
+}
+class SlowPoint extends Point {
+	int xLimit, yLimit;
+	void move(int dx, int dy) {
+		super.move(limit(dx, xLimit), limit(dy, yLimit));
+	}
+	static int limit(int d, int limit) {
+		return d > limit ? limit : d < -limit ? -limit : d;
+	}
+}
+
+the class SlowPoint overrides the declarations of method move of class Point with its own move method, which limits the distance that the point can move on each invocation of the method. When the move method is invoked for an instance of class SlowPoint, the overriding definition in class SlowPoint will always be called, even if the reference to the SlowPoint object is taken from a variable whose type is Point.

+ +

8.4.8.2 Example: Overloading, Overriding, and Hiding

+ +In the example:

+

class Point {
+	int x = 0, y = 0;
+	void move(int dx, int dy) { x += dx; y += dy; }
+	int color;
+}
+class RealPoint extends Point {
+	float x = 0.0f, y = 0.0f;
+	void move(int dx, int dy) { move((float)dx, (float)dy); }
+	void move(float dx, float dy) { x += dx; y += dy; }
+}
+
+the class RealPoint hides the declarations of the int instance variables x and y of class Point with its own float instance variables x and y, and overrides the method move of class Point with its own move method. It also overloads the name move with another method with a different signature (§8.4.2).

+ +In this example, the members of the class RealPoint include the instance variable color inherited from the class Point, the float instance variables x and y declared in RealPoint, and the two move methods declared in RealPoint. + +

+Which of these overloaded move methods of class RealPoint will be chosen for any particular method invocation will be determined at compile time by the overloading resolution procedure described in §15.12. + +

8.4.8.3 Example: Incorrect Overriding

+ +This example is an extended variation of that in the preceding section:

+

class Point {
+	int x = 0, y = 0, color;
+	void move(int dx, int dy) { x += dx; y += dy; }
+	int getX() { return x; }
+	int getY() { return y; }
+}
+class RealPoint extends Point {
+	float x = 0.0f, y = 0.0f;
+	void move(int dx, int dy) { move((float)dx, (float)dy); }
+	void move(float dx, float dy) { x += dx; y += dy; }
+	float getX() { return x; }
+	float getY() { return y; }
+}
+
+Here the class Point provides methods getX and getY that return the values of its fields x and y; the class RealPoint then overrides these methods by declaring methods with the same signature. The result is two errors at compile time, one for each method, because the return types do not match; the methods in class Point return values of type int, but the wanna-be overriding methods in class RealPoint return values of type float.

+ +

8.4.8.4 Example: Overriding versus Hiding

+ +This example corrects the errors of the example in the preceding section:

+

class Point {
+	int x = 0, y = 0;
+	void move(int dx, int dy) { x += dx; y += dy; }
+	int getX() { return x; }
+	int getY() { return y; }
+	int color;
+}
+class RealPoint extends Point {
+	float x = 0.0f, y = 0.0f;
+	void move(int dx, int dy) { move((float)dx, (float)dy); }
+	void move(float dx, float dy) { x += dx; y += dy; }
+	int getX() { return (int)Math.floor(x); }
+	int getY() { return (int)Math.floor(y); }
+}
+
+Here the overriding methods getX and getY in class RealPoint have the same return types as the methods of class Point that they override, so this code can be successfully compiled.

+ +Consider, then, this test program: +

class Test {
+	public static void main(String[] args) {
+		RealPoint rp = new RealPoint();
+		Point p = rp;
+		rp.move(1.71828f, 4.14159f);
+		p.move(1, -1);
+		show(p.x, p.y);
+		show(rp.x, rp.y);
+		show(p.getX(), p.getY());
+		show(rp.getX(), rp.getY());
+	}
+	static void show(int x, int y) {
+		System.out.println("(" + x + ", " + y + ")");
+	}
+	static void show(float x, float y) {
+		System.out.println("(" + x + ", " + y + ")");
+	}
+}
+
+The output from this program is:

+

(0, 0)
+(2.7182798, 3.14159)
+(2, 3)
+(2, 3)
+
+ +The first line of output illustrates the fact that an instance of RealPoint actually contains the two integer fields declared in class Point; it is just that their names are hidden from code that occurs within the declaration of class RealPoint (and those of any subclasses it might have). When a reference to an instance of class RealPoint in a variable of type Point is used to access the field x, the integer field x declared in class Point is accessed. The fact that its value is zero indicates that the method invocation p.move(1, -1) did not invoke the method move of class Point; instead, it invoked the overriding method move of class RealPoint. + +

The second line of output shows that the field access rp.x refers to the field x declared in class RealPoint. This field is of type float, and this second line of output accordingly displays floating-point values. Incidentally, this also illustrates the fact that the method name show is overloaded; the types of the arguments in the method invocation dictate which of the two definitions will be invoked. + +

The last two lines of output show that the method invocations p.getX() and rp.getX() each invoke the getX method declared in class RealPoint. Indeed, there is no way to invoke the getX method of class Point for an instance of class RealPoint from outside the body of RealPoint, no matter what the type of the variable we may use to hold the reference to the object. Thus, we see that fields and methods behave differently: hiding is different from overriding. + +

8.4.8.5 Example: Invocation of Hidden Class Methods

+ +A hidden class (static) method can be invoked by using a reference whose type is the class that actually contains the declaration of the method. In this respect, hiding of static methods is different from overriding of instance methods. The example:

+

class Super {
+	static String greeting() { return "Goodnight"; }
+	String name() { return "Richard"; }
+}
+class Sub extends Super {
+	static String greeting() { return "Hello"; }
+	String name() { return "Dick"; }
+}
+class Test {
+	public static void main(String[] args) {
+		Super s = new Sub();
+		System.out.println(s.greeting() + ", " + s.name());
+	}
+}
+
+produces the output:

+

Goodnight, Dick
+
+because the invocation of greeting uses the type of s, namely Super, to figure out, at compile time, which class method to invoke, whereas the invocation of name uses the class of s, namely Sub, to figure out, at run time, which instance method to invoke.

+ +

8.4.8.6 Large Example of Overriding

+ +Overriding makes it easy for subclasses to extend the behavior of an existing class, as shown in this example:

+

import java.io.OutputStream;
+import java.io.IOException;
+class BufferOutput {
+	private OutputStream o;
+	BufferOutput(OutputStream o) { this.o = o; }
+	protected byte[] buf = new byte[512];
+	protected int pos = 0;
+	public void putchar(char c) throws IOException {
+		if (pos == buf.length)
+			flush();
+		buf[pos++] = (byte)c;
+	}
+	public void putstr(String s) throws IOException {
+		for (int i = 0; i < s.length(); i++)
+			putchar(s.charAt(i));
+	}
+	public void flush() throws IOException {
+		o.write(buf, 0, pos);
+		pos = 0;
+	}
+}
+class LineBufferOutput extends BufferOutput {
+	LineBufferOutput(OutputStream o) { super(o); }
+	public void putchar(char c) throws IOException {
+		super.putchar(c);
+		if (c == '\n')
+			flush();
+	}
+}
+class Test {
+	public static void main(String[] args)
+		throws IOException
+	{
+		LineBufferOutput lbo =
+			new LineBufferOutput(System.out);
+		lbo.putstr("lbo\nlbo");
+		System.out.print("print\n");
+		lbo.putstr("\n");
+	}
+}
+
+This example produces the output:

+

lbo
+print
+lbo
+
+ +The class BufferOutput implements a very simple buffered version of an OutputStream, flushing the output when the buffer is full or flush is invoked. The subclass LineBufferOutput declares only a constructor and a single method putchar, which overrides the method putchar of BufferOutput. It inherits the methods putstr and flush from class BufferOutput. +

+ +In the putchar method of a LineBufferOutput object, if the character argument is a newline, then it invokes the flush method. The critical point about overriding in this example is that the method putstr, which is declared in class BufferOutput, invokes the putchar method defined by the current object this, which is not necessarily the putchar method declared in class BufferOutput. + +

+Thus, when putstr is invoked in main using the LineBufferOutput object lbo, the invocation of putchar in the body of the putstr method is an invocation of the putchar of the object lbo, the overriding declaration of putchar that checks for a newline. This allows a subclass of BufferOutput to change the behavior of the putstr method without redefining it. + +

+Documentation for a class such as BufferOutput, which is designed to be extended, should clearly indicate what is the contract between the class and its subclasses, and should clearly indicate that subclasses may override the putchar method in this way. The implementor of the BufferOutput class would not, therefore, want to change the implementation of putstr in a future implementation of BufferOutput not to use the method putchar, because this would break the preexisting contract with subclasses. See the further discussion of binary compatibility in §13, especially §13.2. + +

8.4.8.7 Example: Incorrect Overriding because of Throws

+ +This example uses the usual and conventional form for declaring a new exception type, in its declaration of the class BadPointException:

+

class BadPointException extends Exception {
+	BadPointException() { super(); }
+	BadPointException(String s) { super(s); }
+}
+class Point {
+	int x, y;
+	void move(int dx, int dy) { x += dx; y += dy; }
+}
+class CheckedPoint extends Point {
+	void move(int dx, int dy) throws BadPointException {
+		if ((x + dx) < 0 || (y + dy) < 0)
+			throw new BadPointException();
+		x += dx; y += dy;
+	}
+}
+
+This example results in a compile-time error, because the override of method move in class CheckedPoint declares that it will throw a checked exception that the move in class Point has not declared. If this were not considered an error, an invoker of the method move on a reference of type Point could find the contract between it and Point broken if this exception were thrown.

+ +Removing the throws clause does not help: +

class CheckedPoint extends Point {
+	void move(int dx, int dy) {
+		if ((x + dx) < 0 || (y + dy) < 0)
+			throw new BadPointException();
+		x += dx; y += dy;
+	}
+}
+
+ +

+A different compile-time error now occurs, because the body of the method move cannot throw a checked exception, namely BadPointException, that does not appear in the throws clause for move. + +

8.5 Member Type Declarations

+ +A member class is a class whose declaration is directly enclosed in another class or interface declaration. Similarly, a member interface is an interface whose declaration is directly enclosed in another class or interface declaration. The scope (§6.3) of a member class or interface is specified in §8.1.5.

+ +If the class declares a member type with a certain name, then the declaration of that type is said to hide any and all accessible declarations of member types with the same name in superclasses and superinterfaces of the class.

+ +Within a class C, a declaration d of a member type named n shadows the declarations of any other types named n that are in scope at the point where d occurs.

+ +If a member class or interface declared with simple name C is directly enclosed within the declaration of a class with fully qualified name N, then the member class or interface has the fully qualified name N.C.

+ +A class may inherit two or more type declarations with the same name, either from two interfaces or from its superclass and an interface. A compile-time error occurs on any attempt to refer to any ambiguously inherited class or interface by its simple name.

+ +If the same type declaration is inherited from an interface by multiple paths, the class or interface is considered to be inherited only once. It may be referred to by its simple name without ambiguity.

+ +

8.5.1 Access Modifiers

+ +The access modifiers public, protected, and private are discussed in §6.6. A compile-time error occurs if a member type declaration has more than one of the access modifiers public, protected, and private.

+ +

8.5.2 Static Member Type Declarations

+ +The static keyword may modify the declaration of a member type C within the body of a non-inner class T. Its effect is to declare that C is not an inner class. Just as a static method of T has no current instance of T in its body, C also has no current instance of T, nor does it have any lexically enclosing instances.

+ +It is a compile-time error if a static class contains a usage of a non-static member of an enclosing class.

+ +Member interfaces are always implicitly static. It is permitted but not required for the declaration of a member interface to explicitly list the static modifier.

+ +

8.6 Instance Initializers

+ +An instance initializer declared in a class is executed when an instance of the class is created (§15.9), as specified in §8.8.5.1.

+

    +InstanceInitializer:
+	Block
+
+An instance initializer of a named class may not throw a checked exception unless that exception or one of its superclasses is explicitly declared in the throws clause of each constructor of its class and the class has at least one explicitly declared constructor. An instance initializer in an anonymous class (§15.9.5) can throw any exceptions.

+ +The rules above distinguish between instance initializers in named and anonymous classes. This distinction is deliberate. A given anonymous class is only instantiated at a single point in a program. It is therefore possible to directly propagate information about what exceptions might be raised by an anonymous class' instance initializer to the surrounding expression. Named classes, on the other hand, can be instantiated in many places. Therefore the only way to propagate information about what exceptions might be raised by an instance initializer of a named class is through the throws clauses of its constructors. It follows that a more liberal rule can be used in the case of anonymous classes. Similar comments apply to instance variable initializers. + +

+It is a compile-time error if an instance initializer cannot complete normally (§14.20). If a return statement (§14.16) appears anywhere within an instance initializer, then a compile-time error occurs.

+ +Use of instance variables whose declarations appear textually after the use is sometimes restricted, even though these instance variables are in scope. See §8.3.2.3 for the precise rules governing forward reference to instance variables.

+ +Instance initializers are permitted to refer to the current object this (§15.8.3) and to use the keyword super (§15.11.2, §15.12).

+ +

8.7 Static Initializers

+ +Any static initializers declared in a class are executed when the class is initialized and, together with any field initializers (§8.3.2) for class variables, may be used to initialize the class variables of the class (§12.4).

+

    +StaticInitializer:
+	static Block
+
+It is a compile-time error for a static initializer to be able to complete abruptly (§14.1, §15.6) with a checked exception (§11.2). It is a compile-time error if a static initializer cannot complete normally (§14.20).

+ +The static initializers and class variable initializers are executed in textual order.

+ +Use of class variables whose declarations appear textually after the use is sometimes restricted, even though these class variables are in scope. See §8.3.2.3 for the precise rules governing forward reference to class variables.

+ +If a return statement (§14.16) appears anywhere within a static initializer, then a compile-time error occurs.

+ +If the keyword this (§15.8.3) or the keyword super (§15.11, §15.12) appears anywhere within a static initializer, then a compile-time error occurs.

+ +

8.8 Constructor Declarations

+ +A constructor is used in the creation of an object that is an instance of a class:

+

    +ConstructorDeclaration:
+	ConstructorModifiersopt ConstructorDeclarator
+		Throwsopt ConstructorBody
+
+ConstructorDeclarator:
+	SimpleTypeName ( FormalParameterListopt )
+
+The SimpleTypeName in the ConstructorDeclarator must be the simple name of the class that contains the constructor declaration; otherwise a compile-time error occurs. In all other respects, the constructor declaration looks just like a method declaration that has no result type.

+ +

+ +Here is a simple example: +

class Point {
+	int x, y;
+	Point(int x, int y) { this.x = x; this.y = y; }
+}
+
+Constructors are invoked by class instance creation expressions (§15.9), by the conversions and concatenations caused by the string concatenation operator + (§15.18.1), and by explicit constructor invocations from other constructors (§8.8.5). Constructors are never invoked by method invocation expressions (§15.12).

+ +Access to constructors is governed by access modifiers (§6.6).

+ +This is useful, for example, in preventing instantiation by declaring an inaccessible constructor (§8.8.8). + +Constructor declarations are not members. They are never inherited and therefore are not subject to hiding or overriding.

+ +

8.8.1 Formal Parameters

+ +The formal parameters of a constructor are identical in structure and behavior to the formal parameters of a method (§8.4.1).

+ +

8.8.2 Constructor Signature

+ +The signature of a constructor consists of the number and types of formal parameters to the constructor. A class may not declare two constructors with the same signature, or a compile-time error occurs.

+ +

8.8.3 Constructor Modifiers

+
    +ConstructorModifiers:
+	ConstructorModifier
+	ConstructorModifiers ConstructorModifier
+
+ConstructorModifier: one of
+	public protected private
+
+The access modifiers public, protected, and private are discussed in §6.6. A compile-time error occurs if the same modifier appears more than once in a constructor declaration, or if a constructor declaration has more than one of the access modifiers public, protected, and private.

+ +Unlike methods, a constructor cannot be abstract, static, final, native, strictfp, or synchronized. A constructor is not inherited, so there is no need to declare it final and an abstract constructor could never be implemented. A constructor is always invoked with respect to an object, so it makes no sense for a constructor to be static. There is no practical need for a constructor to be synchronized, because it would lock the object under construction, which is normally not made available to other threads until all constructors for the object have completed their work. The lack of native constructors is an arbitrary language design choice that makes it easy for an implementation of the Java virtual machine to verify that superclass constructors are always properly invoked during object creation. + +

Note that a ConstructorModifier cannot be declared strictfp. This difference in the definitions for ConstructorModifier and MethodModifier (§8.4.3) is an intentional language design choice; it effectively ensures that a constructor is FP-strict (§15.4) if and only if its class is FP-strict. + + +

8.8.4 Constructor Throws

+ +The throws clause for a constructor is identical in structure and behavior to the throws clause for a method (§8.4.4).

+ +

8.8.5 Constructor Body

+ +The first statement of a constructor body may be an explicit invocation of another constructor of the same class or of the direct superclass (§8.8.5.1).

+

    +ConstructorBody:
+	{ ExplicitConstructorInvocationopt BlockStatementsopt }
+
+It is a compile-time error for a constructor to directly or indirectly invoke itself through a series of one or more explicit constructor invocations involving this.

+ +If a constructor body does not begin with an explicit constructor invocation and the constructor being declared is not part of the primordial class Object, then the constructor body is implicitly assumed by the compiler to begin with a superclass constructor invocation "super();", an invocation of the constructor of its direct superclass that takes no arguments.

+ +Except for the possibility of explicit constructor invocations, the body of a constructor is like the body of a method (§8.4.5). A return statement (§14.16) may be used in the body of a constructor if it does not include an expression.

+ +In the example: +

class Point {
+	int x, y;
+	Point(int x, int y) { this.x = x; this.y = y; }
+}
+class ColoredPoint extends Point {
+	static final int WHITE = 0, BLACK = 1;
+	int color;
+	ColoredPoint(int x, int y) {
+		this(x, y, WHITE);
+	}
+	ColoredPoint(int x, int y, int color) {
+		super(x, y);
+		this.color = color;
+	}
+}
+
+the first constructor of ColoredPoint invokes the second, providing an additional argument; the second constructor of ColoredPoint invokes the constructor of its superclass Point, passing along the coordinates.

+ +§12.5 and §15.9 describe the creation and initialization of new class instances.

+ +

8.8.5.1 Explicit Constructor Invocations

+
    +ExplicitConstructorInvocation:
+	this ( ArgumentListopt ) ;
+	super ( ArgumentListopt ) ;
+	Primary.super ( ArgumentListopt ) ; 
+
+

+ +Explicit constructor invocation statements can be divided into two kinds:

+

    +
  • Alternate constructor invocations begin with the keyword this. They are used to invoke an alternate constructor of the same class. + +
  • Superclass constructor invocations begin with either the keyword super or a Primary expression. They are used to invoke a constructor of the direct superclass. Superclass constructor invocations may be further subdivided: +
      + +
    • Unqualified superclass constructor invocations begin with the keyword super. + +
    • Qualified superclass constructor invocations begin with a Primary expression. They allow a subclass constructor to explicitly specify the newly created object's immediately enclosing instance with respect to the direct superclass (§8.1.2). This may be necessary when the superclass is an inner class. + +Here is an example of a qualified superclass constructor invocation: +
    +
class Outer {
+	class Inner{}
+}
+class ChildOfInner extends Outer.Inner {
+	ChildOfInner(){(new Outer()).super();}
+}
+
+ +An explicit constructor invocation statement in a constructor body may not refer to any instance variables or instance methods declared in this class or any superclass, or use this or super in any expression; otherwise, a compile-time error occurs.

+ +For example, if the first constructor of ColoredPoint in the example above were changed to: +

ColoredPoint(int x, int y) {
+	this(x, y, color);
+}
+
+then a compile-time error would occur, because an instance variable cannot be used within a superclass constructor invocation.

+ +If an anonymous class instance creation expression appears within an explicit constructor invocation statement, then the anonymous class may not refer to any of the enclosing instances of the class whose constructor is being invoked.

+ +For example: +

class Top {
+	int x;
+	class Dummy {
+		Dummy(Object o) {}
+	}
+	class Inside extends Dummy {
+		Inside() {
+			super(new Object() { int r = x; }); // error
+		}
+		Inside(final int y) {
+			super(new Object() { int r = y; }); // correct
+		}
+	}
+}
+
+Let C be the class being instantiated, let S be the direct superclass of C, and let i be the instance being created. The evaluation of an explicit constructor invocation proceeds as follows:

+

    +
  • First, if the constructor invocation statement is a superclass constructor invocation, then the immediately enclosing instance of i with respect to S (if any) must be determined. Whether or not i has an immediately enclosing instance with respect to S is determined by the superclass constructor invocation as follows: +
      + +
    • If S is not an inner class, or if the declaration of S occurs in a static context, no immediately enclosing instance of i with respect to S exists. A compile-time error occurs if the superclass constructor invocation is a qualified superclass constructor invocation. + +
    • Otherwise: +
        + +
      • If the superclass constructor invocation is qualified, then the Primary expression p immediately preceding ".super" is evaluated. If the primary expression evaluates to null, a NullPointerException is raised, and the superclass constructor invocation completes abruptly. Otherwise, the result of this evaluation is the immediately enclosing instance of i with respect to S. Let O be the immediately lexically enclosing class of S; it is a compile-time error if the type of p is not O or a subclass of O. + +
      • Otherwise: +
          + +
        • If S is a local class (§14.3), then S must be declared in a method declared in a lexically enclosing class O. Let n be an integer such that O is the nth lexically enclosing class of C. The immediately enclosing instance of i with respect to S is the nth lexically enclosing instance of this. + +
        • Otherwise, S is an inner member class (§8.5). It is a compile-time error if S is not a member of a lexically enclosing class. Let O be the innermost lexically enclosing class of which S is a member, and let n be an integer such that O is the nth lexically enclosing class of C. The immediately enclosing instance of i with respect to S is the nth lexically enclosing instance of this. +
        +
      +
    + +
  • Second, the arguments to the constructor are evaluated, left-to-right, as in an ordinary method invocation. + +
  • Next, the constructor is invoked. + +
  • Finally, if the constructor invocation statement is a superclass constructor invocation and the constructor invocation statement completes normally, then all instance variable initializers of C and all instance initializers of C are executed. If an instance initializer or instance variable initializer I textually precedes another instance initializer or instance variable initializer J, then I is executed before J. This action is performed regardless of whether the superclass constructor invocation actually appears as an explicit constructor invocation statement or is provided automatically. An alternate constructor invocation does not perform this additional implicit action. +
+

8.8.6 Constructor Overloading

+ +Overloading of constructors is identical in behavior to overloading of methods. The overloading is resolved at compile time by each class instance creation expression (§15.9).

+ +

8.8.7 Default Constructor

+ +If a class contains no constructor declarations, then a default constructor that takes no parameters is automatically provided:

+

    +
  • If the class being declared is the primordial class Object, then the default constructor has an empty body. + +
  • Otherwise, the default constructor takes no parameters and simply invokes the superclass constructor with no arguments. +
+A compile-time error occurs if a default constructor is provided by the compiler but the superclass does not have an accessible constructor that takes no arguments.

+ +

A default constructor has no throws clause.

+It follows that is the nullary constructor of the superclass has a +throws clause, then a compile-time error will occur. +

+ +If the class is declared public, then the default constructor is implicitly given the access modifier public (§6.6); if the class is declared protected, then the default constructor is implicitly given the access modifier protected (§6.6); if the class is declared private, then the default constructor is implicitly given the access modifier private (§6.6); otherwise, the default constructor has the default access implied by no access modifier.

+ +Thus, the example: +

public class Point {
+	int x, y;
+}
+
+is equivalent to the declaration:

+

public class Point {
+	int x, y;
+	public Point() { super(); }
+}
+
+where the default constructor is public because the class Point is public.

+ +The rule that the default constructor of a class has the same access modifier as the class itself is simple and intuitive. Note, however, that this does not imply that the constructor is accessible whenever the class is accessible. Consider +

package p1;
+public class Outer {
+ 	protected class Inner{}
+}
+package p2;
+class SonOfOuter extends p1.Outer {
+	void foo() {
+ 		new Inner(); // compile-time access error
+	}
+}
+
+
+The constructor for Inner is protected. However, the constructor is protected relative to Inner, while Inner is protected relative to Outer. So, Inner is accessible in SonOfOuter, since it is a subclass of Outer. Inner's constructor is not accessible in SonOfOuter, because the class SonOfOuter is not a subclass of Inner! Hence, even though Inner is accessible, its default constructor is not.

+ +

8.8.8 Preventing Instantiation of a Class

+ +A class can be designed to prevent code outside the class declaration from creating instances of the class by declaring at least one constructor, to prevent the creation of an implicit constructor, and declaring all constructors to be private. A public class can likewise prevent the creation of instances outside its package by declaring at least one constructor, to prevent creation of a default constructor with public access, and declaring no constructor that is public.

+ +Thus, in the example: +

class ClassOnly {
+	private ClassOnly() { }
+	static String just = "only the lonely";
+}
+
+the class ClassOnly cannot be instantiated, while in the example:

+

package just;
+public class PackageOnly {
+	PackageOnly() { }
+	String[] justDesserts = { "cheesecake", "ice cream" };
+}
+
+the class PackageOnly can be instantiated only within the package just, in which it is declared. +

+ + +

+

+ + + +
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+Second Edition
+

+Java Language Specification (HTML generated by Suzette Pelouch on May 19, 2000)
+Copyright © 2000 Sun Microsystems, Inc. +All rights reserved +
+Please send any comments or corrections via our feedback form + + + + + Interfaces + + + + + + + + +
Contents | Prev | Next | IndexJava Language Specification
+Second Edition
+


+ + +

+CHAPTER + 9

+ +

Interfaces

+

+ +An interface declaration introduces a new reference type whose members are classes, interfaces, constants and abstract methods. This type has no implementation, but otherwise unrelated classes can implement it by providing implementations for its abstract methods.

+ +A nested interface is any interface whose declaration occurs within the body of another class or interface. A top-level interface is an interface that is not a nested interface.

+ +This chapter discusses the common semantics of all interfaces-top-level (§7.6) and nested (§8.5, §9.5). Details that are specific to particular kinds of interfaces are discussed in the sections dedicated to these constructs.

+ +Programs can use interfaces to make it unnecessary for related classes to share a common abstract superclass or to add methods to Object. + +An interface may be declared to be a direct extension of one or more other interfaces, meaning that it implicitly specifies all the member types, abstract methods and constants of the interfaces it extends, except for any member types and constants that it may hide.

+ +A class may be declared to directly implement one or more interfaces, meaning that any instance of the class implements all the abstract methods specified by the interface or interfaces. A class necessarily implements all the interfaces that its direct superclasses and direct superinterfaces do. This (multiple) interface inheritance allows objects to support (multiple) common behaviors without sharing any implementation.

+ +A variable whose declared type is an interface type may have as its value a reference to any instance of a class which implements the specified interface. It is not sufficient that the class happen to implement all the abstract methods of the interface; the class or one of its superclasses must actually be declared to implement the interface, or else the class is not considered to implement the interface.

+ +

9.1 Interface Declarations

+ +An interface declaration specifies a new named reference type:

+

    +InterfaceDeclaration:
+	InterfaceModifiersopt interface Identifier
+		ExtendsInterfacesopt InterfaceBody
+
+The Identifier in an interface declaration specifies the name of the interface. A compile-time error occurs if an interface has the same simple name as any of its enclosing classes or interfaces.

+ +

9.1.1 Interface Modifiers

+ +An interface declaration may include interface modifiers:

+

    +InterfaceModifiers:
+	InterfaceModifier
+	InterfaceModifiers InterfaceModifier
+
+InterfaceModifier: one of
+	public protected private
+	abstract static strictfp
+
+The access modifier public is discussed in §6.6. Not all modifiers are applicable to all kinds of interface declarations. The access modifiers protected and private pertain only to member interfaces within a directly enclosing class declaration (§8.5) and are discussed in §8.5.1. The access modifier static pertains only to member interfaces (§8.5, §9.5). A compile-time error occurs if the same modifier appears more than once in an interface declaration.

+ +

9.1.1.1 abstract Interfaces

+ +Every interface is implicitly abstract. This modifier is obsolete and should not be used in new programs.

+ +

9.1.1.2 strictfp Interfaces

+ +The effect of the strictfp modifier is to make all float or double expressions within the interface declaration be explicitly FP-strict (§15.4).

+ +This implies that all nested types declared in the interface are implicitly strictfp.

+ +

9.1.2 Superinterfaces and Subinterfaces

+ +If an extends clause is provided, then the interface being declared extends each of the other named interfaces and therefore inherits the member types, methods, and constants of each of the other named interfaces. These other named interfaces are the direct superinterfaces of the interface being declared. Any class that implements the declared interface is also considered to implement all the interfaces that this interface extends.

+

    +ExtendsInterfaces:
+	extends InterfaceType
+	ExtendsInterfaces , InterfaceType
+
+The following is repeated from §4.2 to make the presentation here clearer:

+

    +InterfaceType:
+	TypeName
+
+Each InterfaceType in the extends clause of an interface declaration must name an accessible interface type; otherwise a compile-time error occurs.

+ +An interface I directly depends on a type T if T is mentioned in the extends clause of I either as a superinterface or as a qualifier within a superinterface name. An interface I depends on a reference type T if any of the following conditions hold:

+

    +
  • I directly depends on T. + +
  • I directly depends on a class C that depends (§8.1.3) on T. + +
  • I directly depends on an interface J that depends on T (using this definition recursively). +
+A compile-time error occurs if an interface depends on itself.

+ +While every class is an extension of class Object, there is no single interface of which all interfaces are extensions.

+ +The superinterface relationship is the transitive closure of the direct superinterface relationship. An interface K is a superinterface of interface I if either of the following is true:

+

    +
  • K is a direct superinterface of I. + +
  • There exists an interface J such that K is a superinterface of J, and J is a superinterface of I, applying this definition recursively. +
+Interface I is said to be a subinterface of interface K whenever K is a superinterface of I.

+ +

9.1.3 Interface Body and Member Declarations

+ +The body of an interface may declare members of the interface:

+

    +
+InterfaceBody:
+	{ InterfaceMemberDeclarationsopt }
+
+InterfaceMemberDeclarations:
+	InterfaceMemberDeclaration
+	InterfaceMemberDeclarations InterfaceMemberDeclaration
+
+InterfaceMemberDeclaration:
+	ConstantDeclaration
+	AbstractMethodDeclaration
+	ClassDeclaration 
+	InterfaceDeclaration
+	;
+
+The scope of the declaration of a member m declared in or inherited by an interface type I is the entire body of I, including any nested type declarations.

+ +

9.1.4 Access to Interface Member Names

+ +All interface members are implicitly public. They are accessible outside the package where the interface is declared if the interface is also declared public or protected, in accordance with the rules of §6.6.

+ +

9.2 Interface Members

+ +The members of an interface are:

+

    +
  • Those members declared in the interface. + +
  • Those members inherited from direct superinterfaces. + +
  • If an interface has no direct superinterfaces, then the interface implicitly declares a public abstract member method m with signature s, return type r, and throws clause t corresponding to each public instance method m with signature s, return type r, and throws clause t declared in Object, unless a method with the same signature, same return type, and a compatible throws clause is explicitly declared by the interface. +
+It follows that it is a compile-time error if the interface declares a method with the same signature and different return type or incompatible throws clause.

+ +The interface inherits, from the interfaces it extends, all members of those interfaces, except for fields, classes, and interfaces that it hides and methods that it overrides.

+ +

9.3 Field (Constant) Declarations

+
    +
+ConstantDeclaration:
+	ConstantModifiersopt Type VariableDeclarators
+
+ConstantModifiers: 
+	ConstantModifier
+	ConstantModifiers ConstantModifer 
+
+ConstantModifier: one of
+	public static final
+
+Every field declaration in the body of an interface is implicitly public, static, and final. It is permitted to redundantly specify any or all of these modifiers for such fields.

+ +If the interface declares a field with a certain name, then the declaration of that field is said to hide any and all accessible declarations of fields with the same name in superinterfaces of the interface.

+ +It is a compile-time error for the body of an interface declaration to declare two fields with the same name.

+ +It is possible for an interface to inherit more than one field with the same name (§8.3.3.3). Such a situation does not in itself cause a compile-time error. However, any attempt within the body of the interface to refer to either field by its simple name will result in a compile-time error, because such a reference is ambiguous.

+ +There might be several paths by which the same field declaration might be inherited from an interface. In such a situation, the field is considered to be inherited only once, and it may be referred to by its simple name without ambiguity.

+ +

9.3.1 Initialization of Fields in Interfaces

+ +Every field in the body of an interface must have an initialization expression, which need not be a constant expression. The variable initializer is evaluated and the assignment performed exactly once, when the interface is initialized (§12.4).

+ +A compile-time error occurs if an initialization expression for an interface field contains a reference by simple name to the same field or to another field whose declaration occurs textually later in the same interface.

+ +Thus: +

interface Test {
+	float f = j;
+	int j = 1;
+	int k = k+1;
+}
+
+causes two compile-time errors, because j is referred to in the initialization of f before j is declared and because the initialization of k refers to k itself.

+ +One subtlety here is that, at run time, fields that are initialized with compile-time constant values are initialized first. This applies also to static final fields in classes (§8.3.2.1). This means, in particular, that these fields will never be observed to have their default initial values (§4.5.5), even by devious programs. See §12.4.2 and §13.4.8 for more discussion. +

+ +If the keyword this (§15.8.3) or the keyword super (15.11.2, 15.12) occurs in an initialization expression for a field of an interface, then unless the occurrence is within the body of an anonymous class (§15.9.5), a compile-time error occurs.

+ +

9.3.2 Examples of Field Declarations

+ +The following example illustrates some (possibly subtle) points about field declarations.

+ +

9.3.2.1 Ambiguous Inherited Fields

+ +If two fields with the same name are inherited by an interface because, for example, two of its direct superinterfaces declare fields with that name, then a single ambiguous member results. Any use of this ambiguous member will result in a compile-time error. Thus in the example:

+

interface BaseColors {
+	int RED = 1, GREEN = 2, BLUE = 4;
+}
+interface RainbowColors extends BaseColors {
+	int YELLOW = 3, ORANGE = 5, INDIGO = 6, VIOLET = 7;
+}
+interface PrintColors extends BaseColors {
+	int YELLOW = 8, CYAN = 16, MAGENTA = 32;
+}
+interface LotsOfColors extends RainbowColors, PrintColors {
+	int FUCHSIA = 17, VERMILION = 43, CHARTREUSE = RED+90;
+}
+
+the interface LotsOfColors inherits two fields named YELLOW. This is all right as long as the interface does not contain any reference by simple name to the field YELLOW. (Such a reference could occur within a variable initializer for a field.)

+ +Even if interface PrintColors were to give the value 3 to YELLOW rather than the value 8, a reference to field YELLOW within interface LotsOfColors would still be considered ambiguous. +

+ +

9.3.2.2 Multiply Inherited Fields

+ +If a single field is inherited multiple times from the same interface because, for example, both this interface and one of this interface's direct superinterfaces extend the interface that declares the field, then only a single member results. This situation does not in itself cause a compile-time error.

+ +In the example in the previous section, the fields RED, GREEN, and BLUE are inherited by interface LotsOfColors in more than one way, through interface RainbowColors and also through interface PrintColors, but the reference to field RED in interface LotsOfColors is not considered ambiguous because only one actual declaration of the field RED is involved. +

+ +

9.4 Abstract Method Declarations

+
    +AbstractMethodDeclaration:
+	AbstractMethodModifiersopt ResultType MethodDeclarator Throwsopt ;
+
+AbstractMethodModifiers:
+	AbstractMethodModifier
+	AbstractMethodModifiers AbstractMethodModifier
+
+AbstractMethodModifier: one of
+	public abstract
+
+The access modifier public is discussed in §6.6. A compile-time error occurs if the same modifier appears more than once in an abstract method declaration.

+ +Every method declaration in the body of an interface is implicitly abstract, so its body is always represented by a semicolon, not a block.

+ +Every method declaration in the body of an interface is implicitly public.

+ +For compatibility with older versions of the Java platform, it is permitted but discouraged, as a matter of style, to redundantly specify the abstract modifier for methods declared in interfaces. + +

It is permitted, but strongly discouraged as a matter of style, to redundantly specify the public modifier for interface methods.

+ +Note that a method declared in an interface must not be declared static, or a compile-time error occurs, because static methods cannot be abstract.

+ +Note that a method declared in an interface must not be declared strictfp or native or synchronized, or a compile-time error occurs, because those keywords describe implementation properties rather than interface properties. However, a method declared in an interface may be implemented by a method that is declared strictfp or native or synchronized in a class that implements the interface.

+ +It is a compile-time error for the body of an interface to declare, explicitly or implicitly, two methods with the same signature (name, number of parameters, and types of any parameters) (§8.4.2). However, an interface may inherit several methods with the same signature (§9.4.1).

+ +Note that a method declared in an interface must not be declared final or a compile-time error occurs. However, a method declared in an interface may be implemented by a method that is declared final in a class that implements the interface.

+ +

9.4.1 Inheritance and Overriding

+ +If the interface declares a method, then the declaration of that method is said to override any and all methods with the same signature in the superinterfaces of the interface.

+ +If a method declaration in an interface overrides the declaration of a method in another interface, a compile-time error occurs if the methods have different return types or if one has a return type and the other is void. Moreover, a method declaration must not have a throws clause that conflicts (§8.4.4) with that of any method that it overrides; otherwise, a compile-time error occurs.

+ +Methods are overridden on a signature-by-signature basis. If, for example, an interface declares two public methods with the same name, and a subinterface overrides one of them, the subinterface still inherits the other method.

+ +An interface inherits from its direct superinterfaces all methods of the superinterfaces that are not overridden by a declaration in the interface.

+ +It is possible for an interface to inherit more than one method with the same signature (§8.4.2). Such a situation does not in itself cause a compile-time error. The interface is considered to inherit all the methods. However, a compile-time error occurs if, for any two such inherited methods, either they have different return types or one has a return type and the other is void. (The throws clauses do not cause errors in this case.) There might be several paths by which the same method declaration is inherited from an interface. This fact causes no difficulty and never of itself results in a compile-time error.

+ +

9.4.2 Overloading

+ +If two methods of an interface (whether both declared in the same interface, or both inherited by an interface, or one declared and one inherited) have the same name but different signatures, then the method name is said to be overloaded. This fact causes no difficulty and never of itself results in a compile-time error. There is no required relationship between the return types or between the throws clauses of two methods with the same name but different signatures.

+ +

9.4.3 Examples of Abstract Method Declarations

+ +The following examples illustrate some (possibly subtle) points about abstract method declarations.

+ +

9.4.3.1 Example: Overriding

+ +Methods declared in interfaces are abstract and thus contain no implementation. About all that can be accomplished by an overriding method declaration, other than to affirm a method signature, is to restrict the exceptions that might be thrown by an implementation of the method. Here is a variation of the example shown in §8.4.3.1:

+

class BufferEmpty extends Exception {
+	BufferEmpty() { super(); }
+	BufferEmpty(String s) { super(s); }
+}
+class BufferError extends Exception {
+	BufferError() { super(); }
+	BufferError(String s) { super(s); }
+}
+public interface Buffer {
+	char get() throws BufferEmpty, BufferError;
+}
+public interface InfiniteBuffer extends Buffer {
+	 char get() throws BufferError;												// override
+}
+
+

9.4.3.2 Example: Overloading

+ +In the example code:

+

interface PointInterface {
+	void move(int dx, int dy);
+}
+interface RealPointInterface extends PointInterface {
+	void move(float dx, float dy);
+	void move(double dx, double dy);
+}
+
+the method name move is overloaded in interface RealPointInterface with three different signatures, two of them declared and one inherited. Any non-abstract class that implements interface RealPointInterface must provide implementations of all three method signatures.

+ +

9.5 Member Type Declarations

+ +Interfaces may contain member type declarations (§8.5). A member type declaration in an interface is implicitly static and public.

+ +If a member type declared with simple name C is directly enclosed within the declaration of an interface with fully qualified name N, then the member type has the fully qualified name N.C.

+ +If the interface declares a member type with a certain name, then the declaration of that field is said to hide any and all accessible declarations of member types with the same name in superinterfaces of the interface.

+ +An interface may inherit two or more type declarations with the same name. A compile-time error occurs on any attempt to refer to any ambiguously inherited class or interface by its simple name. If the same type declaration is inherited from an interface by multiple paths, the class or interface is considered to be inherited only once; it may be referred to by its simple name without ambiguity.

+ + +

+ + + +
Contents | Prev | Next | IndexJava Language Specification
+Second Edition
+Copyright © 2000 Sun Microsystems, Inc. +All rights reserved +
+Please send any comments or corrections via our feedback form + + + + + Arrays + + + + + + + + +
Contents | Prev | Next | IndexJava Language Specification
+Second Edition
+

+ + +

+CHAPTER + 10

+ +

Arrays

+

+ +In the Java programming language arrays are objects (§4.3.1), are dynamically created, and may be assigned to variables of type Object (§4.3.2). All methods of class Object may be invoked on an array.

+ +An array object contains a number of variables. The number of variables may be zero, in which case the array is said to be empty. The variables contained in an array have no names; instead they are referenced by array access expressions that use nonnegative integer index values. These variables are called the components of the array. If an array has n components, we say n is the length of the array; the components of the array are referenced using integer indices from 0 to n - 1, inclusive.

+ +All the components of an array have the same type, called the component type of the array. If the component type of an array is T, then the type of the array itself is written T[].

+ +The value of an array component of type float is always an element of the float value set (§4.2.3); similarly, the value of an array component of type double is always an element of the double value set. It is not permitted for the value of an array component of type float to be an element of the float-extended-exponent value set that is not also an element of the float value set, nor for the value of an array component of type double to be an element of the double-extended-exponent value set that is not also an element of the double value set.

+ +The component type of an array may itself be an array type. The components of such an array may contain references to subarrays. If, starting from any array type, one considers its component type, and then (if that is also an array type) the component type of that type, and so on, eventually one must reach a component type that is not an array type; this is called the element type of the original array, and the components at this level of the data structure are called the elements of the original array.

+ +There are some situations in which an element of an array can be an array: if the element type is Object or Cloneable or java.io.Serializable, then some or all of the elements may be arrays, because any array object can be assigned to any variable of these types. + +

10.1 Array Types

+ +An array type is written as the name of an element type followed by some number of empty pairs of square brackets []. The number of bracket pairs indicates the depth of array nesting. An array's length is not part of its type.

+ +The element type of an array may be any type, whether primitive or reference. In particular:

+

    +
  • Arrays with an interface type as the component type are allowed. The elements of such an array may have as their value a null reference or instances of any type that implements the interface. + +
  • Arrays with an abstract class type as the component type are allowed. The elements of such an array may have as their value a null reference or instances of any subclass of the abstract class that is not itself abstract. +
+Array types are used in declarations and in cast expressions (§15.16).

+ +

10.2 Array Variables

+ +A variable of array type holds a reference to an object. Declaring a variable of array type does not create an array object or allocate any space for array components. It creates only the variable itself, which can contain a reference to an array. However, the initializer part of a declarator (§8.3) may create an array, a reference to which then becomes the initial value of the variable.

+ +Because an array's length is not part of its type, a single variable of array type may contain references to arrays of different lengths.

+ +Here are examples of declarations of array variables that do not create arrays: +

+int[] ai;		// array of int
+short[][] as;		// array of array of short
+Object[]    ao,		// array of Object
+	otherAo;	// array of Object
+short	s,		// scalar short 
+	aas[][];	// array of array of short
+
+Here are some examples of declarations of array variables that create array objects:

+

+Exception ae[] = new Exception[3]; 
+Object aao[][] = new Exception[2][3];
+int[] factorial = { 1, 1, 2, 6, 24, 120, 720, 5040 };
+char ac[] = { 'n', 'o', 't', ' ', 'a', ' ',
+				 'S', 't', 'r', 'i', 'n', 'g' }; 
+String[] aas = { "array", "of", "String", };
+
+The [] may appear as part of the type at the beginning of the declaration, or as part of the declarator for a particular variable, or both, as in this example:

+

byte[] rowvector, colvector, matrix[];
+
+This declaration is equivalent to:

+

byte rowvector[], colvector[], matrix[][];
+
+Once an array object is created, its length never changes. To make an array variable refer to an array of different length, a reference to a different array must be assigned to the variable.

+ +If an array variable v has type A[], where A is a reference type, then v can hold a reference to an instance of any array type B[], provided B can be assigned to A. This may result in a run-time exception on a later assignment; see §10.10 for a discussion.

+ +

10.3 Array Creation

+ +An array is created by an array creation expression (§15.10) or an array initializer (§10.6).

+ +An array creation expression specifies the element type, the number of levels of nested arrays, and the length of the array for at least one of the levels of nesting. The array's length is available as a final instance variable length.

+ +An array initializer creates an array and provides initial values for all its components.

+ +

10.4 Array Access

+ +A component of an array is accessed by an array access expression (§15.13) that consists of an expression whose value is an array reference followed by an indexing expression enclosed by [ and ], as in A[i]. All arrays are 0-origin. An array with length n can be indexed by the integers 0 to n-1.

+ +Arrays must be indexed by int values; short, byte, or char values may also be used as index values because they are subjected to unary numeric promotion (§5.6.1) and become int values. An attempt to access an array component with a long index value results in a compile-time error.

+ +All array accesses are checked at run time; an attempt to use an index that is less than zero or greater than or equal to the length of the array causes an ArrayIndexOutOfBoundsException to be thrown.

+ +

10.5 Arrays: A Simple Example

+ +The example:

+

class Gauss {
+	public static void main(String[] args) {
+		int[] ia = new int[101];
+		for (int i = 0; i < ia.length; i++)
+			ia[i] = i;
+		int sum = 0;
+		for (int i = 0; i < ia.length; i++)
+			sum += ia[i];
+		System.out.println(sum);
+	}
+}
+
+that produces the output:

+

5050
+
+declares a variable ia that has type array of int, that is, int[]. The variable ia is initialized to reference a newly created array object, created by an array creation expression (§15.10). The array creation expression specifies that the array should have 101 components. The length of the array is available using the field length, as shown.

+ +The example program fills the array with the integers from 0 to 100, sums these integers, and prints the result. + +

10.6 Array Initializers

+ +An array initializer may be specified in a declaration, or as part of an array creation expression (§15.10), creating an array and providing some initial values:

+

    +ArrayInitializer:
+	{ VariableInitializersopt ,opt }
+
+VariableInitializers:
+	VariableInitializer
+	VariableInitializers , VariableInitializer
+
+The following is repeated from §8.3 to make the presentation here clearer:

+

    +VariableInitializer:
+	Expression
+	ArrayInitializer
+
+An array initializer is written as a comma-separated list of expressions, enclosed by braces "{" and "}".

+ +The length of the constructed array will equal the number of expressions.

+ +The expressions in an array initializer are executed from left to right in the textual order they occur in the source code. The nth variable initializer specifies the value of the n-1st array component. Each expression must be assignment-compatible (§5.2) with the array's component type, or a compile-time error results.

+ +If the component type is itself an array type, then the expression specifying a component may itself be an array initializer; that is, array initializers may be nested.

+ +A trailing comma may appear after the last expression in an array initializer and is ignored.

+ +As an example: +

class Test {
+	public static void main(String[] args) {
+		int ia[][] = { {1, 2}, null };
+		for (int i = 0; i < 2; i++)
+			for (int j = 0; j < 2; j++)
+				System.out.println(ia[i][j]);
+	}
+}
+
+prints:

+

1
+2
+
+before causing a NullPointerException in trying to index the second component of the array ia, which is a null reference.

+ +

10.7 Array Members

+ +The members of an array type are all of the following:

+

    +
  • The public final field length, which contains the number of components of the array (length may be positive or zero) + +
  • The public method clone, which overrides the method of the same name in class Object and throws no checked exceptions + +
  • All the members inherited from class Object; the only method of Object that is not inherited is its clone method +
+

+ +An array thus has the same public fields and methods as the following class:

+

class A implements Cloneable, java.io.Serializable {
+	public final int length = X;
+	public Object clone() {
+		try {
+			return super.clone();
+		} catch (CloneNotSupportedException e) {
+			throw new InternalError(e.getMessage());
+		}
+	}
+}
+
+Every array implements the interfaces Cloneable and java.io.Serializable.

+ +That arrays are cloneable is shown by the test program: +

class Test {
+	public static void main(String[] args) {
+		int ia1[] = { 1, 2 };
+		int ia2[] = (int[])ia1.clone();
+		System.out.print((ia1 == ia2) + " ");
+		ia1[1]++;
+		System.out.println(ia2[1]);
+	}
+}
+
+which prints:

+

false 2
+
+showing that the components of the arrays referenced by ia1 and ia2 are different variables. (In some early implementations of the Java programming language this example failed to compile because the compiler incorrectly believed that the clone method for an array could throw a CloneNotSupportedException.)

+ +A clone of a multidimensional array is shallow, which is to say that it creates only a single new array. Subarrays are shared.

+ +This is shown by the example program: +

class Test {
+	public static void main(String[] args) throws Throwable {
+		int ia[][] = { { 1 , 2}, null };
+		int ja[][] = (int[][])ia.clone();
+		System.out.print((ia == ja) + " ");
+		System.out.println(ia[0] == ja[0] && ia[1] == ja[1]);
+	}
+}
+
+which prints:

+

false true
+
+showing that the int[] array that is ia[0] and the int[] array that is ja[0] are the same array.

+ +

10.8 Class Objects for Arrays

+ +Every array has an associated Class object, shared with all other arrays with the same component type. The direct superclass of an array type is Object. Every array type implements the interfaces Cloneable and java.io.Serializable.

+ +This is shown by the following example code: +

class Test {
+	public static void main(String[] args) {
+		int[] ia = new int[3];
+		System.out.println(ia.getClass());
+		System.out.println(ia.getClass().getSuperclass());
+	}
+}
+
+which prints:

+

class [I
+class java.lang.Object
+
+where the string "[I" is the run-time type signature for the class object "array with component type int".

+ +

10.9 An Array of Characters is Not a String

+ +In Java programming language, unlike C, an array of char is not a String, and neither a String nor an array of char is terminated by '\u0000' (the NUL character).

+ +A String object is immutable, that is, its contents never change, while an array of char has mutable elements. The method toCharArray in class String returns an array of characters containing the same character sequence as a String. The class StringBuffer implements useful methods on mutable arrays of characters.

+ +

10.10 Array Store Exception

+ +If an array variable v has type A[], where A is a reference type, then v can hold a reference to an instance of any array type B[], provided B can be assigned to A.

+ +Thus, the example: +

class Point { int x, y; }
+class ColoredPoint extends Point { int color; }
+class Test {
+	public static void main(String[] args) {
+		ColoredPoint[] cpa = new ColoredPoint[10];
+		Point[] pa = cpa;
+		System.out.println(pa[1] == null);
+		try {
+			pa[0] = new Point();
+		} catch (ArrayStoreException e) {
+			System.out.println(e);
+		}
+	}
+}
+
+produces the output:

+

true
+java.lang.ArrayStoreException
+
+Here the variable pa has type Point[] and the variable cpa has as its value a reference to an object of type ColoredPoint[]. A ColoredPoint can be assigned to a Point; therefore, the value of cpa can be assigned to pa.

+ +A reference to this array pa, for example, testing whether pa[1] is null, will not result in a run-time type error. This is because the element of the array of type ColoredPoint[] is a ColoredPoint, and every ColoredPoint can stand in for a Point, since Point is the superclass of ColoredPoint. +

+ +On the other hand, an assignment to the array pa can result in a run-time error. At compile time, an assignment to an element of pa is checked to make sure that the value assigned is a Point. But since pa holds a reference to an array of ColoredPoint, the assignment is valid only if the type of the value assigned at run-time is, more specifically, a ColoredPoint. + +

+The Java virtual machine checks for such a situation at run-time to ensure that the assignment is valid; if not, an ArrayStoreException is thrown. More formally: an assignment to an element of an array whose type is A[], where A is a reference type, is checked at run-time to ensure that the value assigned can be assigned to the actual element type of the array, where the actual element type may be any reference type that is assignable to A. + +

+ + +

+ + + +
Contents | Prev | Next | IndexJava Language Specification
+Second Edition
+Copyright © 2000 Sun Microsystems, Inc. +All rights reserved +
+Please send any comments or corrections via our feedback form + + + + + Blocks and Statements + + + + + + + + +
Contents | Prev | Next | IndexJava Language Specification
+Second Edition
+

+ + +

+CHAPTER + 14

+ +

Blocks and Statements

+

+ +The sequence of execution of a program is controlled by statements, which are executed for their effect and do not have values.

+ +Some statements contain other statements as part of their structure; such other statements are substatements of the statement. We say that statement S immediately contains statement U if there is no statement T different from S and U such that S contains T and T contains U. In the same manner, some statements contain expressions (§15) as part of their structure.

+ +The first section of this chapter discusses the distinction between normal and abrupt completion of statements (§14.1). Most of the remaining sections explain the various kinds of statements, describing in detail both their normal behavior and any special treatment of abrupt completion.

+ +Blocks are explained first (§14.2), followed by local class declarations (§14.3) and local variable declaration statements (§14.4).

+ +Next a grammatical maneuver that sidesteps the familiar "dangling else" problem (§14.5) is explained.

+ +Statements that will be familiar to C and C++ programmers are the empty (§14.6), labeled (§14.7), expression (§14.8), if (§14.9), switch (§14.10), while (§14.11), do (§14.12), for (§14.13), break (§14.14), continue (§14.15), and return (§14.16) statements.

+ +Unlike C and C++, the Java programming language has no goto statement. However, the break and continue statements are allowed to mention statement labels.

+ +The Java programming language statements that are not in the C language are the throw (§14.17), synchronized (§14.18), and try (§14.19) statements.

+ +The last section (§14.20) of this chapter addresses the requirement that every statement be reachable in a certain technical sense.

+ +

14.1 Normal and Abrupt Completion of Statements

+ +Every statement has a normal mode of execution in which certain computational steps are carried out. The following sections describe the normal mode of execution for each kind of statement.

+ +If all the steps are carried out as described, with no indication of abrupt completion, the statement is said to complete normally. However, certain events may prevent a statement from completing normally:

+

    +
  • The break (§14.14), continue (§14.15), and return (§14.16) statements cause a transfer of control that may prevent normal completion of statements that contain them. + +
  • Evaluation of certain expressions may throw exceptions from the Java virtual machine; these expressions are summarized in §15.6. An explicit throw (§14.17) statement also results in an exception. An exception causes a transfer of control that may prevent normal completion of statements. +
+If such an event occurs, then execution of one or more statements may be terminated before all steps of their normal mode of execution have completed; such statements are said to complete abruptly.

+ +An abrupt completion always has an associated reason, which is one of the following:

+

    +
  • A break with no label + +
  • A break with a given label + +
  • A continue with no label + +
  • A continue with a given label + +
  • A return with no value + +
  • A return with a given value + +
  • A throw with a given value, including exceptions thrown by the Java virtual machine +
+The terms "complete normally" and "complete abruptly" also apply to the evaluation of expressions (§15.6). The only reason an expression can complete abruptly is that an exception is thrown, because of either a throw with a given value (§14.17) or a run-time exception or error (§11, §15.6).

+ +If a statement evaluates an expression, abrupt completion of the expression always causes the immediate abrupt completion of the statement, with the same reason. All succeeding steps in the normal mode of execution are not performed.

+ +Unless otherwise specified in this chapter, abrupt completion of a substatement causes the immediate abrupt completion of the statement itself, with the same reason, and all succeeding steps in the normal mode of execution of the statement are not performed.

+ +Unless otherwise specified, a statement completes normally if all expressions it evaluates and all substatements it executes complete normally.

+ +

14.2 Blocks

+ +A block is a sequence of statements, local class declarations and local variable declaration statements within braces.

+

    +Block:
+	{ BlockStatementsopt }
+
+BlockStatements:
+	BlockStatement
+	BlockStatements BlockStatement
+
+BlockStatement:
+	LocalVariableDeclarationStatement
+	ClassDeclaration
+	Statement
+
+A block is executed by executing each of the local variable declaration statements and other statements in order from first to last (left to right). If all of these block statements complete normally, then the block completes normally. If any of these block statements complete abruptly for any reason, then the block completes abruptly for the same reason.

+ +

14.3 Local Class Declarations

+ +A local class is a nested class (§8) that is not a member of any class and that has a name. All local classes are inner classes (§8.1.2). Every local class declaration statement is immediately contained by a block. Local class declaration statements may be intermixed freely with other kinds of statements in the block.

+ +The scope of a local class declared in a block is the rest of the immediately enclosing block, including its own class declaration.

+ +The name of a local class C may not be redeclared as a local class of the directly enclosing method, constructor, or initializer block within the scope of C, or a compile-time error occurs. However, a local class declaration may be shadowed (§6.3.1) anywhere inside a class declaration nested within the local class declaration's scope. A local class does not have a canonical name, nor does it have a fully qualified name.

+ +It is a compile-time error if a local class declaration contains any one of the following access modifiers: public, protected, private, or static.

+ +Here is an example that illustrates several aspects of the rules given above:

+

class Global {
+	class Cyclic {}
+	void foo() {
+		new Cyclic(); // create a Global.Cyclic
+		class Cyclic extends Cyclic{}; // circular definition
+		{
+			class Local{};
+			{
+				class Local{}; // compile-time error
+			}
+			class Local{}; // compile-time error
+			class AnotherLocal {
+				void bar() {
+					class Local {}; // ok
+				}
+			}
+		}
+		class Local{}; // ok, not in scope of prior Local
+}
+
+The first statement of method foo creates an instance of the member class Global.Cyclic rather than an instance of the local class Cyclic, because the local class declaration is not yet in scope.

+ +The fact that the scope of a local class encompasses its own declaration (not only its body) means that the definition of the local class Cyclic is indeed cyclic because it extends itself rather than Global.Cyclic. Consequently, the declaration of the local class Cyclic will be rejected at compile time.

+ +Since local class names cannot be redeclared within the same method (or constructor or initializer, as the case may be), the second and third declarations of Local result in compile-time errors. However, Local can be redeclared in the context of another, more deeply nested, class such as AnotherLocal.

+ +The fourth and last declaration of Local is legal, since it occurs outside the scope of any prior declaration of Local.

+ +

14.4 Local Variable Declaration Statements

+ +A local variable declaration statement declares one or more local variable names.

+

    +LocalVariableDeclarationStatement:
+	LocalVariableDeclaration ;
+
+LocalVariableDeclaration:
+	finalopt Type VariableDeclarators
+
+The following are repeated from §8.3 to make the presentation here clearer:

+

    +VariableDeclarators:
+	VariableDeclarator
+	VariableDeclarators , VariableDeclarator
+
+VariableDeclarator:
+	VariableDeclaratorId
+	VariableDeclaratorId = VariableInitializer
+
+VariableDeclaratorId:
+	Identifier
+	VariableDeclaratorId [ ]
+
+VariableInitializer:
+	Expression
+	ArrayInitializer
+
+Every local variable declaration statement is immediately contained by a block. Local variable declaration statements may be intermixed freely with other kinds of statements in the block.

+ +A local variable declaration can also appear in the header of a for statement (§14.13). In this case it is executed in the same manner as if it were part of a local variable declaration statement.

+ +

14.4.1 Local Variable Declarators and Types

+ +Each declarator in a local variable declaration declares one local variable, whose name is the Identifier that appears in the declarator.

+ +If the optional keyword final appears at the start of the declarator, the variable being declared is a final variable(§4.5.4).

+ +The type of the variable is denoted by the Type that appears in the local variable declaration, followed by any bracket pairs that follow the Identifier in the declarator.

+ +Thus, the local variable declaration:

+

int a, b[], c[][];
+
+is equivalent to the series of declarations:

+

int a;
+int[] b;
+int[][] c;
+
+Brackets are allowed in declarators as a nod to the tradition of C and C++. The general rule, however, also means that the local variable declaration:

+

float[][] f[][], g[][][], h[];													// Yechh!
+
+is equivalent to the series of declarations:

+

float[][][][] f;
+float[][][][][] g;
+float[][][] h;
+
+We do not recommend such "mixed notation" for array declarations.

+ +A local variable of type float always contains a value that is an element of the float value set (§4.2.3); similarly, a local variable of type double always contains a value that is an element of the double value set. It is not permitted for a local variable of type float to contain an element of the float-extended-exponent value set that is not also an element of the float value set, nor for a local variable of type double to contain an element of the double-extended-exponent value set that is not also an element of the double value set.

+ +

14.4.2 Scope of Local Variable Declarations

+ +The scope of a local variable declaration in a block (§14.2) is the rest of the block in which the declaration appears, starting with its own initializer (§14.4) and including any further declarators to the right in the local variable declaration statement.

+ +The name of a local variable v may not be redeclared as a local variable of the directly enclosing method, constructor or initializer block within the scope of v, or a compile-time error occurs. The name of a local variable v may not be redeclared as an exception parameter of a catch clause in a try statement of the directly enclosing method, constructor or initializer block within the scope of v, or a compile-time error occurs. However, a local variable of a method or initializer block may be shadowed (§6.3.1) anywhere inside a class declaration nested within the scope of the local variable.

+ +A local variable cannot be referred to using a qualified name (§6.6), only a simple name.

+ +The example:

+

class Test {
+	static int x;
+	public static void main(String[] args) {
+		int x = x;
+	}
+}
+
+causes a compile-time error because the initialization of x is within the scope of the declaration of x as a local variable, and the local x does not yet have a value and cannot be used.

+ +The following program does compile:

+

class Test {
+	static int x;
+	public static void main(String[] args) {
+		int x = (x=2)*2;
+		System.out.println(x);
+	}
+}
+
+because the local variable x is definitely assigned (§16) before it is used. It prints:

+

4
+
+Here is another example:

+class Test {
+	public static void main(String[] args) {
+		System.out.print("2+1=");
+		int two = 2, three = two + 1;
+		System.out.println(three);
+	}
+}
+
+which compiles correctly and produces the output:

+

2+1=3
+
+The initializer for three can correctly refer to the variable two declared in an earlier declarator, and the method invocation in the next line can correctly refer to the variable three declared earlier in the block.

+ +The scope of a local variable declared in a for statement is the rest of the for statement, including its own initializer.

+ +If a declaration of an identifier as a local variable of the same method, constructor, or initializer block appears within the scope of a parameter or local variable of the same name, a compile-time error occurs.

+ +Thus the following example does not compile:

+

class Test {
+	public static void main(String[] args) {
+		int i;
+		for (int i = 0; i < 10; i++)
+			System.out.println(i);
+	}
+}
+
+ +This restriction helps to detect some otherwise very obscure bugs. A similar restriction on shadowing of members by local variables was judged impractical, because the addition of a member in a superclass could cause subclasses to have to rename local variables. Related considerations make restrictions on shadowing of local variables by members of nested classes, or on shadowing of local variables by local variables declared within nested classes unattractive as well. Hence, the following example compiles without error:

+

+class Test {
+	public static void main(String[] args) {
+		int i;
+		class Local {
+			{
+				for (int i = 0; i < 10; i++)
+				System.out.println(i);
+			}
+		}
+		new Local();
+	}
+}
+
+ +On the other hand, local variables with the same name may be declared in two separate blocks or for statements neither of which contains the other. Thus:

+

+class Test {
+	public static void main(String[] args) {
+		for (int i = 0; i < 10; i++)
+			System.out.print(i + " ");
+		for (int i = 10; i > 0; i--)
+			System.out.print(i + " ");
+		System.out.println();
+	}
+}
+
+compiles without error and, when executed, produces the output:

+

0 1 2 3 4 5 6 7 8 9 10 9 8 7 6 5 4 3 2 1
+
+

14.4.3 Shadowing of Names by Local Variables

+ +If a name declared as a local variable is already declared as a field name, then that outer declaration is shadowed (§6.3.1) throughout the scope of the local variable. Similarly, if a name is already declared as a variable or parameter name, then that outer declaration is shadowed throughout the scope of the local variable (provided that the shadowing does not cause a compile-time error under the rules of §14.4.2). The shadowed name can sometimes be accessed using an appropriately qualified name.

+ +For example, the keyword this can be used to access a shadowed field x, using the form this.x. Indeed, this idiom typically appears in constructors (§8.8):

+

class Pair {
+	Object first, second;
+	public Pair(Object first, Object second) {
+		this.first = first;
+		this.second = second;
+	}
+}
+
+In this example, the constructor takes parameters having the same names as the fields to be initialized. This is simpler than having to invent different names for the parameters and is not too confusing in this stylized context. In general, however, it is considered poor style to have local variables with the same names as fields.

+ +

14.4.4 Execution of Local Variable Declarations

+ +A local variable declaration statement is an executable statement. Every time it is executed, the declarators are processed in order from left to right. If a declarator has an initialization expression, the expression is evaluated and its value is assigned to the variable. If a declarator does not have an initialization expression, then a Java compiler must prove, using exactly the algorithm given in §16, that every reference to the variable is necessarily preceded by execution of an assignment to the variable. If this is not the case, then a compile-time error occurs.

+ +Each initialization (except the first) is executed only if the evaluation of the preceding initialization expression completes normally. Execution of the local variable declaration completes normally only if evaluation of the last initialization expression completes normally; if the local variable declaration contains no initialization expressions, then executing it always completes normally.

+ +

14.5 Statements

+ +There are many kinds of statements in the Java programming language. Most correspond to statements in the C and C++ languages, but some are unique.

+ +As in C and C++, the if statement of the Java programming language suffers from the so-called "dangling else problem," illustrated by this misleadingly formatted example:

+


+if (door.isOpen())
+	if (resident.isVisible())
+		resident.greet("Hello!");
+else door.bell.ring();	// A "dangling else"
+
+The problem is that both the outer if statement and the inner if statement might conceivably own the else clause. In this example, one might surmise that the programmer intended the else clause to belong to the outer if statement. The Java programming language, like C and C++ and many programming languages before them, arbitrarily decree that an else clause belongs to the innermost if to which it might possibly belong. This rule is captured by the following grammar:

+

    +Statement:
+	StatementWithoutTrailingSubstatement
+	LabeledStatement
+	IfThenStatement
+	IfThenElseStatement
+	WhileStatement
+	ForStatement
+
+StatementWithoutTrailingSubstatement:
+	Block
+	EmptyStatement
+	ExpressionStatement
+	SwitchStatement
+	DoStatement
+	BreakStatement
+	ContinueStatement
+	ReturnStatement
+	SynchronizedStatement
+	ThrowStatement
+	TryStatement
+
+StatementNoShortIf:
+	StatementWithoutTrailingSubstatement
+	LabeledStatementNoShortIf
+	IfThenElseStatementNoShortIf
+	WhileStatementNoShortIf
+	ForStatementNoShortIf
+
+The following are repeated from §14.9 to make the presentation here clearer:

+

    +IfThenStatement:
+	if ( Expression ) Statement
+
+IfThenElseStatement:
+	if ( Expression ) StatementNoShortIf else Statement
+
+IfThenElseStatementNoShortIf:
+	if ( Expression ) StatementNoShortIf else StatementNoShortIf
+
+Statements are thus grammatically divided into two categories: those that might end in an if statement that has no else clause (a "short if statement") and those that definitely do not. Only statements that definitely do not end in a short if statement may appear as an immediate substatement before the keyword else in an if statement that does have an else clause.

+ +This simple rule prevents the "dangling else" problem. The execution behavior of a statement with the "no short if" restriction is identical to the execution behavior of the same kind of statement without the "no short if" restriction; the distinction is drawn purely to resolve the syntactic difficulty.

+ +

14.6 The Empty Statement

+ +An empty statement does nothing.

+

    +EmptyStatement:
+	;
+
+Execution of an empty statement always completes normally.

+ +

14.7 Labeled Statements

+ +Statements may have label prefixes.

+

    +LabeledStatement:
+	Identifier : Statement
+
+LabeledStatementNoShortIf:
+	Identifier : StatementNoShortIf
+
+The Identifier is declared to be the label of the immediately contained Statement.

+ +Unlike C and C++, the Java programming language has no goto statement; identifier statement labels are used with break (§14.14) or continue (§14.15) statements appearing anywhere within the labeled statement.

+ +The scope of a label declared by a labeled statement is the statement immediately enclosed by the labeled statement.

+ + Let l be a label, and let m be the immediately enclosing method, constructor, instance initializer or static initializer. It is a compile-time error if l shadows (§6.3.1) the declaration of another label immediately enclosed in m.

+ +There is no restriction against using the same identifier as a label and as the name of a package, class, interface, method, field, parameter, or local variable. Use of an identifier to label a statement does not obscure (§6.3.2) a package, class, interface, method, field, parameter, or local variable with the same name. Use of an identifier as a class, interface, method, field, local variable or as the parameter of an exception handler (§14.19) does not obscure a statement label with the same name.

+ +A labeled statement is executed by executing the immediately contained Statement. If the statement is labeled by an Identifier and the contained Statement completes abruptly because of a break with the same Identifier, then the labeled statement completes normally. In all other cases of abrupt completion of the Statement, the labeled statement completes abruptly for the same reason.

+ +

14.8 Expression Statements

+ +Certain kinds of expressions may be used as statements by following them with semicolons:

+

    +ExpressionStatement:
+	StatementExpression ;
+
+StatementExpression:
+	Assignment
+	PreIncrementExpression
+	PreDecrementExpression
+	PostIncrementExpression
+	PostDecrementExpression
+	MethodInvocation
+	ClassInstanceCreationExpression
+
+An expression statement is executed by evaluating the expression; if the expression has a value, the value is discarded. Execution of the expression statement completes normally if and only if evaluation of the expression completes normally.

+ +Unlike C and C++, the Java programming language allows only certain forms of expressions to be used as expression statements. Note that the Java programming language does not allow a "cast to void"-void is not a type-so the traditional C trick of writing an expression statement such as:

+

(void) ... ;			// incorrect!
+
+does not work. On the other hand, the language allows all the most useful kinds of expressions in expressions statements, and it does not require a method invocation used as an expression statement to invoke a void method, so such a trick is almost never needed. If a trick is needed, either an assignment statement (§15.26) or a local variable declaration statement (§14.4) can be used instead.

+ +

14.9 The if Statement

+ +The if statement allows conditional execution of a statement or a conditional choice of two statements, executing one or the other but not both.

+

    +IfThenStatement:
+	if ( Expression ) Statement
+
+IfThenElseStatement:
+	if ( Expression ) StatementNoShortIf else Statement
+
+IfThenElseStatementNoShortIf:
+	if ( Expression ) StatementNoShortIf else StatementNoShortIf
+
+The Expression must have type boolean, or a compile-time error occurs.

+ +

14.9.1 The if-then Statement

+ +An if-then statement is executed by first evaluating the Expression. If evaluation of the Expression completes abruptly for some reason, the if-then statement completes abruptly for the same reason. Otherwise, execution continues by making a choice based on the resulting value:

+

    +
  • If the value is true, then the contained Statement is executed; the if-then statement completes normally if and only if execution of the Statement completes normally. + +
  • If the value is false, no further action is taken and the if-then statement completes normally. +
+

14.9.2 The if-then-else Statement

+ +An if-then-else statement is executed by first evaluating the Expression. If evaluation of the Expression completes abruptly for some reason, then the if-then-else statement completes abruptly for the same reason. Otherwise, execution continues by making a choice based on the resulting value:

+

    +
  • If the value is true, then the first contained Statement (the one before the else keyword) is executed; the if-then-else statement completes normally if and only if execution of that statement completes normally. + +
  • If the value is false, then the second contained Statement (the one after the else keyword) is executed; the if-then-else statement completes normally if and only if execution of that statement completes normally. +
+

14.10 The switch Statement

+ +The switch statement transfers control to one of several statements depending on the value of an expression.

+

    +SwitchStatement:
+	switch ( Expression ) SwitchBlock
+
+SwitchBlock:
+	{ SwitchBlockStatementGroupsopt SwitchLabelsopt }
+
+SwitchBlockStatementGroups:
+	SwitchBlockStatementGroup
+	SwitchBlockStatementGroups SwitchBlockStatementGroup
+
+SwitchBlockStatementGroup:
+	SwitchLabels BlockStatements
+
+SwitchLabels:
+	SwitchLabel
+	SwitchLabels SwitchLabel
+
+SwitchLabel:
+	case ConstantExpression :
+	default :
+
+The type of the Expression must be char, byte, short, or int, or a compile-time error occurs.

+ +The body of a switch statement is known as a switch block. Any statement immediately contained by the switch block may be labeled with one or more case or default labels. These labels are said to be associated with the switch statement, as are the values of the constant expressions (§15.28) in the case labels.

+ +All of the following must be true, or a compile-time error will result:

+

    +
  • Every case constant expression associated with a switch statement must be assignable (§5.2) to the type of the switch Expression. + +
  • No two of the case constant expressions associated with a switch statement may have the same value. + +
  • At most one default label may be associated with the same switch statement. + +In C and C++ the body of a switch statement can be a statement and statements with case labels do not have to be immediately contained by that statement. Consider the simple loop:

    +

for (i = 0; i < n; ++i) foo();
+
+where n is known to be positive. A trick known as Duff's device can be used in C or C++ to unroll the loop, but this is not valid code in the Java programming language:

+

int q = (n+7)/8;
+switch (n%8) {
+case 0:		do {	foo();		// Great C hack, Tom,
+case 7:			foo();		// but it's not valid here.
+case 6:			foo();
+case 5:			foo();
+case 4:			foo();
+case 3:			foo();
+case 2:			foo();
+case 1:			foo();
+		} while (--q >= 0);
+}
+
+Fortunately, this trick does not seem to be widely known or used. Moreover, it is less needed nowadays; this sort of code transformation is properly in the province of state-of-the-art optimizing compilers.

+ +When the switch statement is executed, first the Expression is evaluated. If evaluation of the Expression completes abruptly for some reason, the switch statement completes abruptly for the same reason. Otherwise, execution continues by comparing the value of the Expression with each case constant. Then there is a choice:

+

    +
  • If one of the case constants is equal to the value of the expression, then we say that the case matches, and all statements after the matching case label in the switch block, if any, are executed in sequence. If all these statements complete normally, or if there are no statements after the matching case label, then the entire switch statement completes normally. + +
  • If no case matches but there is a default label, then all statements after the matching default label in the switch block, if any, are executed in sequence. If all these statements complete normally, or if there are no statements after the default label, then the entire switch statement completes normally. + +
  • If no case matches and there is no default label, then no further action is taken and the switch statement completes normally. +
+If any statement immediately contained by the Block body of the switch statement completes abruptly, it is handled as follows:

+

    +
  • If execution of the Statement completes abruptly because of a break with no label, no further action is taken and the switch statement completes normally. + +
  • If execution of the Statement completes abruptly for any other reason, the switch statement completes abruptly for the same reason. The case of abrupt completion because of a break with a label is handled by the general rule for labeled statements (§14.7). +
+As in C and C++, execution of statements in a switch block "falls through labels."

+ +For example, the program:

+

class Toomany {
+	static void howMany(int k) {
+		switch (k) {
+		case 1:			System.out.print("one ");
+		case 2:			System.out.print("too ");
+		case 3:			System.out.println("many");
+		}
+	}
+	public static void main(String[] args) {
+		howMany(3);
+		howMany(2);
+		howMany(1);
+	}
+}
+
+contains a switch block in which the code for each case falls through into the code for the next case. As a result, the program prints:

+

many
+too many
+one too many
+
+If code is not to fall through case to case in this manner, then break statements should be used, as in this example:

+

class Twomany {
+	static void howMany(int k) {
+		switch (k) {
+		case 1:			System.out.println("one");
+					break;					// exit the switch
+		case 2:			System.out.println("two");
+					break;					// exit the switch
+		case 3:			System.out.println("many");
+					break;					// not needed, but good style
+		}
+	}
+	public static void main(String[] args) {
+		howMany(1);
+		howMany(2);
+		howMany(3);
+	}
+}
+
+This program prints:

+

one
+two
+many
+
+

14.11 The while Statement

+ +The while statement executes an Expression and a Statement repeatedly until the value of the Expression is false.

+

    +WhileStatement:
+	while ( Expression ) Statement
+
+WhileStatementNoShortIf:
+	while ( Expression ) StatementNoShortIf
+
+The Expression must have type boolean, or a compile-time error occurs.

+ +A while statement is executed by first evaluating the Expression. If evaluation of the Expression completes abruptly for some reason, the while statement completes abruptly for the same reason. Otherwise, execution continues by making a choice based on the resulting value:

+

    +
  • If the value is true, then the contained Statement is executed. Then there is a choice: +
      + +
    • If execution of the Statement completes normally, then the entire while statement is executed again, beginning by re-evaluating the Expression. + +
    • If execution of the Statement completes abruptly, see §14.11.1 below. +
    + +
  • If the value of the Expression is false, no further action is taken and the while statement completes normally. +
+If the value of the Expression is false the first time it is evaluated, then the Statement is not executed.

+ +

14.11.1 Abrupt Completion

+ +Abrupt completion of the contained Statement is handled in the following manner:

+

    +
  • If execution of the Statement completes abruptly because of a break with no label, no further action is taken and the while statement completes normally. +
      + +
    • If execution of the Statement completes abruptly because of a continue with no label, then the entire while statement is executed again. + +
    • If execution of the Statement completes abruptly because of a continue with label L, then there is a choice: +
        + +
      • If the while statement has label L, then the entire while statement is executed again. + +
      • If the while statement does not have label L, the while statement completes abruptly because of a continue with label L. +
      + +
    • If execution of the Statement completes abruptly for any other reason, the while statement completes abruptly for the same reason. Note that the case of abrupt completion because of a break with a label is handled by the general rule for labeled statements (§14.7). +
    +
+

14.12 The do Statement

+ +The do statement executes a Statement and an Expression repeatedly until the value of the Expression is false.

+

    +DoStatement:
+	do Statement while ( Expression ) ;
+
+The Expression must have type boolean, or a compile-time error occurs.

+ +A do statement is executed by first executing the Statement. Then there is a choice:

+

    +
  • If execution of the Statement completes normally, then the Expression is evaluated. If evaluation of the Expression completes abruptly for some reason, the do statement completes abruptly for the same reason. Otherwise, there is a choice based on the resulting value: +
      + +
    • If the value is true, then the entire do statement is executed again. + +
    • If the value is false, no further action is taken and the do statement completes normally. +
    + +
  • If execution of the Statement completes abruptly, see §14.12.1 below. +
+Executing a do statement always executes the contained Statement at least once.

+ +

14.12.1 Abrupt Completion

+ +Abrupt completion of the contained Statement is handled in the following manner:

+

    +
  • If execution of the Statement completes abruptly because of a break with no label, then no further action is taken and the do statement completes normally. + +
  • If execution of the Statement completes abruptly because of a continue with no label, then the Expression is evaluated. Then there is a choice based on the resulting value: +
      + +
    • If the value is true, then the entire do statement is executed again. + +
    • If the value is false, no further action is taken and the do statement completes normally. +
    + +
  • If execution of the Statement completes abruptly because of a continue with label L, then there is a choice: +
      + +
    • If the do statement has label L, then the Expression is evaluated. Then there is a choice: +
        + +
      • If the value of the Expression is true, then the entire do statement is executed again. + +
      • If the value of the Expression is false, no further action is taken and the do statement completes normally. +
      + +
    • If the do statement does not have label L, the do statement completes abruptly because of a continue with label L. +
    + +
  • If execution of the Statement completes abruptly for any other reason, the do statement completes abruptly for the same reason. The case of abrupt completion because of a break with a label is handled by the general rule (§14.7). +
+

14.12.2 Example of do statement

+ +The following code is one possible implementation of the toHexString method of class Integer:

+

public static String toHexString(int i) {
+	StringBuffer buf = new StringBuffer(8);
+	do {
+		buf.append(Character.forDigit(i & 0xF, 16));
+		i >>>= 4;
+	} while (i != 0);
+	return buf.reverse().toString();
+}
+
+Because at least one digit must be generated, the do statement is an appropriate control structure.

+ +

14.13 The for Statement

+ +The for statement executes some initialization code, then executes an Expression, a Statement, and some update code repeatedly until the value of the Expression is false.

+

    +ForStatement:
+	for ( ForInitopt ; Expressionopt ; ForUpdateopt )
+		Statement
+
+ForStatementNoShortIf:
+	for ( ForInitopt ; Expressionopt ; ForUpdateopt )
+		StatementNoShortIf
+
+ForInit:
+	StatementExpressionList
+	LocalVariableDeclaration
+
+ForUpdate:
+	StatementExpressionList
+
+StatementExpressionList:
+	StatementExpression
+	StatementExpressionList , StatementExpression
+
+The Expression must have type boolean, or a compile-time error occurs.

+ +

14.13.1 Initialization of for statement

+ +A for statement is executed by first executing the ForInit code:

+

    +
  • If the ForInit code is a list of statement expressions (§14.8), the expressions are evaluated in sequence from left to right; their values, if any, are discarded. If evaluation of any expression completes abruptly for some reason, the for statement completes abruptly for the same reason; any ForInit statement expressions to the right of the one that completed abruptly are not evaluated. +
+If the ForInit code is a local variable declaration, it is executed as if it were a local variable declaration statement (§14.4) appearing in a block. The scope of a local variable declared in the ForInit part of a for statement (§14.13) includes all of the following:

+

    +
  • Its own initializer + +
  • Any further declarators to the right in the ForInit part of the for statement + +
  • The Expression and ForUpdate parts of the for statement + +
  • The contained Statement +
+If execution of the local variable declaration completes abruptly for any reason, the for statement completes abruptly for the same reason.

+

    +
  • If the ForInit part is not present, no action is taken. +
+

14.13.2 Iteration of for statement

+ +Next, a for iteration step is performed, as follows:

+

    +
  • If the Expression is present, it is evaluated, and if evaluation of the Expression completes abruptly, the for statement completes abruptly for the same reason. Otherwise, there is then a choice based on the presence or absence of the Expression and the resulting value if the Expression is present: +
      + +
    • If the Expression is not present, or it is present and the value resulting from its evaluation is true, then the contained Statement is executed. Then there is a choice: +
        + +
      • If execution of the Statement completes normally, then the following two steps are performed in sequence: +
          + +
        • First, if the ForUpdate part is present, the expressions are evaluated in sequence from left to right; their values, if any, are discarded. If evaluation of any expression completes abruptly for some reason, the for statement completes abruptly for the same reason; any ForUpdate statement expressions to the right of the one that completed abruptly are not evaluated. If the ForUpdate part is not present, no action is taken. + +
        • Second, another for iteration step is performed. +
        + +
      • If execution of the Statement completes abruptly, see §14.13.3 below. +
      + +
    • If the Expression is present and the value resulting from its evaluation is false, no further action is taken and the for statement completes normally. +
    +
+If the value of the Expression is false the first time it is evaluated, then the Statement is not executed.

+ +If the Expression is not present, then the only way a for statement can complete normally is by use of a break statement.

+ +

14.13.3 Abrupt Completion of for statement

+ +Abrupt completion of the contained Statement is handled in the following manner:

+

    +
  • If execution of the Statement completes abruptly because of a break with no label, no further action is taken and the for statement completes normally. + +
  • If execution of the Statement completes abruptly because of a continue with no label, then the following two steps are performed in sequence: +
      + +
    • First, if the ForUpdate part is present, the expressions are evaluated in sequence from left to right; their values, if any, are discarded. If the ForUpdate part is not present, no action is taken. + +
    • Second, another for iteration step is performed. +
    + +
  • If execution of the Statement completes abruptly because of a continue with label L, then there is a choice: +
      + +
    • If the for statement has label L, then the following two steps are performed in sequence: +
        + +
      • First, if the ForUpdate part is present, the expressions are evaluated in sequence from left to right; their values, if any, are discarded. If the ForUpdate is not present, no action is taken. + +
      • Second, another for iteration step is performed. +
      + +
    • If the for statement does not have label L, the for statement completes abruptly because of a continue with label L. +
    + +
  • If execution of the Statement completes abruptly for any other reason, the for statement completes abruptly for the same reason. Note that the case of abrupt completion because of a break with a label is handled by the general rule for labeled statements (§14.7). +
+

14.14 The break Statement

+ +A break statement transfers control out of an enclosing statement.

+

    +BreakStatement:
+	break Identifieropt ;
+
+A break statement with no label attempts to transfer control to the innermost enclosing switch, while, do, or for statement of the immediately enclosing method or initializer block; this statement, which is called the break target, then immediately completes normally.

+ +To be precise, a break statement with no label always completes abruptly, the reason being a break with no label. If no switch, while, do, or for statement encloses the break statement, a compile-time error occurs.

+ +A break statement with label Identifier attempts to transfer control to the enclosing labeled statement (§14.7) that has the same Identifier as its label; this statement, which is called the break target, then immediately completes normally. In this case, the break target need not be a while, do, for, or switch statement. A break statement must refer to a label within the immediately enclosing method or initializer block. There are no non-local jumps.

+ +To be precise, a break statement with label Identifier always completes abruptly, the reason being a break with label Identifier. If no labeled statement with Identifier as its label encloses the break statement, a compile-time error occurs.

+ +It can be seen, then, that a break statement always completes abruptly.

+ +The preceding descriptions say "attempts to transfer control" rather than just "transfers control" because if there are any try statements (§14.19) within the break target whose try blocks contain the break statement, then any finally clauses of those try statements are executed, in order, innermost to outermost, before control is transferred to the break target. Abrupt completion of a finally clause can disrupt the transfer of control initiated by a break statement.

+ +In the following example, a mathematical graph is represented by an array of arrays. A graph consists of a set of nodes and a set of edges; each edge is an arrow that points from some node to some other node, or from a node to itself. In this example it is assumed that there are no redundant edges; that is, for any two nodes P and Q, where Q may be the same as P, there is at most one edge from P to Q. Nodes are represented by integers, and there is an edge from node i to node edges[i][j] for every i and j for which the array reference edges[i][j] does not throw an IndexOutOfBoundsException.

+ +The task of the method loseEdges, given integers i and j, is to construct a new graph by copying a given graph but omitting the edge from node i to node j, if any, and the edge from node j to node i, if any:

+

class Graph {
+	int edges[][];
+	public Graph(int[][] edges) { this.edges = edges; }
+	public Graph loseEdges(int i, int j) {
+		int n = edges.length;
+		int[][] newedges = new int[n][];
+		for (int k = 0; k < n; ++k) {
+			edgelist: {
+				int z;
+				search: {
+					if (k == i) {
+						for (z = 0; z < edges[k].length; ++z)
+							if (edges[k][z] == j)
+								break search;
+					} else if (k == j) {
+						for (z = 0; z < edges[k].length; ++z)
+							if (edges[k][z] == i)
+								break search;
+					}
+					// No edge to be deleted; share this list.
+					newedges[k] = edges[k];
+					break edgelist;
+				} //search
+				// Copy the list, omitting the edge at position z.
+				int m = edges[k].length - 1;
+				int ne[] = new int[m];
+				System.arraycopy(edges[k], 0, ne, 0, z);
+				System.arraycopy(edges[k], z+1, ne, z, m-z);
+				newedges[k] = ne;
+			} //edgelist
+		}
+		return new Graph(newedges);
+	}
+}
+
+Note the use of two statement labels, edgelist and search, and the use of break statements. This allows the code that copies a list, omitting one edge, to be shared between two separate tests, the test for an edge from node i to node j, and the test for an edge from node j to node i.

+ +

14.15 The continue Statement

+ +A continue statement may occur only in a while, do, or for statement; statements of these three kinds are called iteration statements. Control passes to the loop-continuation point of an iteration statement.

+

    +ContinueStatement:
+	continue Identifieropt ;
+
+A continue statement with no label attempts to transfer control to the innermost enclosing while, do, or for statement of the immediately enclosing method or initializer block; this statement, which is called the continue target, then immediately ends the current iteration and begins a new one.

+ +To be precise, such a continue statement always completes abruptly, the reason being a continue with no label. If no while, do, or for statement of the immediately enclosing method or initializer block encloses the continue statement, a compile-time error occurs.

+ +A continue statement with label Identifier attempts to transfer control to the enclosing labeled statement (§14.7) that has the same Identifier as its label; that statement, which is called the continue target, then immediately ends the current iteration and begins a new one. The continue target must be a while, do, or for statement or a compile-time error occurs. A continue statement must refer to a label within the immediately enclosing method or initializer block. There are no non-local jumps.

+ +More precisely, a continue statement with label Identifier always completes abruptly, the reason being a continue with label Identifier. If no labeled statement with Identifier as its label contains the continue statement, a compile-time error occurs.

+ +It can be seen, then, that a continue statement always completes abruptly.

+ +See the descriptions of the while statement (§14.11), do statement (§14.12), and for statement (§14.13) for a discussion of the handling of abrupt termination because of continue.

+ +The preceding descriptions say "attempts to transfer control" rather than just "transfers control" because if there are any try statements (§14.19) within the continue target whose try blocks contain the continue statement, then any finally clauses of those try statements are executed, in order, innermost to outermost, before control is transferred to the continue target. Abrupt completion of a finally clause can disrupt the transfer of control initiated by a continue statement.

+ +In the Graph example in the preceding section, one of the break statements is used to finish execution of the entire body of the outermost for loop. This break can be replaced by a continue if the for loop itself is labeled:

+

class Graph {
+	. . .
+	public Graph loseEdges(int i, int j) {
+		int n = edges.length;
+		int[][] newedges = new int[n][];
+		edgelists: for (int k = 0; k < n; ++k) {
+			int z;
+			search: {
+				if (k == i) {
+					. . .
+				} else if (k == j) {
+					. . .
+				}
+				newedges[k] = edges[k];
+				continue edgelists;
+			} // search
+			. . .
+		} // edgelists
+		return new Graph(newedges);
+	}
+}
+
+Which to use, if either, is largely a matter of programming style.

+ +

14.16 The return Statement

+ +A return statement returns control to the invoker of a method (§8.4, §15.12) or constructor (§8.8, §15.9).

+

    +ReturnStatement:
+	return Expressionopt ;
+
+A return statement with no Expression must be contained in the body of a method that is declared, using the keyword void, not to return any value (§8.4), or in the body of a constructor (§8.8). A compile-time error occurs if a return statement appears within an instance initializer or a static initializer (§8.7). A return statement with no Expression attempts to transfer control to the invoker of the method or constructor that contains it.

+ +To be precise, a return statement with no Expression always completes abruptly, the reason being a return with no value.

+ +A return statement with an Expression must be contained in a method declaration that is declared to return a value (§8.4) or a compile-time error occurs. The Expression must denote a variable or value of some type T, or a compile-time error occurs. The type T must be assignable (§5.2) to the declared result type of the method, or a compile-time error occurs.

+ +A return statement with an Expression attempts to transfer control to the invoker of the method that contains it; the value of the Expression becomes the value of the method invocation. More precisely, execution of such a return statement first evaluates the Expression. If the evaluation of the Expression completes abruptly for some reason, then the return statement completes abruptly for that reason. If evaluation of the Expression completes normally, producing a value V, then the return statement completes abruptly, the reason being a return with value V. If the expression is of type float and is not FP-strict (§15.4), then the value may be an element of either the float value set or the float-extended-exponent value set (§4.2.3). If the expression is of type double and is not FP-strict, then the value may be an element of either the double value set or the double-extended-exponent value set.

+ +It can be seen, then, that a return statement always completes abruptly.

+ +The preceding descriptions say "attempts to transfer control" rather than just "transfers control" because if there are any try statements (§14.19) within the method or constructor whose try blocks contain the return statement, then any finally clauses of those try statements will be executed, in order, innermost to outermost, before control is transferred to the invoker of the method or constructor. Abrupt completion of a finally clause can disrupt the transfer of control initiated by a return statement.

+ +

14.17 The throw Statement

+ +A throw statement causes an exception (§11) to be thrown. The result is an immediate transfer of control (§11.3) that may exit multiple statements and multiple constructor, instance initializer, static initializer and field initializer evaluations, and method invocations until a try statement (§14.19) is found that catches the thrown value. If no such try statement is found, then execution of the thread (§17) that executed the throw is terminated (§11.3) after invocation of the uncaughtException method for the thread group to which the thread belongs.

+

    +ThrowStatement:
+	throw Expression ;
+
+The Expression in a throw statement must denote a variable or value of a reference type which is assignable (§5.2) to the type Throwable, or a compile-time error occurs. Moreover, at least one of the following three conditions must be true, or a compile-time error occurs:

+

    +
  • The exception is not a checked exception (§11.2)-specifically, one of the following situations is true: +
      + +
    • The type of the Expression is the class RuntimeException or a subclass of RuntimeException. + +
    • The type of the Expression is the class Error or a subclass of Error. +
    + +
  • The throw statement is contained in the try block of a try statement (§14.19) and the type of the Expression is assignable (§5.2) to the type of the parameter of at least one catch clause of the try statement. (In this case we say the thrown value is caught by the try statement.) + +
  • The throw statement is contained in a method or constructor declaration and the type of the Expression is assignable (§5.2) to at least one type listed in the throws clause (§8.4.4, §8.8.4) of the declaration. +
+A throw statement first evaluates the Expression. If the evaluation of the Expression completes abruptly for some reason, then the throw completes abruptly for that reason. If evaluation of the Expression completes normally, producing a non-null value V, then the throw statement completes abruptly, the reason being a throw with value V. If evaluation of the Expression completes normally, producing a null value, then an instance V' of class NullPointerException is created and thrown instead of null. The throw statement then completes abruptly, the reason being a throw with value V'.

+ +It can be seen, then, that a throw statement always completes abruptly.

+ +If there are any enclosing try statements (§14.19) whose try blocks contain the throw statement, then any finally clauses of those try statements are executed as control is transferred outward, until the thrown value is caught. Note that abrupt completion of a finally clause can disrupt the transfer of control initiated by a throw statement.

+ +If a throw statement is contained in a method declaration, but its value is not caught by some try statement that contains it, then the invocation of the method completes abruptly because of the throw.

+ +If a throw statement is contained in a constructor declaration, but its value is not caught by some try statement that contains it, then the class instance creation expression that invoked the constructor will complete abruptly because of the throw.

+ +If a throw statement is contained in a static initializer (§8.7), then a compile-time check ensures that either its value is always an unchecked exception or its value is always caught by some try statement that contains it. If at run-time, despite this check, the value is not caught by some try statement that contains the throw statement, then the value is rethrown if it is an instance of class Error or one of its subclasses; otherwise, it is wrapped in an ExceptionInInitializerError object, which is then thrown (§12.4.2).

+ +If a throw statement is contained in an instance initializer (§8.6), then a compile-time check ensures that either its value is always an unchecked exception or its value is always caught by some try statement that contains it, or the type of the thrown exception (or one of its superclasses) occurs in the throws clause of every constructor of the class.

+ +By convention, user-declared throwable types should usually be declared to be subclasses of class Exception, which is a subclass of class Throwable (§11.5).

+ +

14.18 The synchronized Statement

+ +A synchronized statement acquires a mutual-exclusion lock (§17.13) on behalf of the executing thread, executes a block, then releases the lock. While the executing thread owns the lock, no other thread may acquire the lock.

+

    +SynchronizedStatement:
+	synchronized ( Expression ) Block
+
+The type of Expression must be a reference type, or a compile-time error occurs.

+ +A synchronized statement is executed by first evaluating the Expression.

+ +If evaluation of the Expression completes abruptly for some reason, then the synchronized statement completes abruptly for the same reason.

+ +Otherwise, if the value of the Expression is null, a NullPointerException is thrown.

+ +Otherwise, let the non-null value of the Expression be V. The executing thread locks the lock associated with V. Then the Block is executed. If execution of the Block completes normally, then the lock is unlocked and the synchronized statement completes normally. If execution of the Block completes abruptly for any reason, then the lock is unlocked and the synchronized statement then completes abruptly for the same reason.

+ +Acquiring the lock associated with an object does not of itself prevent other threads from accessing fields of the object or invoking unsynchronized methods on the object. Other threads can also use synchronized methods or the synchronized statement in a conventional manner to achieve mutual exclusion.

+ +The locks acquired by synchronized statements are the same as the locks that are acquired implicitly by synchronized methods; see §8.4.3.6. A single thread may hold a lock more than once.

+ +The example:

+

class Test {
+	public static void main(String[] args) {
+		Test t = new Test();
+		synchronized(t) {
+			synchronized(t) {
+				System.out.println("made it!");
+			}
+		}
+	}
+}
+
+prints:

+

made it!
+
+This example would deadlock if a single thread were not permitted to lock a lock more than once.

+ +

14.19 The try statement

+ +A try statement executes a block. If a value is thrown and the try statement has one or more catch clauses that can catch it, then control will be transferred to the first such catch clause. If the try statement has a finally clause, then another block of code is executed, no matter whether the try block completes normally or abruptly, and no matter whether a catch clause is first given control.

+

    +TryStatement:
+	try Block Catches
+	try Block Catchesopt Finally
+
+Catches:
+	CatchClause
+	Catches CatchClause
+
+CatchClause:
+	catch ( FormalParameter ) Block
+
+Finally:
+	finally Block
+
+The following is repeated from §8.4.1 to make the presentation here clearer:

+

    +FormalParameter:
+	finalopt Type VariableDeclaratorId
+
+The following is repeated from §8.3 to make the presentation here clearer:

+

    +VariableDeclaratorId:
+	Identifier
+	VariableDeclaratorId [ ]
+
+The Block immediately after the keyword try is called the try block of the try statement. The Block immediately after the keyword finally is called the finally block of the try statement.

+ +A try statement may have catch clauses (also called exception handlers). A catch clause must have exactly one parameter (which is called an exception parameter); the declared type of the exception parameter must be the class Throwable or a subclass of Throwable, or a compile-time error occurs. The scope of the parameter variable is the Block of the catch clause.

+ +An exception parameter of a catch clause must not have the same name as a local variable or parameter of the method or initializer block immediately enclosing the catch clause, or a compile-time error occurs.

+ +The scope of a parameter of an exception handler that is declared in a catch clause of a try statement (§14.19) is the entire block associated with the catch.

+ +Within the Block of the catch clause, the name of the parameter may not be redeclared as a local variable of the directly enclosing method or initializer block, nor may it be redeclared as an exception parameter of a catch clause in a try statement of the directly enclosing method or initializer block, or a compile-time error occurs. However, an exception parameter may be shadowed (§6.3.1) anywhere inside a class declaration nested within the Block of the catch clause.

+ +It is a compile-time error if an exception parameter that is declared final is assigned to within the body of the catch clause.

+ +Exception parameters cannot be referred to using qualified names (§6.6), only by simple names.

+ +Exception handlers are considered in left-to-right order: the earliest possible catch clause accepts the exception, receiving as its actual argument the thrown exception object.

+ +A finally clause ensures that the finally block is executed after the try block and any catch block that might be executed, no matter how control leaves the try block or catch block.

+ +Handling of the finally block is rather complex, so the two cases of a try statement with and without a finally block are described separately.

+ +

14.19.1 Execution of try-catch

+ +A try statement without a finally block is executed by first executing the try block. Then there is a choice:

+

    +
  • If execution of the try block completes normally, then no further action is taken and the try statement completes normally. + +
  • If execution of the try block completes abruptly because of a throw of a value V, then there is a choice: +
      + +
    • If the run-time type of V is assignable (§5.2) to the Parameter of any catch clause of the try statement, then the first (leftmost) such catch clause is selected. The value V is assigned to the parameter of the selected catch clause, and the Block of that catch clause is executed. If that block completes normally, then the try statement completes normally; if that block completes abruptly for any reason, then the try statement completes abruptly for the same reason. + +
    • If the run-time type of V is not assignable to the parameter of any catch clause of the try statement, then the try statement completes abruptly because of a throw of the value V. +
    + +
  • If execution of the try block completes abruptly for any other reason, then the try statement completes abruptly for the same reason. + +In the example:

    +

class BlewIt extends Exception {
+	BlewIt() { }
+	BlewIt(String s) { super(s); }
+}
+class Test {
+	static void blowUp() throws BlewIt { throw new BlewIt(); }
+	public static void main(String[] args) {
+		try {
+			blowUp();
+		} catch (RuntimeException r) {
+			System.out.println("RuntimeException:" + r);
+		} catch (BlewIt b) {
+			System.out.println("BlewIt");
+		}
+	}
+}
+
+the exception BlewIt is thrown by the method blowUp. The try-catch statement in the body of main has two catch clauses. The run-time type of the exception is BlewIt which is not assignable to a variable of type RuntimeException, but is assignable to a variable of type BlewIt, so the output of the example is:

+

BlewIt
+
+

14.19.2 Execution of try-catch-finally

+ +A try statement with a finally block is executed by first executing the try block. Then there is a choice:

+

    +
  • If execution of the try block completes normally, then the finally block is executed, and then there is a choice: +
      + +
    • If the finally block completes normally, then the try statement completes normally. + +
    • If the finally block completes abruptly for reason S, then the try statement completes abruptly for reason S. +
    + +
  • If execution of the try block completes abruptly because of a throw of a value V, then there is a choice: +
      + +
    • If the run-time type of V is assignable to the parameter of any catch clause of the try statement, then the first (leftmost) such catch clause is selected. The value V is assigned to the parameter of the selected catch clause, and the Block of that catch clause is executed. Then there is a choice: +
        + +
      • If the catch block completes normally, then the finally block is executed. Then there is a choice: +
          + +
        • If the finally block completes normally, then the try statement completes normally. + +
        • If the finally block completes abruptly for any reason, then the try statement completes abruptly for the same reason. +
        + +
      • If the catch block completes abruptly for reason R, then the finally block is executed. Then there is a choice: +
          + +
        • If the finally block completes normally, then the try statement completes abruptly for reason R. + +
        • If the finally block completes abruptly for reason S, then the try statement completes abruptly for reason S (and reason R is discarded). +
        +
      + +
    • If the run-time type of V is not assignable to the parameter of any catch clause of the try statement, then the finally block is executed. Then there is a choice: +
        + +
      • If the finally block completes normally, then the try statement completes abruptly because of a throw of the value V. + +
      • If the finally block completes abruptly for reason S, then the try statement completes abruptly for reason S (and the throw of value V is discarded and forgotten). +
      +
    + +
  • If execution of the try block completes abruptly for any other reason R, then the finally block is executed. Then there is a choice: +
      + +
    • If the finally block completes normally, then the try statement completes abruptly for reason R. + +
    • If the finally block completes abruptly for reason S, then the try statement completes abruptly for reason S (and reason R is discarded). + +The example:

      +

    +
class BlewIt extends Exception {
+	BlewIt() { }
+	BlewIt(String s) { super(s); }
+}
+class Test {
+	static void blowUp() throws BlewIt {
+		throw new NullPointerException();
+	}
+	public static void main(String[] args) {
+		try {
+			blowUp();
+		} catch (BlewIt b) {
+			System.out.println("BlewIt");
+		} finally {
+			System.out.println("Uncaught Exception");
+		}
+	}
+}
+
+produces the output:

+

Uncaught Exception
+java.lang.NullPointerException
+	at Test.blowUp(Test.java:7)
+	at Test.main(Test.java:11)
+
+The NullPointerException (which is a kind of RuntimeException) that is thrown by method blowUp is not caught by the try statement in main, because a NullPointerException is not assignable to a variable of type BlewIt. This causes the finally clause to execute, after which the thread executing main, which is the only thread of the test program, terminates because of an uncaught exception, which typically results in printing the exception name and a simple backtrace.

+ +

14.20 Unreachable Statements

+ +It is a compile-time error if a statement cannot be executed because it is unreachable. Every Java compiler must carry out the conservative flow analysis specified here to make sure all statements are reachable.

+ +This section is devoted to a precise explanation of the word "reachable." The idea is that there must be some possible execution path from the beginning of the constructor, method, instance initializer or static initializer that contains the statement to the statement itself. The analysis takes into account the structure of statements. Except for the special treatment of while, do, and for statements whose condition expression has the constant value true, the values of expressions are not taken into account in the flow analysis.

+ +For example, a Java compiler will accept the code:

+

{
+	int n = 5;
+	while (n > 7) k = 2;
+}
+
+even though the value of n is known at compile time and in principle it can be known at compile time that the assignment to k can never be executed.

+ +A Java compiler must operate according to the rules laid out in this section.

+ +The rules in this section define two technical terms:

+

    +
  • whether a statement is reachable + +
  • whether a statement can complete normally +
+The definitions here allow a statement to complete normally only if it is reachable.

+ +To shorten the description of the rules, the customary abbreviation "iff" is used to mean "if and only if."

+ +The rules are as follows:

+

    +
  • The block that is the body of a constructor, method, instance initializer or static initializer is reachable. + +
  • An empty block that is not a switch block can complete normally iff it is reachable. A nonempty block that is not a switch block can complete normally iff the last statement in it can complete normally. The first statement in a nonempty block that is not a switch block is reachable iff the block is reachable. Every other statement S in a nonempty block that is not a switch block is reachable iff the statement preceding S can complete normally. + +
  • A local class declaration statement can complete normally iff it is reachable. + +
  • A local variable declaration statement can complete normally iff it is reachable. + +
  • An empty statement can complete normally iff it is reachable. + +
  • A labeled statement can complete normally if at least one of the following is true: +
      + +
    • The contained statement can complete normally. + +
    • There is a reachable break statement that exits the labeled statement. +
    +
    +The contained statement is reachable iff the labeled statement is reachable. +
    +
  • An expression statement can complete normally iff it is reachable. + +
  • The if statement, whether or not it has an else part, is handled in an unusual manner. For this reason, it is discussed separately at the end of this section. + +
  • A switch statement can complete normally iff at least one of the following is true: +
      + +
    • The last statement in the switch block can complete normally. + +
    • The switch block is empty or contains only switch labels. + +
    • There is at least one switch label after the last switch block statement group. + +
    • The switch block does not contain a default label. + +
    • There is a reachable break statement that exits the switch statement. +
    + +
  • A switch block is reachable iff its switch statement is reachable. + +
  • A statement in a switch block is reachable iff its switch statement is reachable and at least one of the following is true: +
      + +
    • It bears a case or default label. + +
    • There is a statement preceding it in the switch block and that preceding statement can complete normally. +
    + +
  • A while statement can complete normally iff at least one of the following is true: +
      + +
    • The while statement is reachable and the condition expression is not a constant expression with value true. + +
    • There is a reachable break statement that exits the while statement. +
    +
    +The contained statement is reachable iff the while statement is reachable and the condition expression is not a constant expression whose value is false. +
    +
  • A do statement can complete normally iff at least one of the following is true: +
      + +
    • The contained statement can complete normally and the condition expression is not a constant expression with value true. + +
    • The do statement contains a reachable continue statement with no label, and the do statement is the innermost while, do, or for statement that contains that continue statement, and the condition expression is not a constant expression with value true. + +
    • The do statement contains a reachable continue statement with a label L, and the do statement has label L, and the condition expression is not a constant expression with value true. + +
    • There is a reachable break statement that exits the do statement. +
    +
    +The contained statement is reachable iff the do statement is reachable. +
    +
  • A for statement can complete normally iff at least one of the following is true: +
      + +
    • The for statement is reachable, there is a condition expression, and the condition expression is not a constant expression with value true. + +
    • There is a reachable break statement that exits the for statement. +
    +
    +The contained statement is reachable iff the for statement is reachable and the condition expression is not a constant expression whose value is false. +
    +
  • A break, continue, return, or throw statement cannot complete normally. + +
  • A synchronized statement can complete normally iff the contained statement can complete normally. The contained statement is reachable iff the synchronized statement is reachable. + +
  • A try statement can complete normally iff both of the following are true: +
      + +
    • The try block can complete normally or any catch block can complete normally. + +
    • If the try statement has a finally block, then the finally block can complete normally. +
    + +
  • The try block is reachable iff the try statement is reachable. + +
  • A catch block C is reachable iff both of the following are true: +
      + +
    • Some expression or throw statement in the try block is reachable and can throw an exception whose type is assignable to the parameter of the catch clause C. (An expression is considered reachable iff the innermost statement containing it is reachable.) + +
    • There is no earlier catch block A in the try statement such that the type of C's parameter is the same as or a subclass of the type of A's parameter. +
    + +
  • If a finally block is present, it is reachable iff the try statement is reachable. + +One might expect the if statement to be handled in the following manner, but these are not the rules that the Java programming language actually uses:

    + +

  • HYPOTHETICAL: An if-then statement can complete normally iff at least one of the following is true: +
      + +
    • The if-then statement is reachable and the condition expression is not a constant expression whose value is true. + +
    • The then-statement can complete normally. +
    +
+The then-statement is reachable iff the if-then statement is reachable and the condition expression is not a constant expression whose value is false.

+

    +
  • HYPOTHETICAL: An if-then-else statement can complete normally iff the then-statement can complete normally or the else-statement can complete normally. The then-statement is reachable iff the if-then-else statement is reachable and the condition expression is not a constant expression whose value is false. The else statement is reachable iff the if-then-else statement is reachable and the condition expression is not a constant expression whose value is true. + +This approach would be consistent with the treatment of other control structures. However, in order to allow the if statement to be used conveniently for "conditional compilation" purposes, the actual rules differ.

    +

+The actual rules for the if statement are as follows:

+

    +
  • ACTUAL: An if-then statement can complete normally iff it is reachable. The then-statement is reachable iff the if-then statement is reachable. + +
  • ACTUAL: An if-then-else statement can complete normally iff the then-statement can complete normally or the else-statement can complete normally. The then-statement is reachable iff the if-then-else statement is reachable. The else-statement is reachable iff the if-then-else statement is reachable. + +As an example, the following statement results in a compile-time error:

    +

while (false) { x=3; }
+
+because the statement x=3; is not reachable; but the superficially similar case:

+

if (false) { x=3; }
+
+does not result in a compile-time error. An optimizing compiler may realize that the statement x=3; will never be executed and may choose to omit the code for that statement from the generated class file, but the statement x=3; is not regarded as "unreachable" in the technical sense specified here.

+ +The rationale for this differing treatment is to allow programmers to define "flag variables" such as:

+

static final boolean DEBUG = false;
+
+and then write code such as:

+

if (DEBUG) { x=3; }
+
+The idea is that it should be possible to change the value of DEBUG from false to true or from true to false and then compile the code correctly with no other changes to the program text.

+ +This ability to "conditionally compile" has a significant impact on, and relationship to, binary compatibility (§13). If a set of classes that use such a "flag" variable are compiled and conditional code is omitted, it does not suffice later to distribute just a new version of the class or interface that contains the definition of the flag. A change to the value of a flag is, therefore, not binary compatible with preexisting binaries (§13.4.8). (There are other reasons for such incompatibility as well, such as the use of constants in case labels in switch statements; see §13.4.8.)

+ + +

+ + + +
Contents | Prev | Next | IndexJava Language Specification
+Second Edition
+Copyright © 2000 Sun Microsystems, Inc. +All rights reserved +
+Please send any comments or corrections via our feedback form + + + + + Expressions + + + + + + + + +
Contents | Prev | Next | IndexJava Language Specification
+Second Edition
+

+ + +

+CHAPTER + 15

+ +

Expressions

+

+ +Much of the work in a program is done by evaluating expressions, either for their side effects, such as assignments to variables, or for their values, which can be used as arguments or operands in larger expressions, or to affect the execution sequence in statements, or both.

+ +This chapter specifies the meanings of expressions and the rules for their evaluation.

+ +

15.1 Evaluation, Denotation, and Result

+ +When an expression in a program is evaluated (executed), the result denotes one of three things:

+

    +
  • A variable (§4.5) (in C, this would be called an lvalue) + +
  • A value (§4.2, §4.3) + +
  • Nothing (the expression is said to be void) +
+Evaluation of an expression can also produce side effects, because expressions may contain embedded assignments, increment operators, decrement operators, and method invocations.

+ +An expression denotes nothing if and only if it is a method invocation (§15.12) that invokes a method that does not return a value, that is, a method declared void (§8.4). Such an expression can be used only as an expression statement (§14.8), because every other context in which an expression can appear requires the expression to denote something. An expression statement that is a method invocation may also invoke a method that produces a result; in this case the value returned by the method is quietly discarded.

+ +Value set conversion (§5.1.8) is applied to the result of every expression that produces a value.

+ +Each expression occurs in the declaration of some (class or interface) type that is being declared: in a field initializer, in a static initializer, in a constructor declaration, or in the code for a method.

+ +

15.2 Variables as Values

+ +If an expression denotes a variable, and a value is required for use in further evaluation, then the value of that variable is used. In this context, if the expression denotes a variable or a value, we may speak simply of the value of the expression.

+ +If the value of a variable of type float or double is used in this manner, then value set conversion (§5.1.8) is applied to the value of the variable.

+ +

15.3 Type of an Expression

+ +If an expression denotes a variable or a value, then the expression has a type known at compile time. The rules for determining the type of an expression are explained separately below for each kind of expression.

+ +The value of an expression is always assignment compatible (§5.2) with the type of the expression, just as the value stored in a variable is always compatible with the type of the variable.

+ +In other words, the value of an expression whose type is T is always suitable for assignment to a variable of type T.

+ +Note that an expression whose type is a class type F that is declared final is guaranteed to have a value that is either a null reference or an object whose class is F itself, because final types have no subclasses.

+ +

15.4 FP-strict Expressions

+ +If the type of an expression is float or double, then there is a question as to what value set (§4.2.3) the value of the expression is drawn from. This is governed by the rules of value set conversion (§5.1.8); these rules in turn depend on whether or not the expression is FP-strict.

+ +Every compile-time constant expression (§15.28) is FP-strict. If an expression is not a compile-time constant expression, then consider all the class declarations, interface declarations, and method declarations that contain the expression. If any such declaration bears the strictfp modifier, then the expression is FP-strict.

+ +If a class, interface, or method, X, is declared strictfp, then X and any class, interface, method, constructor, instance initializer, static initializer or variable initializer within X is said to be FP-strict.

+ +It follows that an expression is not FP-strict if and only if it is not a compile-time constant expression and it does not appear within any declaration that has the strictfp modifier.

+ +Within an FP-strict expression, all intermediate values must be elements of the float value set or the double value set, implying that the results of all FP-strict expressions must be those predicted by IEEE 754 arithmetic on operands represented using single and double formats. Within an expression that is not FP-strict, some leeway is granted for an implementation to use an extended exponent range to represent intermediate results; the net effect, roughly speaking, is that a calculation might produce "the correct answer" in situations where exclusive use of the float value set or double value set might result in overflow or underflow.

+ +

15.5 Expressions and Run-Time Checks

+ +If the type of an expression is a primitive type, then the value of the expression is of that same primitive type. But if the type of an expression is a reference type, then the class of the referenced object, or even whether the value is a reference to an object rather than null, is not necessarily known at compile time. There are a few places in the Java programming language where the actual class of a referenced object affects program execution in a manner that cannot be deduced from the type of the expression. They are as follows:

+

    +
  • Method invocation (§15.12). The particular method used for an invocation o.m(...) is chosen based on the methods that are part of the class or interface that is the type of o. For instance methods, the class of the object referenced by the run-time value of o participates because a subclass may override a specific method already declared in a parent class so that this overriding method is invoked. (The overriding method may or may not choose to further invoke the original overridden m method.) + +
  • The instanceof operator (§15.20.2). An expression whose type is a reference type may be tested using instanceof to find out whether the class of the object referenced by the run-time value of the expression is assignment compatible (§5.2) with some other reference type. + +
  • Casting (§5.5, §15.16). The class of the object referenced by the run-time value of the operand expression might not be compatible with the type specified by the cast. For reference types, this may require a run-time check that throws an exception if the class of the referenced object, as determined at run time, is not assignment compatible (§5.2) with the target type. + +
  • Assignment to an array component of reference type (§10.10, §15.13, §15.26.1). The type-checking rules allow the array type S[] to be treated as a subtype of T[] if S is a subtype of T, but this requires a run-time check for assignment to an array component, similar to the check performed for a cast. + +
  • Exception handling (§14.19). An exception is caught by a catch clause only if the class of the thrown exception object is an instanceof the type of the formal parameter of the catch clause. +
+The first two of the cases just listed ought never to result in detecting a type error. Thus, a run-time type error can occur only in these situations:

+

    +
  • In a cast, when the actual class of the object referenced by the value of the operand expression is not compatible with the target type specified by the cast operator (§5.5, §15.16); in this case a ClassCastException is thrown. + +
  • In an assignment to an array component of reference type, when the actual class of the object referenced by the value to be assigned is not compatible with the actual run-time component type of the array (§10.10, §15.13, §15.26.1); in this case an ArrayStoreException is thrown. + +
  • When an exception is not caught by any catch handler (§11.3); in this case the thread of control that encountered the exception first invokes the method uncaughtException for its thread group and then terminates. +
+

15.6 Normal and Abrupt Completion of Evaluation

+ +Every expression has a normal mode of evaluation in which certain computational steps are carried out. The following sections describe the normal mode of evaluation for each kind of expression. If all the steps are carried out without an exception being thrown, the expression is said to complete normally.

+ +If, however, evaluation of an expression throws an exception, then the expression is said to complete abruptly. An abrupt completion always has an associated reason, which is always a throw with a given value.

+ +Run-time exceptions are thrown by the predefined operators as follows:

+

    +
  • A class instance creation expression (§15.9), array creation expression (§15.10), or string concatenation operatior expression (§15.18.1) throws an OutOfMemoryError if there is insufficient memory available. + +
  • An array creation expression throws a NegativeArraySizeException if the value of any dimension expression is less than zero (§15.10). + +
  • A field access (§15.11) throws a NullPointerException if the value of the object reference expression is null. + +
  • A method invocation expression (§15.12) that invokes an instance method throws a NullPointerException if the target reference is null. + +
  • An array access (§15.13) throws a NullPointerException if the value of the array reference expression is null. + +
  • An array access (§15.13) throws an ArrayIndexOutOfBoundsException if the value of the array index expression is negative or greater than or equal to the length of the array. + +
  • A cast (§15.16) throws a ClassCastException if a cast is found to be impermissible at run time. + +
  • An integer division (§15.17.2) or integer remainder (§15.17.3) operator throws an ArithmeticException if the value of the right-hand operand expression is zero. + +
  • An assignment to an array component of reference type (§15.26.1) throws an ArrayStoreException when the value to be assigned is not compatible with the component type of the array. +
+A method invocation expression can also result in an exception being thrown if an exception occurs that causes execution of the method body to complete abruptly. A class instance creation expression can also result in an exception being thrown if an exception occurs that causes execution of the constructor to complete abruptly. Various linkage and virtual machine errors may also occur during the evaluation of an expression. By their nature, such errors are difficult to predict and difficult to handle.

+ +If an exception occurs, then evaluation of one or more expressions may be terminated before all steps of their normal mode of evaluation are complete; such expressions are said to complete abruptly. The terms "complete normally" and "complete abruptly" are also applied to the execution of statements (§14.1). A statement may complete abruptly for a variety of reasons, not just because an exception is thrown.

+ +If evaluation of an expression requires evaluation of a subexpression, abrupt completion of the subexpression always causes the immediate abrupt completion of the expression itself, with the same reason, and all succeeding steps in the normal mode of evaluation are not performed.

+ +

15.7 Evaluation Order

+ +The Java programming language guarantees that the operands of operators appear to be evaluated in a specific evaluation order, namely, from left to right.

+ +It is recommended that code not rely crucially on this specification. Code is usually clearer when each expression contains at most one side effect, as its outermost operation, and when code does not depend on exactly which exception arises as a consequence of the left-to-right evaluation of expressions.

+ +

15.7.1 Evaluate Left-Hand Operand First

+ +The left-hand operand of a binary operator appears to be fully evaluated before any part of the right-hand operand is evaluated. For example, if the left-hand operand contains an assignment to a variable and the right-hand operand contains a reference to that same variable, then the value produced by the reference will reflect the fact that the assignment occurred first.

+ +Thus:

+

class Test {
+	public static void main(String[] args) {
+		int i = 2;
+		int j = (i=3) * i;
+		System.out.println(j);
+	}
+}
+
+prints:

+

9
+
+It is not permitted for it to print 6 instead of 9.

+ +If the operator is a compound-assignment operator (§15.26.2), then evaluation of the left-hand operand includes both remembering the variable that the left-hand operand denotes and fetching and saving that variable's value for use in the implied combining operation. So, for example, the test program:

+

class Test {
+	public static void main(String[] args) {
+		int a = 9;
+		a += (a = 3);									// first example
+		System.out.println(a);
+		int b = 9;
+		b = b + (b = 3);									// second example
+		System.out.println(b);
+	}
+}
+
+prints:

+

12
+12
+
+because the two assignment statements both fetch and remember the value of the left-hand operand, which is 9, before the right-hand operand of the addition is evaluated, thereby setting the variable to 3. It is not permitted for either example to produce the result 6. Note that both of these examples have unspecified behavior in C, according to the ANSI/ISO standard.

+ +If evaluation of the left-hand operand of a binary operator completes abruptly, no part of the right-hand operand appears to have been evaluated.

+ +Thus, the test program:

+

class Test {
+	public static void main(String[] args) {
+		int j = 1;
+		try {
+			int i = forgetIt() / (j = 2);
+		} catch (Exception e) {
+			System.out.println(e);
+			System.out.println("Now j = " + j);
+		}
+	}
+	static int forgetIt() throws Exception {
+		throw new Exception("I'm outta here!");
+	}
+}
+
+prints:

+

java.lang.Exception: I'm outta here!
+Now j = 1
+
+That is, the left-hand operand forgetIt() of the operator / throws an exception before the right-hand operand is evaluated and its embedded assignment of 2 to j occurs.

+ +

15.7.2 Evaluate Operands before Operation

+ +The Java programming language also guarantees that every operand of an operator (except the conditional operators &&, ||, and ? :) appears to be fully evaluated before any part of the operation itself is performed.

+ +If the binary operator is an integer division / (§15.17.2) or integer remainder % (§15.17.3), then its execution may raise an ArithmeticException, but this exception is thrown only after both operands of the binary operator have been evaluated and only if these evaluations completed normally.

+ +So, for example, the program:

+

class Test {
+	public static void main(String[] args) {
+		int divisor = 0;
+		try {
+			int i = 1 / (divisor * loseBig());
+		} catch (Exception e) {
+			System.out.println(e);
+		}
+	}
+	static int loseBig() throws Exception {
+		throw new Exception("Shuffle off to Buffalo!");
+	}
+}
+
+always prints:

+

java.lang.Exception: Shuffle off to Buffalo!
+
+and not:

+

java.lang.ArithmeticException: / by zero
+
+since no part of the division operation, including signaling of a divide-by-zero exception, may appear to occur before the invocation of loseBig completes, even though the implementation may be able to detect or infer that the division operation would certainly result in a divide-by-zero exception.

+ +

15.7.3 Evaluation Respects Parentheses and Precedence

+ +Java programming language implementations must respect the order of evaluation as indicated explicitly by parentheses and implicitly by operator precedence. An implementation may not take advantage of algebraic identities such as the associative law to rewrite expressions into a more convenient computational order unless it can be proven that the replacement expression is equivalent in value and in its observable side effects, even in the presence of multiple threads of execution (using the thread execution model in §17), for all possible computational values that might be involved.

+ +In the case of floating-point calculations, this rule applies also for infinity and not-a-number (NaN) values. For example, !(x<y) may not be rewritten as x>=y, because these expressions have different values if either x or y is NaN or both are NaN.

+ +Specifically, floating-point calculations that appear to be mathematically associative are unlikely to be computationally associative. Such computations must not be naively reordered.

+ +For example, it is not correct for a Java compiler to rewrite 4.0*x*0.5 as 2.0*x; while roundoff happens not to be an issue here, there are large values of x for which the first expression produces infinity (because of overflow) but the second expression produces a finite result.

+ +So, for example, the test program:

+

strictfp class Test {
+	public static void main(String[] args) {
+		double d = 8e+307;
+		System.out.println(4.0 * d * 0.5);
+		System.out.println(2.0 * d);
+	}
+}
+
+prints:

+

Infinity
+1.6e+308
+
+because the first expression overflows and the second does not.

+ +In contrast, integer addition and multiplication are provably associative in the Java programming language.

+ +For example a+b+c, where a, b, and c are local variables (this simplifying assumption avoids issues involving multiple threads and volatile variables), will always produce the same answer whether evaluated as (a+b)+c or a+(b+c); if the expression b+c occurs nearby in the code, a smart compiler may be able to use this common subexpression.

+ +

15.7.4 Argument Lists are Evaluated Left-to-Right

+ +In a method or constructor invocation or class instance creation expression, argument expressions may appear within the parentheses, separated by commas. Each argument expression appears to be fully evaluated before any part of any argument expression to its right.

+ +Thus:

+

class Test {
+	public static void main(String[] args) {
+		String s = "going, ";
+		print3(s, s, s = "gone");
+	}
+	static void print3(String a, String b, String c) {
+		System.out.println(a + b + c);
+	}
+}
+
+always prints:

+

going, going, gone
+
+because the assignment of the string "gone" to s occurs after the first two arguments to print3 have been evaluated.

+ +If evaluation of an argument expression completes abruptly, no part of any argument expression to its right appears to have been evaluated.

+ +Thus, the example:

+

class Test {
+	static int id;
+	public static void main(String[] args) {
+		try {
+			test(id = 1, oops(), id = 3);
+		} catch (Exception e) {
+			System.out.println(e + ", id=" + id);
+		}
+	}
+	static int oops() throws Exception {
+		throw new Exception("oops");
+	}
+	static int test(int a, int b, int c) {
+		return a + b + c;
+	}
+}
+
+prints:

+

java.lang.Exception: oops, id=1
+
+because the assignment of 3 to id is not executed.

+ +

15.7.5 Evaluation Order for Other Expressions

+ +The order of evaluation for some expressions is not completely covered by these general rules, because these expressions may raise exceptional conditions at times that must be specified. See, specifically, the detailed explanations of evaluation order for the following kinds of expressions:

+

+

15.8 Primary Expressions

+ +Primary expressions include most of the simplest kinds of expressions, from which all others are constructed: literals, class literals, field accesses, method invocations, and array accesses. A parenthesized expression is also treated syntactically as a primary expression.

+

    +Primary:
+	PrimaryNoNewArray
+	ArrayCreationExpression
+
+PrimaryNoNewArray:
+	Literal
+	Type . class 
+	void . class 
+	this
+	ClassName.this
+	( Expression )
+	ClassInstanceCreationExpression
+	FieldAccess
+	MethodInvocation
+	ArrayAccess
+
+

15.8.1 Lexical Literals

+ +A literal (§3.10) denotes a fixed, unchanging value.

+ +The following production from §3.10 is repeated here for convenience:

+

    +Literal: 
+	IntegerLiteral
+	FloatingPointLiteral
+	BooleanLiteral
+	CharacterLiteral
+	StringLiteral
+	NullLiteral
+
+The type of a literal is determined as follows:

+

    +
  • The type of an integer literal that ends with L or l is long; the type of any other integer literal is int. + +
  • The type of a floating-point literal that ends with F or f is float and its value must be an element of the float value set (§4.2.3). The type of any other floating-point literal is double and its value must be an element of the double value set. + +
  • The type of a boolean literal is boolean. + +
  • The type of a character literal is char. + +
  • The type of a string literal is String. + +
  • The type of the null literal null is the null type; its value is the null reference. +
+Evaluation of a lexical literal always completes normally.

+ +

15.8.2 Class Literals

+ +A class literal is an expression consisting of the name of a class, interface, array, or primitive type followed by a `.' and the token class. The type of a class literal is Class. It evaluates to the Class object for the named type (or for void) as defined by the defining class loader of the class of the current instance.

+ +

15.8.3 this

+ +The keyword this may be used only in the body of an instance method, instance initializer or constructor, or in the initializer of an instance variable of a class. If it appears anywhere else, a compile-time error occurs.

+ +When used as a primary expression, the keyword this denotes a value, that is a reference to the object for which the instance method was invoked (§15.12), or to the object being constructed. The type of this is the class C within which the keyword this occurs. At run time, the class of the actual object referred to may be the class C or any subclass of C.

+ +In the example:

+

class IntVector {
+	int[] v;
+	boolean equals(IntVector other) {
+		if (this == other)
+			return true;
+		if (v.length != other.v.length)
+			return false;
+		for (int i = 0; i < v.length; i++)
+			if (v[i] != other.v[i])
+				return false;
+		return true;
+	}
+}
+
+the class IntVector implements a method equals, which compares two vectors. If the other vector is the same vector object as the one for which the equals method was invoked, then the check can skip the length and value comparisons. The equals method implements this check by comparing the reference to the other object to this.

+ +The keyword this is also used in a special explicit constructor invocation statement, which can appear at the beginning of a constructor body (§8.8.5).

+ +

15.8.4 Qualified this

+ +Any lexically enclosing instance can be referred to by explicitly qualifying the keyword this.

+ +Let C be the class denoted by ClassName. Let n be an integer such that C is the nth lexically enclosing class of the class in which the qualified this expression appears. The value of an expression of the form ClassName.this is the nth lexically enclosing instance of this (§8.1.2). The type of the expression is C. It is a compile-time error if the current class is not an inner class of class C or C itself.

+ +

15.8.5 Parenthesized Expressions

+ +A parenthesized expression is a primary expression whose type is the type of the contained expression and whose value at run time is the value of the contained expression. If the contained expression denotes a variable then the parenthesized expression also denotes that variable.

+ +Parentheses do not affect in any way the choice of value set (§4.2.3) for the value of an expression of type float or double.

+ +

15.9 Class Instance Creation Expressions

+ +A class instance creation expression is used to create new objects that are instances of classes.

+

    +ClassInstanceCreationExpression:
+	new ClassOrInterfaceType ( ArgumentListopt ) ClassBodyopt
+	Primary.new Identifier ( ArgumentListopt ) ClassBodyopt
+
+ArgumentList:
+	Expression
+	ArgumentList , Expression
+
+Class instance creation expressions have two forms:

+

    +
  • Unqualified class instance creation expressions begin with the keyword new. An unqualified class instance creation expression may be used to create an instance of a class, regardless of whether the class is a top-level (§7.6), member (§8.5, §9.5), local (§14.3) or anonymous class (§15.9.5). + +
  • Qualified class instance creation expressions begin with a Primary. A qualified class instance creation expression enables the creation of instances of inner member classes and their anonymous subclasses. +
+Both unqualified and qualified class instance creation expressions may optionally end with a class body. Such a class instance creation expression declares an anonymous class (§15.9.5) and creates an instance of it.

+ +We say that a class is instantiated when an instance of the class is created by a class instance creation expression. Class instantiation involves determining what class is to be instantiated, what the enclosing instances (if any) of the newly created instance are, what constructor should be invoked to create the new instance and what arguments should be passed to that constructor.

+ +

15.9.1 Determining the Class being Instantiated

+ +If the class instance creation expression ends in a class body, then the class being instantiated is an anonymous class. Then:

+

    +
  • If the class instance creation expression is an unqualified class instance creation expression, then let T be the ClassOrInterfaceType after the new token. It is a compile-time error if the class or interface named by T is not accessible (§6.6). If T is the name of a class, then an anonymous direct subclass of the class named by T is declared. It is a compile-time error if the class named by T is a final class. If T is the name of an interface then an anonymous direct subclass of Object that implements the interface named by T is declared. In either case, the body of the subclass is the ClassBody given in the class instance creation expression. The class being instantiated is the anonymous subclass. + +
  • Otherwise, the class instance creation expression is a qualified class instance creation expression. Let T be the name of the Identifier after the new token. It is a compile-time error if T is not the simple name (§6.2) of an accessible (§6.6) non-final inner class (§8.1.2) that is a member of the compile-time type of the Primary. It is also a compile-time error if T is ambiguous (§8.5). An anonymous direct subclass of the class named by T is declared. The body of the subclass is the ClassBody given in the class instance creation expression. The class being instantiated is the anonymous subclass. +
+If a class instance creation expression does not declare an anonymous class, then:

+

    +
  • If the class instance creation expression is an unqualified class instance creation expression, then the ClassOrInterfaceType must name a class that is accessible (§6.6) and not abstract, or a compile-time error occurs. In this case, the class being instantiated is the class denoted by ClassOrInterfaceType. + +
  • Otherwise, the class instance creation expression is a qualified class instance creation expression. It is a compile-time error if Identifier is not the simple name (§6.2) of an accessible (§6.6) non-abstract inner class (§8.1.2) T that is a member of the compile-time type of the Primary. It is also a compile-time error if Identifier is ambiguous (§8.5). The class being instantiated is the class denoted by Identifier. +
+The type of the class instance creation expression is the class type being instantiated.

+ +

15.9.2 Determining Enclosing Instances

+ +Let C be the class being instantiated, and let i the instance being created. If C is an inner class then i may have an immediately enclosing instance. The immediately enclosing instance of i (§8.1.2) is determined as follows:

+

    +
  • If C is an anonymous class, then: +
      + +
    • If the class instance creation expression occurs in a static context (§8.1.2), then i has no immediately enclosing instance. + +
    • Otherwise, the immediately enclosing instance of i is this. +
    + +
  • If C is a local class (§14.3), C must be declared in a method declared in a lexically enclosing class O. Let n be an integer such that O is the nth lexically enclosing class of the class in which the class instance creation expression appears. Then: +
      + +
    • If C occurs in a static context, then i has no immediately enclosing instance. + +
    • Otherwise, if the class instance creation expression occurs in a static context, then a compile-time error occurs. + +
    • Otherwise, the immediately enclosing instance of i is the nth lexically enclosing instance of this (§8.1.2). +
    + +
  • Otherwise, C is an inner member class (§8.5). +
      + +
    • If the class instance creation expression is an unqualified class instance creation expression, then: +
        + +
      • If the class instance creation expression occurs in a static context, then a compile-time error occurs. + +
      • Otherwise, if C is a member of an enclosing class then let O be the innermost lexically enclosing class of which C is a member, and let n be an integer such that O is the nth lexically enclosing class of the class in which the class instance creation expression appears. The immediately enclosing instance of i is the nth lexically enclosing instance of this. + +
      • Otherwise, a compile-time error occurs. +
      + +
    • Otherwise, the class instance creation expression is a qualified class instance creation expression. The immediately enclosing instance of i is the object that is the value of the Primary expression. +
    +
+In addition, if C is an anonymous class, and the direct superclass of C, S, is an inner class then i may have an immediately enclosing instance with respect to S which is determined as follows:

+

    +
  • If S is a local class (§14.3), then S must be declared in a method declared in a lexically enclosing class O. Let n be an integer such that O is the nth lexically enclosing class of the class in which the class instance creation expression appears. Then: +
      + +
    • If S occurs within a static context, then i has no immediately enclosing instance with respect to S. + +
    • Otherwise, if the class instance creation expression occurs in a static context, then a compile-time error occurs. + +
    • Otherwise, the immediately enclosing instance of i with respect to S is the nth lexically enclosing instance of this. +
    + +
  • Otherwise, S is an inner member class (§8.5). +
      + +
    • If the class instance creation expression is an unqualified class instance creation expression, then: +
        + +
      • If the class instance creation expression occurs in a static context, then a compile-time error occurs. + +
      • Otherwise, if S is a member of an enclosing class then let O be the innermost lexically enclosing class of which S is a member, and let n be an integer such that O is the nth lexically enclosing class of the class in which the class instance creation expression appears. The immediately enclosing instance of i with respect to S is the nth lexically enclosing instance of this. + +
      • Otherwise, a compile-time error occurs. +
      + +
    • Otherwise, the class instance creation expression is a qualified class instance creation expression. The immediately enclosing instance of i with respect to S is the object that is the value of the Primary expression. +
    +
+

15.9.3 Choosing the Constructor and its Arguments

+ +Let C be the class type being instantiated. To create an instance of C, i, a constructor of C is chosen at compile-time by the following rules:

+

    +
  • First, the actual arguments to the constructor invocation are determined. +
      + +
    • If C is an anonymous class, and the direct superclass of C, S, is an inner class, then: +
        + +
      • If the S is a local class and S occurs in a static context, then the arguments in the argument list, if any, are the arguments to the constructor, in the order they appear in the expression. + +
      • Otherwise, the immediately enclosing instance of i with respect to S is the first argument to the constructor, followed by the arguments in the argument list of the class instance creation expression, if any, in the order they appear in the expression. +
      + +
    • Otherwise the arguments in the argument list, if any, are the arguments to the constructor, in the order they appear in the expression. +
    + +
  • Once the actual arguments have been determined, they are used to select a constructor of C, using the same rules as for method invocations (§15.12). As in method invocations, a compile-time method matching error results if there is no unique most-specific constructor that is both applicable and accessible. +
+Note that the type of the class instance creation expression may be an anonymous class type, in which case the constructor being invoked is an anonymous constructor.

+ +

15.9.4 Run-time Evaluation of Class Instance Creation Expressions

+ +At run time, evaluation of a class instance creation expression is as follows.

+ +First, if the class instance creation expression is a qualified class instance creation expression, the qualifying primary expression is evaluated. If the qualifying expression evaluates to null, a NullPointerException is raised, and the class instance creation expression completes abruptly. If the qualifying expression completes abruptly, the class instance creation expression completes abruptly for the same reason.

+ +Next, space is allocated for the new class instance. If there is insufficient space to allocate the object, evaluation of the class instance creation expression completes abruptly by throwing an OutOfMemoryError (§15.9.6).

+ +The new object contains new instances of all the fields declared in the specified class type and all its superclasses. As each new field instance is created, it is initialized to its default value (§4.5.5).

+ +Next, the actual arguments to the constructor are evaluated, left-to-right. If any of the argument evaluations completes abruptly, any argument expressions to its right are not evaluated, and the class instance creation expression completes abruptly for the same reason.

+ +Next, the selected constructor of the specified class type is invoked. This results in invoking at least one constructor for each superclass of the class type. This process can be directed by explicit constructor invocation statements (§8.8) and is described in detail in §12.5.

+ +The value of a class instance creation expression is a reference to the newly created object of the specified class. Every time the expression is evaluated, a fresh object is created.

+ +

15.9.5 Anonymous Class Declarations

+ +An anonymous class declaration is automatically derived from a class instance creation expression by the compiler.

+ +An anonymous class is never abstract (§8.1.1.1). An anonymous class is always an inner class (§8.1.2); it is never static (§8.1.1, §8.5.2). An anonymous class is always implicitly final (§8.1.1.2).

+ +

15.9.5.1 Anonymous Constructors

+ +An anonymous class cannot have an explicitly declared constructor. Instead, the compiler must automatically provide an anonymous constructor for the anonymous class. The form of the anonymous constructor of an anonymous class C with direct superclass S is as follows:

+

    +
  • If S is not an inner class, or if S is a local class that occurs in a static context, then the anonymous constructor has one formal parameter for each actual argument to the class instance creation expression in which C is declared. The actual arguments to the class instance creation expression are used to determine a constructor cs of S, using the same rules as for method invocations (§15.12). The type of each formal parameter of the anonymous constructor must be identical to the corresponding formal parameter of cs. + +The body of the constructor consists of an explicit constructor invocation (§8.8.5.1) of the form super(...), where the actual arguments are the formal parameters of the constructor, in the order they were declared.

    + +

  • Otherwise, the first formal parameter of the constructor of C represents the value of the immediately enclosing instance of i with respect to S. The type of this parameter is the class type that immediately encloses the declaration of S. The constructor has an additional formal parameter for each actual argument to the class instance creation expression that declared the anonymous class. The nth formal parameter e corresponds to the n - 1st actual argument. The actual arguments to the class instance creation expression are used to determine a constructor cs of S, using the same rules as for method invocations (§15.12). The type of each formal parameter of the anonymous constructor must be identical to the corresponding formal parameter of cs. The body of the constructor consists of an explicit constructor invocation (§8.8.5.1) of the form o.super(...), where o is the first formal parameter of the constructor, and the actual arguments are the subsequent formal parameters of the constructor, in the order they were declared. +
+In all cases, the throws clause of an anonymous constructor must list all the checked exceptions thrown by the explicit superclass constructor invocation statement contained within the anonymous constructor, and all checked exceptions thrown by any instance initializers or instance variable initializers of the anonymous class.

+ +Note that it is possible for the signature of the anonymous constructor to refer to an inaccessible type (for example, if such a type occurred in the signature of the superclass constructor cs). This does not, in itself, cause any errors at either compile time or run time.

+ +

15.9.6 Example: Evaluation Order and Out-of-Memory Detection

+ +If evaluation of a class instance creation expression finds there is insufficient memory to perform the creation operation, then an OutOfMemoryError is thrown. This check occurs before any argument expressions are evaluated.

+ +So, for example, the test program:

+

class List {
+	int value;
+	List next;
+	static List head = new List(0);
+	List(int n) { value = n; next = head; head = this; }
+}
+class Test {
+	public static void main(String[] args) {
+		int id = 0, oldid = 0;
+		try {
+			for (;;) {
+				++id;
+				new List(oldid = id);
+			}
+		} catch (Error e) {
+			System.out.println(e + ", " + (oldid==id));
+		}
+	}
+}
+
+prints:

+

java.lang.OutOfMemoryError: List, false
+
+because the out-or-memory condition is detected before the argument expression oldid = id is evaluated.

+ +Compare this to the treatment of array creation expressions (§15.10), for which the out-of-memory condition is detected after evaluation of the dimension expressions (§15.10.3).

+ +

15.10 Array Creation Expressions

+ +An array instance creation expression is used to create new arrays (§10).

+

    +ArrayCreationExpression:
+	new PrimitiveType DimExprs Dimsopt
+	new TypeName DimExprs Dimsopt
+	new PrimitiveType Dims ArrayInitializer 
+	new TypeName Dims ArrayInitializer
+
+

+

    +DimExprs:
+	DimExpr
+	DimExprs DimExpr
+
+DimExpr:
+	[ Expression ]
+
+Dims:
+	[ ]
+	Dims [ ]
+
+An array creation expression creates an object that is a new array whose elements are of the type specified by the PrimitiveType or TypeName. The TypeName may name any named reference type, even an abstract class type (§8.1.1.1) or an interface type (§9).

+ +The type of the creation expression is an array type that can denoted by a copy of the creation expression from which the new keyword and every DimExpr expression and array initializer have been deleted.

+ +For example, the type of the creation expression:

+

new double[3][3][]
+
+is:

+

double[][][]
+
+The type of each dimension expression within a DimExpr must be an integral type, or a compile-time error occurs. Each expression undergoes unary numeric promotion (§5.6.1). The promoted type must be int, or a compile-time error occurs; this means, specifically, that the type of a dimension expression must not be long.

+ +If an array initializer is provided, the newly allocated array will be initialized with the values provided by the array initializer as described in §10.6.

+ +

15.10.1 Run-time Evaluation of Array Creation Expressions

+ +At run time, evaluation of an array creation expression behaves as follows. If there are no dimension expressions, then there must be an array initializer. The value of the array initializer is the value of the array creation expression. Otherwise:

+ +First, the dimension expressions are evaluated, left-to-right. If any of the expression evaluations completes abruptly, the expressions to the right of it are not evaluated.

+ +Next, the values of the dimension expressions are checked. If the value of any DimExpr expression is less than zero, then an NegativeArraySizeException is thrown.

+ +Next, space is allocated for the new array. If there is insufficient space to allocate the array, evaluation of the array creation expression completes abruptly by throwing an OutOfMemoryError.

+ +Then, if a single DimExpr appears, a single-dimensional array is created of the specified length, and each component of the array is initialized to its default value (§4.5.5).

+ +If an array creation expression contains N DimExpr expressions, then it effectively executes a set of nested loops of depth N - 1 to create the implied arrays of arrays.

+ +For example, the declaration:

+

float[][] matrix = new float[3][3];
+
+is equivalent in behavior to:

+

float[][] matrix = new float[3][];
+for (int d = 0; d < matrix.length; d++)
+	matrix[d] = new float[3];
+
+and:

+

Age[][][][][] Aquarius = new Age[6][10][8][12][];
+
+is equivalent to:

+

Age[][][][][] Aquarius = new Age[6][][][][];
+for (int d1 = 0; d1 < Aquarius.length; d1++) {
+	Aquarius[d1] = new Age[10][][][];
+	for (int d2 = 0; d2 < Aquarius[d1].length; d2++) {
+		Aquarius[d1][d2] = new Age[8][][];
+		for (int d3 = 0; d3 < Aquarius[d1][d2].length; d3++) {
+			Aquarius[d1][d2][d3] = new Age[12][];
+		}
+	}
+}
+
+with d, d1, d2 and d3 replaced by names that are not already locally declared. Thus, a single new expression actually creates one array of length 6, 6 arrays of length 10, 6 x 10 = 60 arrays of length 8, and 6 x 10 x 8 = 480 arrays of length 12. This example leaves the fifth dimension, which would be arrays containing the actual array elements (references to Age objects), initialized only to null references. These arrays can be filled in later by other code, such as:

+

Age[] Hair = { new Age("quartz"), new Age("topaz") };
+Aquarius[1][9][6][9] = Hair;
+
+A multidimensional array need not have arrays of the same length at each level. 

+
+Thus, a triangular matrix may be created by:

+float triang[][] = new float[100][];
+for (int i = 0; i < triang.length; i++)
+	triang[i] = new float[i+1];
+
+

15.10.2 Example: Array Creation Evaluation Order

+ +In an array creation expression (§15.10), there may be one or more dimension expressions, each within brackets. Each dimension expression is fully evaluated before any part of any dimension expression to its right.

+ +Thus:

+

class Test {
+	public static void main(String[] args) {
+		int i = 4;
+		int ia[][] = new int[i][i=3];
+		System.out.println(
+			"[" + ia.length + "," + ia[0].length + "]");
+	}
+}
+
+prints:

+

[4,3]
+
+because the first dimension is calculated as 4 before the second dimension expression sets i to 3.

+ +If evaluation of a dimension expression completes abruptly, no part of any dimension expression to its right will appear to have been evaluated. Thus, the example:

+

class Test {
+	public static void main(String[] args) {
+		int[][] a = { { 00, 01 }, { 10, 11 } };
+		int i = 99;
+		try {
+			a[val()][i = 1]++;
+		} catch (Exception e) {
+			System.out.println(e + ", i=" + i);
+		}
+	}
+	static int val() throws Exception {
+		throw new Exception("unimplemented");
+	}
+}
+
+prints:

+

java.lang.Exception: unimplemented, i=99
+
+because the embedded assignment that sets i to 1 is never executed.

+ +

15.10.3 Example: Array Creation and Out-of-Memory Detection

+ +If evaluation of an array creation expression finds there is insufficient memory to perform the creation operation, then an OutOfMemoryError is thrown. This check occurs only after evaluation of all dimension expressions has completed normally.

+ +So, for example, the test program:

+

class Test {
+	public static void main(String[] args) {
+		int len = 0, oldlen = 0;
+		Object[] a = new Object[0];
+		try {
+			for (;;) {
+				++len;
+				Object[] temp = new Object[oldlen = len];
+				temp[0] = a;
+				a = temp;
+			}
+		} catch (Error e) {
+			System.out.println(e + ", " + (oldlen==len));
+		}
+	}
+}
+
+prints:

+

java.lang.OutOfMemoryError, true
+
+because the out-of-memory condition is detected after the dimension expression oldlen = len is evaluated.

+ +Compare this to class instance creation expressions (§15.9), which detect the out-of-memory condition before evaluating argument expressions (§15.9.6).

+ +

15.11 Field Access Expressions

+ +A field access expression may access a field of an object or array, a reference to which is the value of either an expression or the special keyword super. (It is also possible to refer to a field of the current instance or current class by using a simple name; see §6.5.6.)

+

    +FieldAccess: 
+	Primary . Identifier
+	super . Identifier
+	ClassName .super . Identifier
+
+The meaning of a field access expression is determined using the same rules as for qualified names (§6.6), but limited by the fact that an expression cannot denote a package, class type, or interface type.

+ +

15.11.1 Field Access Using a Primary

+ +The type of the Primary must be a reference type T, or a compile-time error occurs. The meaning of the field access expression is determined as follows:

+

    +
  • If the identifier names several accessible member fields of type T, then the field access is ambiguous and a compile-time error occurs. + +
  • If the identifier does not name an accessible member field of type T, then the field access is undefined and a compile-time error occurs. + +
  • Otherwise, the identifier names a single accessible member field of type T and the type of the field access expression is the declared type of the field. At run time, the result of the field access expression is computed as follows: +
      + +
    • If the field is static: +
        + +
      • If the field is final, then the result is the value of the specified class variable in the class or interface that is the type of the Primary expression. + +
      • If the field is not final, then the result is a variable, namely, the specified class variable in the class that is the type of the Primary expression. +
      + +
    • If the field is not static: +
        + +
      • If the value of the Primary is null, then a NullPointerException is thrown. + +
      • If the field is final, then the result is the value of the specified instance variable in the object referenced by the value of the Primary. + +
      • If the field is not final, then the result is a variable, namely, the specified instance variable in the object referenced by the value of the Primary. +
      +
    +
+Note, specifically, that only the type of the Primary expression, not the class of the actual object referred to at run time, is used in determining which field to use.

+ +Thus, the example:

+

class S { int x = 0; }
+class T extends S { int x = 1; }
+class Test {
+	public static void main(String[] args) {
+		T t = new T();
+		System.out.println("t.x=" + t.x + when("t", t));
+		S s = new S();
+		System.out.println("s.x=" + s.x + when("s", s));
+		s = t;
+		System.out.println("s.x=" + s.x + when("s", s));
+	}
+	static String when(String name, Object t) {
+		return " when " + name + " holds a "
+			+ t.getClass() + " at run time.";
+	}
+}
+
+produces the output:

+

t.x=1 when t holds a class T at run time.
+s.x=0 when s holds a class S at run time.
+s.x=0 when s holds a class T at run time.
+
+The last line shows that, indeed, the field that is accessed does not depend on the run-time class of the referenced object; even if s holds a reference to an object of class T, the expression s.x refers to the x field of class S, because the type of the expression s is S. Objects of class T contain two fields named x, one for class T and one for its superclass S.

+ +This lack of dynamic lookup for field accesses allows programs to be run efficiently with straightforward implementations. The power of late binding and overriding is available in, but only when instance methods are used. Consider the same example using instance methods to access the fields:

+

class S { int x = 0; int z() { return x; } }
+class T extends S { int x = 1; int z() { return x; } }
+class Test {
+	public static void main(String[] args) {
+		T t = new T();
+		System.out.println("t.z()=" + t.z() + when("t", t));
+		S s = new S();
+		System.out.println("s.z()=" + s.z() + when("s", s));
+		s = t;
+		System.out.println("s.z()=" + s.z() + when("s", s));
+	}
+	static String when(String name, Object t) {
+		return " when " + name + " holds a "
+			+ t.getClass() + " at run time.";
+	}
+}
+
+Now the output is:

+

t.z()=1 when t holds a class T at run time.
+s.z()=0 when s holds a class S at run time.
+s.z()=1 when s holds a class T at run time.
+
+The last line shows that, indeed, the method that is accessed does depend on the run-time class of referenced object; when s holds a reference to an object of class T, the expression s.z() refers to the z method of class T, despite the fact that the type of the expression s is S. Method z of class T overrides method z of class S.

+ +The following example demonstrates that a null reference may be used to access a class (static) variable without causing an exception:

+

class Test {
+	static String mountain = "Chocorua";
+	static Test favorite(){
+		System.out.print("Mount ");
+		return null;
+	}
+	public static void main(String[] args) {
+		System.out.println(favorite().mountain);
+	}
+}
+
+It compiles, executes, and prints:

+

Mount Chocorua
+
+ +Even though the result of favorite() is null, a NullPointerException is not thrown. That "Mount " is printed demonstrates that the Primary expression is indeed fully evaluated at run time, despite the fact that only its type, not its value, is used to determine which field to access (because the field mountain is static).

+ +

15.11.2 Accessing Superclass Members using super

+ +The special forms using the keyword super are valid only in an instance method, instance initializer or constructor, or in the initializer of an instance variable of a class; these are exactly the same situations in which the keyword this may be used (§15.8.3). The forms involving super may not be used anywhere in the class Object, since Object has no superclass; if super appears in class Object, then a compile-time error results.

+ +Suppose that a field access expression super.name appears within class C, and the immediate superclass of C is class S. Then super.name is treated exactly as if it had been the expression ((S)this).name; thus, it refers to the field named name of the current object, but with the current object viewed as an instance of the superclass. Thus it can access the field named name that is visible in class S, even if that field is hidden by a declaration of a field named name in class C.

+ +The use of super is demonstrated by the following example:

+

interface I { int x = 0; }
+class T1 implements I { int x = 1; }
+class T2 extends T1 { int x = 2; }
+class T3 extends T2 {
+	int x = 3;
+	void test() {
+		System.out.println("x=\t\t"+x);
+		System.out.println("super.x=\t\t"+super.x);
+		System.out.println("((T2)this).x=\t"+((T2)this).x);
+		System.out.println("((T1)this).x=\t"+((T1)this).x);
+		System.out.println("((I)this).x=\t"+((I)this).x);
+	}
+}
+class Test {
+	public static void main(String[] args) {
+		new T3().test();
+	}
+}
+
+which produces the output:

+

x=					3
+super.x=					2
+((T2)this).x=					2
+((T1)this).x=					1
+((I)this).x=					0
+
+Within class T3, the expression super.x is treated exactly as if it were:

+

((T2)this).x
+
+Suppose that a field access expression T.super.name appears within class C, and the immediate superclass of the class denoted by T is a class whose fully qualified name is S. Then T.super.name is treated exactly as if it had been the expression ((S)T.this).name.

+ +Thus the expression T.super.name can access the field named name that is visible in the class named by S, even if that field is hidden by a declaration of a field named name in the class named by T.

+ +It is a compile-time error if the class denoted by T is not a lexically enclosing class of the current class.

+ +

15.12 Method Invocation Expressions

+ +A method invocation expression is used to invoke a class or instance method.

+

    +MethodInvocation:
+	MethodName ( ArgumentListopt )
+	Primary . Identifier ( ArgumentListopt )
+	super . Identifier ( ArgumentListopt )
+	ClassName . super . Identifier ( ArgumentListopt )
+
+The definition of ArgumentList from §15.9 is repeated here for convenience:

+

    +ArgumentList:
+	Expression
+	ArgumentList , Expression
+
+ +Resolving a method name at compile time is more complicated than resolving a field name because of the possibility of method overloading. Invoking a method at run time is also more complicated than accessing a field because of the possibility of instance method overriding.

+ +Determining the method that will be invoked by a method invocation expression involves several steps. The following three sections describe the compile-time processing of a method invocation; the determination of the type of the method invocation expression is described in §15.12.3.

+ +

15.12.1 Compile-Time Step 1: Determine Class or Interface to Search

+ +The first step in processing a method invocation at compile time is to figure out the name of the method to be invoked and which class or interface to check for definitions of methods of that name. There are several cases to consider, depending on the form that precedes the left parenthesis, as follows:

+

    +
  • If the form is MethodName, then there are three subcases: +
      + +
    • If it is a simple name, that is, just an Identifier, then the name of the method is the Identifier. If the Identifier appears within the scope (§6.3) of a visible method declaration with that name, then there must be an enclosing type declaration of which that method is a member. Let T be the innermost such type declaration. The class or interface to search is T. + +
    • If it is a qualified name of the form TypeName . Identifier, then the name of the method is the Identifier and the class to search is the one named by the TypeName. If TypeName is the name of an interface rather than a class, then a compile-time error occurs, because this form can invoke only static methods and interfaces have no static methods. + +
    • In all other cases, the qualified name has the form FieldName . Identifier; then the name of the method is the Identifier and the class or interface to search is the declared type of the field named by the FieldName. +
    + +
  • If the form is Primary . Identifier, then the name of the method is the Identifier and the class or interface to be searched is the type of the Primary expression. + +
  • If the form is super . Identifier, then the name of the method is the Identifier and the class to be searched is the superclass of the class whose declaration contains the method invocation. Let T be the type declaration immediately enclosing the method invocation. It is a compile-time error if any of the following situations occur: +
      + +
    • T is the class Object. + +
    • T is an interface. +
    + +
  • If the form is ClassName.super . Identifier, then the name of the method is the Identifier and the class to be searched is the superclass of the class C denoted by ClassName. It is a compile-time error if C is not a lexically enclosing class of the current class. It is a compile-time error if C is the class Object. Let T be the type declaration immediately enclosing the method invocation. It is a compile-time error if any of the following situations occur: +
      + +
    • T is the class Object. + +
    • T is an interface. +
    +
+

15.12.2 Compile-Time Step 2: Determine Method Signature

+ +The second step searches the class or interface determined in the previous step for method declarations. This step uses the name of the method and the types of the argument expressions to locate method declarations that are both applicable and accessible, that is, declarations that can be correctly invoked on the given arguments. There may be more than one such method declaration, in which case the most specific one is chosen. The descriptor (signature plus return type) of the most specific method declaration is one used at run time to do the method dispatch.

+ +

15.12.2.1 Find Methods that are Applicable and Accessible

+ +A method declaration is applicable to a method invocation if and only if both of the following are true:

+

    +
  • The number of parameters in the method declaration equals the number of argument expressions in the method invocation. + +
  • The type of each actual argument can be converted by method invocation conversion (§5.3) to the type of the corresponding parameter. Method invocation conversion is the same as assignment conversion (§5.2), except that constants of type int are never implicitly narrowed to byte, short, or char. +
+The class or interface determined by the process described in §15.12.1 is searched for all method declarations applicable to this method invocation; method definitions inherited from superclasses and superinterfaces are included in this search.

+ +Whether a method declaration is accessible (§6.6) at a method invocation depends on the access modifier (public, none, protected, or private) in the method declaration and on where the method invocation appears.

+ +If the class or interface has no method declaration that is both applicable and accessible, then a compile-time error occurs.

+ +In the example program:

+

public class Doubler {
+	static int two() { return two(1); }
+	private static int two(int i) { return 2*i; }
+}
+class Test extends Doubler {	
+	public static long two(long j) {return j+j; }
+	public static void main(String[] args) {
+		System.out.println(two(3));
+		System.out.println(Doubler.two(3)); // compile-time error
+	}
+}
+
+for the method invocation two(1) within class Doubler, there are two accessible methods named two, but only the second one is applicable, and so that is the one invoked at run time. For the method invocation two(3) within class Test, there are two applicable methods, but only the one in class Test is accessible, and so that is the one to be invoked at run time (the argument 3 is converted to type long). For the method invocation Doubler.two(3), the class Doubler, not class Test, is searched for methods named two; the only applicable method is not accessible, and so this method invocation causes a compile-time error.

+ +Another example is:

+

class ColoredPoint {
+	int x, y;
+	byte color;
+	void setColor(byte color) { this.color = color; }
+}
+class Test {
+	public static void main(String[] args) {
+		ColoredPoint cp = new ColoredPoint();
+		byte color = 37;
+		cp.setColor(color);
+		cp.setColor(37);											// compile-time error
+	}
+}
+
+Here, a compile-time error occurs for the second invocation of setColor, because no applicable method can be found at compile time. The type of the literal 37 is int, and int cannot be converted to byte by method invocation conversion. Assignment conversion, which is used in the initialization of the variable color, performs an implicit conversion of the constant from type int to byte, which is permitted because the value 37 is small enough to be represented in type byte; but such a conversion is not allowed for method invocation conversion.

+ +If the method setColor had, however, been declared to take an int instead of a byte, then both method invocations would be correct; the first invocation would be allowed because method invocation conversion does permit a widening conversion from byte to int. However, a narrowing cast would then be required in the body of setColor:

+

	void setColor(int color) { this.color = (byte)color; }
+
+

15.12.2.2 Choose the Most Specific Method

+ +If more than one method declaration is both accessible and applicable to a method invocation, it is necessary to choose one to provide the descriptor for the run-time method dispatch. The Java programming language uses the rule that the most specific method is chosen.

+ +The informal intuition is that one method declaration is more specific than another if any invocation handled by the first method could be passed on to the other one without a compile-time type error.

+ +The precise definition is as follows. Let m be a name and suppose that there are two declarations of methods named m, each having n parameters. Suppose that one declaration appears within a class or interface T and that the types of the parameters are T1, . . . , Tn; suppose moreover that the other declaration appears within a class or interface U and that the types of the parameters are U1, . . . , Un. Then the method m declared in T is more specific than the method m declared in U if and only if both of the following are true:

+

    +
  • T can be converted to U by method invocation conversion. + +
  • Tj can be converted to Uj by method invocation conversion, for all j from 1 to n. +
+A method is said to be maximally specific for a method invocation if it is applicable and accessible and there is no other applicable and accessible method that is more specific.

+ +If there is exactly one maximally specific method, then it is in fact the most specific method; it is necessarily more specific than any other method that is applicable and accessible. It is then subjected to some further compile-time checks as described in §15.12.3.

+ +It is possible that no method is the most specific, because there are two or more maximally specific methods. In this case:

+

    +
  • If all the maximally specific methods have the same signature, then: +
      + +
    • If one of the maximally specific methods is not declared abstract, it is the most specific method. + +
    • Otherwise, all the maximally specific methods are necessarily declared abstract. The most specific method is chosen arbitrarily among the maximally specific methods. However, the most specific method is considered to throw a checked exception if and only if that exception is declared in the throws clauses of each of the maximally specific methods. +
    + +
  • Otherwise, we say that the method invocation is ambiguous, and a compile-time error occurs. +
+

15.12.2.3 Example: Overloading Ambiguity

+ +Consider the example:

+

class Point { int x, y; }
+class ColoredPoint extends Point { int color; }
+
+class Test {
+	static void test(ColoredPoint p, Point q) {
+		System.out.println("(ColoredPoint, Point)");
+	}
+	static void test(Point p, ColoredPoint q) {
+		System.out.println("(Point, ColoredPoint)");
+	}
+	public static void main(String[] args) {
+		ColoredPoint cp = new ColoredPoint();
+		test(cp, cp);											// compile-time error
+	}
+}
+
+This example produces an error at compile time. The problem is that there are two declarations of test that are applicable and accessible, and neither is more specific than the other. Therefore, the method invocation is ambiguous.

+ +If a third definition of test were added:

+

	static void test(ColoredPoint p, ColoredPoint q) {
+		System.out.println("(ColoredPoint, ColoredPoint)");
+	}
+
+then it would be more specific than the other two, and the method invocation would no longer be ambiguous.

+ +

15.12.2.4 Example: Return Type Not Considered

+ +As another example, consider:

+

class Point { int x, y; }
+class ColoredPoint extends Point { int color; }
+class Test {
+	static int test(ColoredPoint p) {
+		return p.color;
+	}
+	static String test(Point p) {
+		return "Point";
+	}
+	public static void main(String[] args) {
+		ColoredPoint cp = new ColoredPoint();
+		String s = test(cp);											// compile-time error
+	}
+}
+
+Here the most specific declaration of method test is the one taking a parameter of type ColoredPoint. Because the result type of the method is int, a compile-time error occurs because an int cannot be converted to a String by assignment conversion. This example shows that the result types of methods do not participate in resolving overloaded methods, so that the second test method, which returns a String, is not chosen, even though it has a result type that would allow the example program to compile without error.

+ +

15.12.2.5 Example: Compile-Time Resolution

+ +The most applicable method is chosen at compile time; its descriptor determines what method is actually executed at run time. If a new method is added to a class, then source code that was compiled with the old definition of the class might not use the new method, even if a recompilation would cause this method to be chosen.

+ +So, for example, consider two compilation units, one for class Point:

+

package points;
+public class Point {
+	public int x, y;
+	public Point(int x, int y) { this.x = x; this.y = y; }
+	public String toString() { return toString(""); }
+	public String toString(String s) {
+		return "(" + x + "," + y + s + ")";
+	}
+}
+
+and one for class ColoredPoint:

+

package points;
+public class ColoredPoint extends Point {
+	public static final int
+		RED = 0, GREEN = 1, BLUE = 2;
+	public static String[] COLORS =
+		{ "red", "green", "blue" };
+	public byte color;
+	public ColoredPoint(int x, int y, int color) {
+		super(x, y); this.color = (byte)color;
+	}
+	/** Copy all relevant fields of the argument into
+		    this ColoredPoint object. */
+	public void adopt(Point p) { x = p.x; y = p.y; }
+	public String toString() {
+		String s = "," + COLORS[color];
+		return super.toString(s);
+	}
+}
+
+Now consider a third compilation unit that uses ColoredPoint:

+

import points.*;
+class Test {
+	public static void main(String[] args) {
+		ColoredPoint cp =
+			new ColoredPoint(6, 6, ColoredPoint.RED);
+		ColoredPoint cp2 =
+			new ColoredPoint(3, 3, ColoredPoint.GREEN);
+		cp.adopt(cp2);
+		System.out.println("cp: " + cp);
+	}
+}
+
+The output is:

+

cp: (3,3,red)
+
+ +The application programmer who coded class Test has expected to see the word green, because the actual argument, a ColoredPoint, has a color field, and color would seem to be a "relevant field" (of course, the documentation for the package Points ought to have been much more precise!).

+ +Notice, by the way, that the most specific method (indeed, the only applicable method) for the method invocation of adopt has a signature that indicates a method of one parameter, and the parameter is of type Point. This signature becomes part of the binary representation of class Test produced by the compiler and is used by the method invocation at run time.

+ +Suppose the programmer reported this software error and the maintainer of the points package decided, after due deliberation, to correct it by adding a method to class ColoredPoint:

+

+public void adopt(ColoredPoint p) {
+	adopt((Point)p); color = p.color;
+}
+
+ +If the application programmer then runs the old binary file for Test with the new binary file for ColoredPoint, the output is still:

+

+cp: (3,3,red)
+
+because the old binary file for Test still has the descriptor "one parameter, whose type is Point; void" associated with the method call cp.adopt(cp2). If the source code for Test is recompiled, the compiler will then discover that there are now two applicable adopt methods, and that the signature for the more specific one is "one parameter, whose type is ColoredPoint; void"; running the program will then produce the desired output:

+

cp: (3,3,green)
+
+ +With forethought about such problems, the maintainer of the points package could fix the ColoredPoint class to work with both newly compiled and old code, by adding defensive code to the old adopt method for the sake of old code that still invokes it on ColoredPoint arguments:

+

+public void adopt(Point p) {
+	if (p instanceof ColoredPoint)
+		color = ((ColoredPoint)p).color;
+	x = p.x; y = p.y;
+}
+
+ +Ideally, source code should be recompiled whenever code that it depends on is changed. However, in an environment where different classes are maintained by different organizations, this is not always feasible. Defensive programming with careful attention to the problems of class evolution can make upgraded code much more robust. See §13 for a detailed discussion of binary compatibility and type evolution.

+ +

15.12.3 Compile-Time Step 3: Is the Chosen Method Appropriate?

+ +If there is a most specific method declaration for a method invocation, it is called the compile-time declaration for the method invocation. Three further checks must be made on the compile-time declaration:

+

    +
  • If the method invocation has, before the left parenthesis, a MethodName of the form Identifier, and the method is an instance method, then: +
      + +
    • If the invocation appears within a static context (§8.1.2), then a compile-time error occurs. (The reason is that a method invocation of this form cannot be used to invoke an instance method in places where this (§15.8.3) is not defined.) + +
    • Otherwise, let C be the innermost enclosing class of which the method is a member. If the invocation is not directly enclosed by C or an inner class of C, then a compile-time error occurs +
    + +
  • If the method invocation has, before the left parenthesis, a MethodName of the form TypeName . Identifier, then the compile-time declaration should be static. If the compile-time declaration for the method invocation is for an instance method, then a compile-time error occurs. (The reason is that a method invocation of this form does not specify a reference to an object that can serve as this within the instance method.) + +
  • If the method invocation has, before the left parenthesis, a MethodName of the form super . Identifier, then: +
      + +
    • If the method is abstract, a compile-time error occurs + +
    • If the method invocation occurs in a static context, a compile-time error occurs +
    + +
  • If the method invocation has, before the left parenthesis, a MethodName of the form ClassName.super . Identifier, then: +
      + +
    • If the method is abstract, a compile-time error occurs + +
    • If the method invocation occurs in a static context, a compile-time error occurs + +
    • Otherwise, let C be the class denoted by ClassName. If the invocation is not directly enclosed by C or an inner class of C, then a compile-time error occurs +
    + +
  • If the compile-time declaration for the method invocation is void, then the method invocation must be a top-level expression, that is, the Expression in an expression statement (§14.8) or in the ForInit or ForUpdate part of a for statement (§14.13), or a compile-time error occurs. (The reason is that such a method invocation produces no value and so must be used only in a situation where a value is not needed.) +
+The following compile-time information is then associated with the method invocation for use at run time:

+

    +
  • The name of the method. + +
  • The qualifying type of the method invocation (§13.1). + +
  • The number of parameters and the types of the parameters, in order. + +
  • The result type, or void, as declared in the compile-time declaration. + +
  • The invocation mode, computed as follows: +
      + +
    • If the compile-time declaration has the static modifier, then the invocation mode is static. + +
    • Otherwise, if the compile-time declaration has the private modifier, then the invocation mode is nonvirtual. + +
    • Otherwise, if the part of the method invocation before the left parenthesis is of the form super . Identifier or of the form ClassName.super.Identifier then the invocation mode is super. + +
    • Otherwise, if the compile-time declaration is in an interface, then the invocation mode is interface. + +
    • Otherwise, the invocation mode is virtual. +
    +
+If the compile-time declaration for the method invocation is not void, then the type of the method invocation expression is the result type specified in the compile-time declaration.

+ +

15.12.4 Runtime Evaluation of Method Invocation

+ +At run time, method invocation requires five steps. First, a target reference may be computed. Second, the argument expressions are evaluated. Third, the accessibility of the method to be invoked is checked. Fourth, the actual code for the method to be executed is located. Fifth, a new activation frame is created, synchronization is performed if necessary, and control is transferred to the method code.

+ +

15.12.4.1 Compute Target Reference (If Necessary)

+ +There are several cases to consider, depending on which of the four productions for MethodInvocation (§15.12) is involved:

+

    +
  • If the first production for MethodInvocation, which includes a MethodName, is involved, then there are three subcases: +
      + +
    • If the MethodName is a simple name, that is, just an Identifier, then there are two subcases: +
        + +
      • If the invocation mode is static, then there is no target reference. + +
      • Otherwise, let T be the enclosing type declaration of which the method is a member, and let n be an integer such that T is the nth lexically enclosing type declaration (§8.1.2) of the class whose declaration immediately contains the method invocation. Then the target reference is the nth lexically enclosing instance (§8.1.2) of this. It is a compile-time error if the nth lexically enclosing instance (§8.1.2) of this does not exist. +
      + +
    • If the MethodName is a qualified name of the form TypeName . Identifier, then there is no target reference. + +
    • If the MethodName is a qualified name of the form FieldName . Identifier, then there are two subcases: +
        + +
      • If the invocation mode is static, then there is no target reference. + +
      • Otherwise, the target reference is the value of the expression FieldName. +
      +
    + +
  • If the second production for MethodInvocation, which includes a Primary, is involved, then there are two subcases: +
      + +
    • If the invocation mode is static, then there is no target reference. The expression Primary is evaluated, but the result is then discarded. + +
    • Otherwise, the expression Primary is evaluated and the result is used as the target reference. +
    +
    +In either case, if the evaluation of the Primary expression completes abruptly, then no part of any argument expression appears to have been evaluated, and the method invocation completes abruptly for the same reason. +
    +
  • If the third production for MethodInvocation, which includes the keyword super, is involved, then the target reference is the value of this. + +
  • If the fourth production for MethodInvocation, ClassName.super, is involved, then the target reference is the value of ClassName.this. +
+

15.12.4.2 Evaluate Arguments

+ +The argument expressions are evaluated in order, from left to right. If the evaluation of any argument expression completes abruptly, then no part of any argument expression to its right appears to have been evaluated, and the method invocation completes abruptly for the same reason.

+ +

15.12.4.3 Check Accessibility of Type and Method

+ +Let C be the class containing the method invocation, and let T be the qualifying type of the method invocation (§13.1), and m be the name of the method, as determined at compile time (§15.12.3). An implementation of the Java programming language must insure, as part of linkage, that the method m still exists in the type T. If this is not true, then a NoSuchMethodError (which is a subclass of IncompatibleClassChangeError) occurs. If the invocation mode is interface, then the implementation must also check that the target reference type still implements the specified interface. If the target reference type does not still implement the interface, then an IncompatibleClassChangeError occurs.

+ +The implementation must also insure, during linkage, that the type T and the method m are accessible. For the type T:

+

    +
  • If T is in the same package as C, then T is accessible. + +
  • If T is in a different package than C, and T is public, then T is accessible. + +
  • If T is in a different package than C, and T is protected, then T is accessible if and only if C is a subclass of T. +
+For the method m:

+

    +
  • If m is public, then m is accessible. (All members of interfaces are public (§9.2)). + +
  • If m is protected, then m is accessible if and only if either T is in the same package as C, or C is T or a subclass of T. + +
  • If m has default (package) access, then m is accessible if and only if T is in the same package as C. + +
  • If m is private, then m is accessible if and only if C is T, or C encloses T, or T encloses C, or T and C are both enclosed by a third class. +
+If either T or m is not accessible, then an IllegalAccessError occurs (§12.3).

+ +

15.12.4.4 Locate Method to Invoke

+ +The strategy for method lookup depends on the invocation mode.

+ +If the invocation mode is static, no target reference is needed and overriding is not allowed. Method m of class T is the one to be invoked.

+ +Otherwise, an instance method is to be invoked and there is a target reference. If the target reference is null, a NullPointerException is thrown at this point. Otherwise, the target reference is said to refer to a target object and will be used as the value of the keyword this in the invoked method. The other four possibilities for the invocation mode are then considered.

+ +If the invocation mode is nonvirtual, overriding is not allowed. Method m of class T is the one to be invoked.

+ +Otherwise, the invocation mode is interface, virtual, or super, and overriding may occur. A dynamic method lookup is used. The dynamic lookup process starts from a class S, determined as follows:

+

    +
  • If the invocation mode is interface or virtual, then S is initially the actual run-time class R of the target object. This is true even if the target object is an array instance. (Note that for invocation mode interface, R necessarily implements T; for invocation mode virtual, R is necessarily either T or a subclass of T.) + +
  • If the invocation mode is super, then S is initially the qualifying type (§13.1) of the method invocation. +
+The dynamic method lookup uses the following procedure to search class S, and then the superclasses of class S, as necessary, for method m.

+ +Let X be the compile-time type of the target reference of the method invocation.

+

    + +
  1. If class S contains a declaration for a non-abstract method named m with the same descriptor (same number of parameters, the same parameter types, and the same return type) required by the method invocation as determined at compile time (§15.12.3), then: +
      + +
    • If the invocation mode is super or interface, then this is the method to be invoked, and the procedure terminates. + +
    • If the invocation mode is virtual, and the declaration in S overrides (§8.4.6.1) X.m, then the method declared in S is the method to be invoked, and the procedure terminates. +
    + +
  2. Otherwise, if S has a superclass, this same lookup procedure is performed recursively using the direct superclass of S in place of S; the method to be invoked is the result of the recursive invocation of this lookup procedure. +
+ +The above procedure will always find a non-abstract, accessible method to invoke, provided that all classes and interfaces in the program have been consistently compiled. However, if this is not the case, then various errors may occur. The specification of the behavior of a Java virtual machine under these circumstances is given by The Java Virtual Machine Specification, Second Edition.

+ +We note that the dynamic lookup process, while described here explicitly, will often be implemented implicitly, for example as a side-effect of the construction and use of per-class method dispatch tables, or the construction of other per-class structures used for efficient dispatch.

+ +

15.12.4.5 Create Frame, Synchronize, Transfer Control

+ +A method m in some class S has been identified as the one to be invoked.

+ +Now a new activation frame is created, containing the target reference (if any) and the argument values (if any), as well as enough space for the local variables and stack for the method to be invoked and any other bookkeeping information that may be required by the implementation (stack pointer, program counter, reference to previous activation frame, and the like). If there is not sufficient memory available to create such an activation frame, an OutOfMemoryError is thrown.

+ +The newly created activation frame becomes the current activation frame. The effect of this is to assign the argument values to corresponding freshly created parameter variables of the method, and to make the target reference available as this, if there is a target reference. Before each argument value is assigned to its corresponding parameter variable, it is subjected to method invocation conversion (§5.3), which includes any required value set conversion (§5.1.8).

+ +If the method m is a native method but the necessary native, implementation-dependent binary code has not been loaded or otherwise cannot be dynamically linked, then an UnsatisfiedLinkError is thrown.

+ +If the method m is not synchronized, control is transferred to the body of the method m to be invoked.

+ +If the method m is synchronized, then an object must be locked before the transfer of control. No further progress can be made until the current thread can obtain the lock. If there is a target reference, then the target must be locked; otherwise the Class object for class S, the class of the method m, must be locked. Control is then transferred to the body of the method m to be invoked. The object is automatically unlocked when execution of the body of the method has completed, whether normally or abruptly. The locking and unlocking behavior is exactly as if the body of the method were embedded in a synchronized statement (§14.18).

+ +

15.12.4.6 Example: Target Reference and Static Methods

+ +When a target reference is computed and then discarded because the invocation mode is static, the reference is not examined to see whether it is null:

+

class Test {
+	static void mountain() {
+		System.out.println("Monadnock");
+	}
+	static Test favorite(){
+		System.out.print("Mount ");
+		return null;
+	}
+	public static void main(String[] args) {
+		favorite().mountain();
+	}
+}
+
+which prints:

+

Mount Monadnock
+
+Here favorite returns null, yet no NullPointerException is thrown.

+ +

15.12.4.7 Example: Evaluation Order

+ +As part of an instance method invocation (§15.12), there is an expression that denotes the object to be invoked. This expression appears to be fully evaluated before any part of any argument expression to the method invocation is evaluated.

+ +So, for example, in:

+

class Test {
+	public static void main(String[] args) {
+		String s = "one";
+		if (s.startsWith(s = "two"))
+			System.out.println("oops");
+	}
+}
+
+the occurrence of s before ".startsWith" is evaluated first, before the argument expression s="two". Therefore, a reference to the string "one" is remembered as the target reference before the local variable s is changed to refer to the string "two". As a result, the startsWith method is invoked for target object "one" with argument "two", so the result of the invocation is false, as the string "one" does not start with "two". It follows that the test program does not print "oops".

+ +

15.12.4.8 Example: Overriding

+ +In the example:

+

class Point {
+	final int EDGE = 20;
+	int x, y;
+	void move(int dx, int dy) {
+		x += dx; y += dy;
+		if (Math.abs(x) >= EDGE || Math.abs(y) >= EDGE)
+			clear();
+	}
+	void clear() {
+		System.out.println("\tPoint clear");
+		x = 0; y = 0;
+	}
+}
+class ColoredPoint extends Point {
+	int color;
+	void clear() {
+		System.out.println("\tColoredPoint clear");
+		super.clear();
+		color = 0;
+	}
+}
+
+the subclass ColoredPoint extends the clear abstraction defined by its superclass Point. It does so by overriding the clear method with its own method, which invokes the clear method of its superclass, using the form super.clear.

+ +This method is then invoked whenever the target object for an invocation of clear is a ColoredPoint. Even the method move in Point invokes the clear method of class ColoredPoint when the class of this is ColoredPoint, as shown by the output of this test program:

+

class Test {
+	public static void main(String[] args) {
+		Point p = new Point();
+		System.out.println("p.move(20,20):");
+		p.move(20, 20);
+		ColoredPoint cp = new ColoredPoint();
+		System.out.println("cp.move(20,20):");
+		cp.move(20, 20);
+		p = new ColoredPoint();
+		System.out.println("p.move(20,20), p colored:");
+		p.move(20, 20);
+	}
+}
+
+which is:

+

p.move(20,20):
+	Point clear
+cp.move(20,20):
+	ColoredPoint clear
+	Point clear
+p.move(20,20), p colored:
+	ColoredPoint clear
+	Point clear
+
+ +Overriding is sometimes called "late-bound self-reference"; in this example it means that the reference to clear in the body of Point.move (which is really syntactic shorthand for this.clear) invokes a method chosen "late" (at run time, based on the run-time class of the object referenced by this) rather than a method chosen "early" (at compile time, based only on the type of this). This provides the programmer a powerful way of extending abstractions and is a key idea in object-oriented programming.

+ +

15.12.4.9 Example: Method Invocation using super

+ +An overridden instance method of a superclass may be accessed by using the keyword super to access the members of the immediate superclass, bypassing any overriding declaration in the class that contains the method invocation.

+ +When accessing an instance variable, super means the same as a cast of this (§15.11.2), but this equivalence does not hold true for method invocation. This is demonstrated by the example:

+

class T1 {
+	String s() { return "1"; }
+}
+class T2 extends T1 {
+	String s() { return "2"; }
+}
+class T3 extends T2 {
+	String s() { return "3"; }
+	void test() {
+		System.out.println("s()=\t\t"+s());
+		System.out.println("super.s()=\t"+super.s());
+		System.out.print("((T2)this).s()=\t");
+			System.out.println(((T2)this).s());
+		System.out.print("((T1)this).s()=\t");
+			System.out.println(((T1)this).s());
+	}
+}
+class Test {
+	public static void main(String[] args) {
+		T3 t3 = new T3();
+		t3.test();
+	}
+}
+
+which produces the output:

+

s()=					3
+super.s()=					2
+((T2)this).s()=					3
+((T1)this).s()=					3
+
+ +The casts to types T1 and T2 do not change the method that is invoked, because the instance method to be invoked is chosen according to the run-time class of the object referred to be this. A cast does not change the class of an object; it only checks that the class is compatible with the specified type.

+ +

15.13 Array Access Expressions

+ +An array access expression refers to a variable that is a component of an array.

+

    +ArrayAccess:
+	ExpressionName [ Expression ]
+	PrimaryNoNewArray [ Expression ]
+
+An array access expression contains two subexpressions, the array reference expression (before the left bracket) and the index expression (within the brackets). Note that the array reference expression may be a name or any primary expression that is not an array creation expression (§15.10).

+ +The type of the array reference expression must be an array type (call it T[], an array whose components are of type T) or a compile-time error results. Then the type of the array access expression is T.

+ +The index expression undergoes unary numeric promotion (§5.6.1); the promoted type must be int.

+ +The result of an array reference is a variable of type T, namely the variable within the array selected by the value of the index expression. This resulting variable, which is a component of the array, is never considered final, even if the array reference was obtained from a final variable.

+ +

15.13.1 Runtime Evaluation of Array Access

+ +An array access expression is evaluated using the following procedure:

+

    +
  • First, the array reference expression is evaluated. If this evaluation completes abruptly, then the array access completes abruptly for the same reason and the index expression is not evaluated. + +
  • Otherwise, the index expression is evaluated. If this evaluation completes abruptly, then the array access completes abruptly for the same reason. + +
  • Otherwise, if the value of the array reference expression is null, then a NullPointerException is thrown. + +
  • Otherwise, the value of the array reference expression indeed refers to an array. If the value of the index expression is less than zero, or greater than or equal to the array's length, then an ArrayIndexOutOfBoundsException is thrown. + +
  • Otherwise, the result of the array access is the variable of type T, within the array, selected by the value of the index expression. (Note that this resulting variable, which is a component of the array, is never considered final, even if the array reference expression is a final variable.) +
+

15.13.2 Examples: Array Access Evaluation Order

+ +In an array access, the expression to the left of the brackets appears to be fully evaluated before any part of the expression within the brackets is evaluated. For example, in the (admittedly monstrous) expression a[(a=b)[3]], the expression a is fully evaluated before the expression (a=b)[3]; this means that the original value of a is fetched and remembered while the expression (a=b)[3] is evaluated. This array referenced by the original value of a is then subscripted by a value that is element 3 of another array (possibly the same array) that was referenced by b and is now also referenced by a.

+ +Thus, the example:

+

class Test {
+	public static void main(String[] args) {
+		int[] a = { 11, 12, 13, 14 };
+		int[] b = { 0, 1, 2, 3 };
+		System.out.println(a[(a=b)[3]]);
+	}
+}
+
+prints:

+

14
+
+because the monstrous expression's value is equivalent to a[b[3]] or a[3] or 14.

+ +If evaluation of the expression to the left of the brackets completes abruptly, no part of the expression within the brackets will appear to have been evaluated. Thus, the example:

+

class Test {
+	public static void main(String[] args) {
+		int index = 1;
+		try {
+			skedaddle()[index=2]++;
+		} catch (Exception e) {
+			System.out.println(e + ", index=" + index);
+		}
+	}
+	static int[] skedaddle() throws Exception {
+		throw new Exception("Ciao");
+	}
+}
+
+prints:

+

java.lang.Exception: Ciao, index=1
+
+because the embedded assignment of 2 to index never occurs.

+ +If the array reference expression produces null instead of a reference to an array, then a NullPointerException is thrown at run time, but only after all parts of the array access expression have been evaluated and only if these evaluations completed normally. Thus, the example:

+

class Test {
+	public static void main(String[] args) {
+		int index = 1;
+		try {
+			nada()[index=2]++;
+		} catch (Exception e) {
+			System.out.println(e + ", index=" + index);
+		}
+	}
+	static int[] nada() { return null; }
+}
+
+prints:

+

java.lang.NullPointerException, index=2
+
+because the embedded assignment of 2 to index occurs before the check for a null pointer. As a related example, the program:

+

class Test {
+	public static void main(String[] args) {
+		int[] a = null;
+		try {
+			int i = a[vamoose()];
+			System.out.println(i);
+		} catch (Exception e) {
+			System.out.println(e);
+		}
+	}
+	static int vamoose() throws Exception {
+		throw new Exception("Twenty-three skidoo!");
+	}
+}
+
+always prints:

+

java.lang.Exception: Twenty-three skidoo!
+
+ +A NullPointerException never occurs, because the index expression must be completely evaluated before any part of the indexing operation occurs, and that includes the check as to whether the value of the left-hand operand is null.

+ +

15.14 Postfix Expressions

+ +Postfix expressions include uses of the postfix ++ and -- operators. Also, as discussed in §15.8, names are not considered to be primary expressions, but are handled separately in the grammar to avoid certain ambiguities. They become interchangeable only here, at the level of precedence of postfix expressions.

+

    +PostfixExpression:
+	Primary
+	ExpressionName
+	PostIncrementExpression
+	PostDecrementExpression
+
+

15.14.1 Postfix Increment Operator ++

+
    +PostIncrementExpression:
+	PostfixExpression ++
+
+A postfix expression followed by a ++ operator is a postfix increment expression. The result of the postfix expression must be a variable of a numeric type, or a compile-time error occurs. The type of the postfix increment expression is the type of the variable. The result of the postfix increment expression is not a variable, but a value.

+ +At run time, if evaluation of the operand expression completes abruptly, then the postfix increment expression completes abruptly for the same reason and no incrementation occurs. Otherwise, the value 1 is added to the value of the variable and the sum is stored back into the variable. Before the addition, binary numeric promotion (§5.6.2) is performed on the value 1 and the value of the variable. If necessary, the sum is narrowed by a narrowing primitive conversion (§5.1.3) to the type of the variable before it is stored. The value of the postfix increment expression is the value of the variable before the new value is stored.

+ +Note that the binary numeric promotion mentioned above may include value set conversion (§5.1.8). If necessary, value set conversion is applied to the sum prior to its being stored in the variable.

+ +A variable that is declared final cannot be incremented, because when an access of a final variable is used as an expression, the result is a value, not a variable. Thus, it cannot be used as the operand of a postfix increment operator.

+ +

15.14.2 Postfix Decrement Operator --

+
    +PostDecrementExpression:
+	PostfixExpression --
+
+A postfix expression followed by a -- operator is a postfix decrement expression. The result of the postfix expression must be a variable of a numeric type, or a compile-time error occurs. The type of the postfix decrement expression is the type of the variable. The result of the postfix decrement expression is not a variable, but a value.

+ +At run time, if evaluation of the operand expression completes abruptly, then the postfix decrement expression completes abruptly for the same reason and no decrementation occurs. Otherwise, the value 1 is subtracted from the value of the variable and the difference is stored back into the variable. Before the subtraction, binary numeric promotion (§5.6.2) is performed on the value 1 and the value of the variable. If necessary, the difference is narrowed by a narrowing primitive conversion (§5.1.3) to the type of the variable before it is stored. The value of the postfix decrement expression is the value of the variable before the new value is stored.

+ +Note that the binary numeric promotion mentioned above may include value set conversion (§5.1.8). If necessary, value set conversion is applied to the difference prior to its being stored in the variable.

+ +A variable that is declared final cannot be decremented, because when an access of a final variable is used as an expression, the result is a value, not a variable. Thus, it cannot be used as the operand of a postfix decrement operator.

+ +

15.15 Unary Operators

+ +The unary operators include +, -, ++, --, ~, !, and cast operators. Expressions with unary operators group right-to-left, so that -~x means the same as -(~x).

+

    +UnaryExpression:
+	PreIncrementExpression
+	PreDecrementExpression
+	+ UnaryExpression
+	- UnaryExpression
+	UnaryExpressionNotPlusMinus
+
+PreIncrementExpression:
+	++ UnaryExpression
+
+PreDecrementExpression:
+	-- UnaryExpression
+
+UnaryExpressionNotPlusMinus:
+	PostfixExpression
+	~ UnaryExpression
+	! UnaryExpression
+	CastExpression
+
+The following productions from §15.16 are repeated here for convenience:

+

    +CastExpression:
+	( PrimitiveType ) UnaryExpression
+	( ReferenceType ) UnaryExpressionNotPlusMinus
+
+

15.15.1 Prefix Increment Operator ++

+ +A unary expression preceded by a ++ operator is a prefix increment expression. The result of the unary expression must be a variable of a numeric type, or a compile-time error occurs. The type of the prefix increment expression is the type of the variable. The result of the prefix increment expression is not a variable, but a value.

+ +At run time, if evaluation of the operand expression completes abruptly, then the prefix increment expression completes abruptly for the same reason and no incrementation occurs. Otherwise, the value 1 is added to the value of the variable and the sum is stored back into the variable. Before the addition, binary numeric promotion (§5.6.2) is performed on the value 1 and the value of the variable. If necessary, the sum is narrowed by a narrowing primitive conversion (§5.1.3) to the type of the variable before it is stored. The value of the prefix increment expression is the value of the variable after the new value is stored.

+ +Note that the binary numeric promotion mentioned above may include value set conversion (§5.1.8). If necessary, value set conversion is applied to the sum prior to its being stored in the variable.

+ +A variable that is declared final cannot be incremented, because when an access of a final variable is used as an expression, the result is a value, not a variable. Thus, it cannot be used as the operand of a prefix increment operator.

+ +

15.15.2 Prefix Decrement Operator --

+ +A unary expression preceded by a -- operator is a prefix decrement expression. The result of the unary expression must be a variable of a numeric type, or a compile-time error occurs. The type of the prefix decrement expression is the type of the variable. The result of the prefix decrement expression is not a variable, but a value.

+ +At run time, if evaluation of the operand expression completes abruptly, then the prefix decrement expression completes abruptly for the same reason and no decrementation occurs. Otherwise, the value 1 is subtracted from the value of the variable and the difference is stored back into the variable. Before the subtraction, binary numeric promotion (§5.6.2) is performed on the value 1 and the value of the variable. If necessary, the difference is narrowed by a narrowing primitive conversion (§5.1.3) to the type of the variable before it is stored. The value of the prefix decrement expression is the value of the variable after the new value is stored.

+ +Note that the binary numeric promotion mentioned above may include value set conversion (§5.1.8). If necessary, format conversion is applied to the difference prior to its being stored in the variable.

+ +A variable that is declared final cannot be decremented, because when an access of a final variable is used as an expression, the result is a value, not a variable. Thus, it cannot be used as the operand of a prefix decrement operator.

+ +

15.15.3 Unary Plus Operator +

+ +The type of the operand expression of the unary + operator must be a primitive numeric type, or a compile-time error occurs. Unary numeric promotion (§5.6.1) is performed on the operand. The type of the unary plus expression is the promoted type of the operand. The result of the unary plus expression is not a variable, but a value, even if the result of the operand expression is a variable.

+ +At run time, the value of the unary plus expression is the promoted value of the operand.

+ +

15.15.4 Unary Minus Operator -

+ +The type of the operand expression of the unary - operator must be a primitive numeric type, or a compile-time error occurs. Unary numeric promotion (§5.6.1) is performed on the operand. The type of the unary minus expression is the promoted type of the operand.

+ +Note that unary numeric promotion performs value set conversion (§5.1.8). Whatever value set the promoted operand value is drawn from, the unary negation operation is carried out and the result is drawn from that same value set. That result is then subject to further value set conversion.

+ +At run time, the value of the unary minus expression is the arithmetic negation of the promoted value of the operand.

+ +For integer values, negation is the same as subtraction from zero. The Java programming language uses two's-complement representation for integers, and the range of two's-complement values is not symmetric, so negation of the maximum negative int or long results in that same maximum negative number. Overflow occurs in this case, but no exception is thrown. For all integer values x, -x equals (~x)+1.

+ +For floating-point values, negation is not the same as subtraction from zero, because if x is +0.0, then 0.0-x is +0.0, but -x is -0.0. Unary minus merely inverts the sign of a floating-point number. Special cases of interest:

+

    +
  • If the operand is NaN, the result is NaN (recall that NaN has no sign). + +
  • If the operand is an infinity, the result is the infinity of opposite sign. + +
  • If the operand is a zero, the result is the zero of opposite sign. +
+

15.15.5 Bitwise Complement Operator ~

+ +The type of the operand expression of the unary ~ operator must be a primitive integral type, or a compile-time error occurs. Unary numeric promotion (§5.6.1) is performed on the operand. The type of the unary bitwise complement expression is the promoted type of the operand.

+ +At run time, the value of the unary bitwise complement expression is the bitwise complement of the promoted value of the operand; note that, in all cases, ~x equals (-x)-1.

+ +

15.15.6 Logical Complement Operator !

+ +The type of the operand expression of the unary ! operator must be boolean, or a compile-time error occurs. The type of the unary logical complement expression is boolean.

+ +At run time, the value of the unary logical complement expression is true if the operand value is false and false if the operand value is true.

+ +

15.16 Cast Expressions

+ +A cast expression converts, at run time, a value of one numeric type to a similar value of another numeric type; or confirms, at compile time, that the type of an expression is boolean; or checks, at run time, that a reference value refers to an object whose class is compatible with a specified reference type.

+

    +CastExpression:
+	( PrimitiveType Dimsopt ) UnaryExpression
+	( ReferenceType ) UnaryExpressionNotPlusMinus
+
+See §15.15 for a discussion of the distinction between UnaryExpression and UnaryExpressionNotPlusMinus.

+ +The type of a cast expression is the type whose name appears within the parentheses. (The parentheses and the type they contain are sometimes called the cast operator.) The result of a cast expression is not a variable, but a value, even if the result of the operand expression is a variable.

+ +A cast operator has no effect on the choice of value set (§4.2.3) for a value of type float or type double. Consequently, a cast to type float within an expression that is not FP-strict (§15.4) does not necessarily cause its value to be converted to an element of the float value set, and a cast to type double within an expression that is not FP-strict does not necessarily cause its value to be converted to an element of the double value set.

+ +At run time, the operand value is converted by casting conversion (§5.5) to the type specified by the cast operator.

+ +Not all casts are permitted by the language. Some casts result in an error at compile time. For example, a primitive value may not be cast to a reference type. Some casts can be proven, at compile time, always to be correct at run time. For example, it is always correct to convert a value of a class type to the type of its superclass; such a cast should require no special action at run time. Finally, some casts cannot be proven to be either always correct or always incorrect at compile time. Such casts require a test at run time.

+ +A ClassCastException is thrown if a cast is found at run time to be impermissible.

+ +

15.17 Multiplicative Operators

+ +The operators *, /, and % are called the multiplicative operators. They have the same precedence and are syntactically left-associative (they group left-to-right).

+

    +MultiplicativeExpression:
+	UnaryExpression
+	MultiplicativeExpression * UnaryExpression
+	MultiplicativeExpression / UnaryExpression
+	MultiplicativeExpression % UnaryExpression
+
+The type of each of the operands of a multiplicative operator must be a primitive numeric type, or a compile-time error occurs. Binary numeric promotion is performed on the operands (§5.6.2). The type of a multiplicative expression is the promoted type of its operands. If this promoted type is int or long, then integer arithmetic is performed; if this promoted type is float or double, then floating-point arithmetic is performed.

+ +Note that binary numeric promotion performs value set conversion (§5.1.8).

+ +

15.17.1 Multiplication Operator *

+ +The binary * operator performs multiplication, producing the product of its operands. Multiplication is a commutative operation if the operand expressions have no side effects. While integer multiplication is associative when the operands are all of the same type, floating-point multiplication is not associative.

+ +If an integer multiplication overflows, then the result is the low-order bits of the mathematical product as represented in some sufficiently large two's-complement format. As a result, if overflow occurs, then the sign of the result may not be the same as the sign of the mathematical product of the two operand values.

+ +The result of a floating-point multiplication is governed by the rules of IEEE 754 arithmetic:

+

    +
  • If either operand is NaN, the result is NaN. + +
  • If the result is not NaN, the sign of the result is positive if both operands have the same sign, and negative if the operands have different signs. + +
  • Multiplication of an infinity by a zero results in NaN. + +
  • Multiplication of an infinity by a finite value results in a signed infinity. The sign is determined by the rule stated above. + +
  • In the remaining cases, where neither an infinity nor NaN is involved, the exact mathematical product is computed. A floating-point value set is then chosen: +
      + +
    • If the multiplication expression is FP-strict (§15.4): +
        + +
      • If the type of the multiplication expression is float, then the float value set must be chosen. + +
      • If the type of the multiplication expression is double, then the double value set must be chosen. +
      + +
    • If the multiplication expression is not FP-strict: +
        + +
      • If the type of the multiplication expression is float, then either the float value set or the float-extended-exponent value set may be chosen, at the whim of the implementation. + +
      • If the type of the multiplication expression is double, then either the double value set or the double-extended-exponent value set may be chosen, at the whim of the implementation. +
      +
    +
    +

    Next, a value must be chosen from the chosen value set to represent the product. If the magnitude of the product is too large to represent, we say the operation overflows; the result is then an infinity of appropriate sign. Otherwise, the product is rounded to the nearest value in the chosen value set using IEEE 754 round-to-nearest mode. The Java programming language requires support of gradual underflow as defined by IEEE 754 (§4.2.4). +
+Despite the fact that overflow, underflow, or loss of information may occur, evaluation of a multiplication operator * never throws a run-time exception.

+ +

15.17.2 Division Operator /

+ +The binary / operator performs division, producing the quotient of its operands. The left-hand operand is the dividend and the right-hand operand is the divisor.

+ +Integer division rounds toward 0. That is, the quotient produced for operands n and d that are integers after binary numeric promotion (§5.6.2) is an integer value q whose magnitude is as large as possible while satisfying ; moreover, q is positive when and n and d have the same sign, but q is negative when and n and d have opposite signs. There is one special case that does not satisfy this rule: if the dividend is the negative integer of largest possible magnitude for its type, and the divisor is -1, then integer overflow occurs and the result is equal to the dividend. Despite the overflow, no exception is thrown in this case. On the other hand, if the value of the divisor in an integer division is 0, then an ArithmeticException is thrown.

+ +The result of a floating-point division is determined by the specification of IEEE arithmetic:

+

    +
  • If either operand is NaN, the result is NaN. + +
  • If the result is not NaN, the sign of the result is positive if both operands have the same sign, negative if the operands have different signs. + +
  • Division of an infinity by an infinity results in NaN. + +
  • Division of an infinity by a finite value results in a signed infinity. The sign is determined by the rule stated above. + +
  • Division of a finite value by an infinity results in a signed zero. The sign is determined by the rule stated above. + +
  • Division of a zero by a zero results in NaN; division of zero by any other finite value results in a signed zero. The sign is determined by the rule stated above. + +
  • Division of a nonzero finite value by a zero results in a signed infinity. The sign is determined by the rule stated above. + +
  • In the remaining cases, where neither an infinity nor NaN is involved, the exact mathematical quotient is computed. A floating-point value set is then chosen: +
      + +
    • If the division expression is FP-strict (§15.4): +
        + +
      • If the type of the division expression is float, then the float value set must be chosen. + +
      • If the type of the division expression is double, then the double value set must be chosen. +
      + +
    • If the division expression is not FP-strict: +
        + +
      • If the type of the division expression is float, then either the float value set or the float-extended-exponent value set may be chosen, at the whim of the implementation. + +
      • If the type of the division expression is double, then either the double value set or the double-extended-exponent value set may be chosen, at the whim of the implementation. +
      +
    +
    +Next, a value must be chosen from the chosen value set to represent the quotient. If the magnitude of the quotient is too large to represent, we say the operation overflows; the result is then an infinity of appropriate sign. Otherwise, the quotient is rounded to the nearest value in the chosen value set using IEEE 754 round-to-nearest mode. The Java programming language requires support of gradual underflow as defined by IEEE 754 (§4.2.4). +
+Despite the fact that overflow, underflow, division by zero, or loss of information may occur, evaluation of a floating-point division operator / never throws a run-time exception

+ +

15.17.3 Remainder Operator %

+ +The binary % operator is said to yield the remainder of its operands from an implied division; the left-hand operand is the dividend and the right-hand operand is the divisor.

+ +In C and C++, the remainder operator accepts only integral operands, but in the Java programming language, it also accepts floating-point operands.

+ +The remainder operation for operands that are integers after binary numeric promotion (§5.6.2) produces a result value such that (a/b)*b+(a%b) is equal to a. This identity holds even in the special case that the dividend is the negative integer of largest possible magnitude for its type and the divisor is -1 (the remainder is 0). It follows from this rule that the result of the remainder operation can be negative only if the dividend is negative, and can be positive only if the dividend is positive; moreover, the magnitude of the result is always less than the magnitude of the divisor. If the value of the divisor for an integer remainder operator is 0, then an ArithmeticException is thrown.Examples:

+

+5%3 produces 2				(note that 5/3 produces 1)
+5%(-3) produces 2			(note that 5/(-3) produces -1)
+(-5)%3 produces -2			(note that (-5)/3 produces -1)
+(-5)%(-3) produces -2			(note that (-5)/(-3) produces 1)
+
+The result of a floating-point remainder operation as computed by the % operator is not the same as that produced by the remainder operation defined by IEEE 754. The IEEE 754 remainder operation computes the remainder from a rounding division, not a truncating division, and so its behavior is not analogous to that of the usual integer remainder operator. Instead, the Java programming language defines % on floating-point operations to behave in a manner analogous to that of the integer remainder operator; this may be compared with the C library function fmod. The IEEE 754 remainder operation may be computed by the library routine Math.IEEEremainder.

+ +The result of a floating-point remainder operation is determined by the rules of IEEE arithmetic:

+

    +
  • If either operand is NaN, the result is NaN. + +
  • If the result is not NaN, the sign of the result equals the sign of the dividend. + +
  • If the dividend is an infinity, or the divisor is a zero, or both, the result is NaN. + +
  • If the dividend is finite and the divisor is an infinity, the result equals the dividend. + +
  • If the dividend is a zero and the divisor is finite, the result equals the dividend. + +
  • In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the floating-point remainder r from the division of a dividend n by a divisor d is defined by the mathematical relation where q is an integer that is negative only if is negative and positive only if is positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d. +
+Evaluation of a floating-point remainder operator % never throws a run-time exception, even if the right-hand operand is zero. Overflow, underflow, or loss of precision cannot occur.

+ +Examples:

+

5.0%3.0 produces 2.0
+5.0%(-3.0) produces 2.0
+(-5.0)%3.0 produces -2.0
+(-5.0)%(-3.0) produces -2.0
+
+

15.18 Additive Operators

+ +The operators + and - are called the additive operators. They have the same precedence and are syntactically left-associative (they group left-to-right).

+

    +AdditiveExpression:
+	MultiplicativeExpression
+	AdditiveExpression + MultiplicativeExpression
+	AdditiveExpression - MultiplicativeExpression
+
+If the type of either operand of a + operator is String, then the operation is string concatenation.

+ +Otherwise, the type of each of the operands of the + operator must be a primitive numeric type, or a compile-time error occurs.

+ +In every case, the type of each of the operands of the binary - operator must be a primitive numeric type, or a compile-time error occurs.

+ +

15.18.1 String Concatenation Operator +

+ +If only one operand expression is of type String, then string conversion is performed on the other operand to produce a string at run time. The result is a reference to a newly created String object that is the concatenation of the two operand strings. The characters of the left-hand operand precede the characters of the right-hand operand in the newly created string.

+ +

15.18.1.1 String Conversion

+ +Any type may be converted to type String by string conversion.

+ +A value x of primitive type T is first converted to a reference value as if by giving it as an argument to an appropriate class instance creation expression:

+

    +
  • If T is boolean, then use new Boolean(x). + +
  • If T is char, then use new Character(x). + +
  • If T is byte, short, or int, then use new Integer(x). + +
  • If T is long, then use new Long(x). + +
  • If T is float, then use new Float(x). + +
  • If T is double, then use new Double(x). +
+This reference value is then converted to type String by string conversion.

+ +Now only reference values need to be considered. If the reference is null, it is converted to the string "null" (four ASCII characters n, u, l, l). Otherwise, the conversion is performed as if by an invocation of the toString method of the referenced object with no arguments; but if the result of invoking the toString method is null, then the string "null" is used instead.

+ +The toString method is defined by the primordial class Object; many classes override it, notably Boolean, Character, Integer, Long, Float, Double, and String.

+ +

15.18.1.2 Optimization of String Concatenation

+ +An implementation may choose to perform conversion and concatenation in one step to avoid creating and then discarding an intermediate String object. To increase the performance of repeated string concatenation, a Java compiler may use the StringBuffer class or a similar technique to reduce the number of intermediate String objects that are created by evaluation of an expression.

+ +For primitive types, an implementation may also optimize away the creation of a wrapper object by converting directly from a primitive type to a string.

+ +

15.18.1.3 Examples of String Concatenation

+ +The example expression:

+

"The square root of 2 is " + Math.sqrt(2)
+
+produces the result:

+

"The square root of 2 is 1.4142135623730952"
+
+ +The + operator is syntactically left-associative, no matter whether it is later determined by type analysis to represent string concatenation or addition. In some cases care is required to get the desired result. For example, the expression:

+

+a + b + c
+
+is always regarded as meaning:

+

(a + b) + c
+
+Therefore the result of the expression:

+

1 + 2 + " fiddlers"
+
+is:

+

"3 fiddlers"
+
+but the result of:

+

"fiddlers " + 1 + 2
+
+is:

+

"fiddlers 12"
+
+ +In this jocular little example:

+

+class Bottles {
+	static void printSong(Object stuff, int n) {
+		String plural = (n == 1) ? "" : "s";
+		loop: while (true) {
+			System.out.println(n + " bottle" + plural
+				+ " of " + stuff + " on the wall,");
+			System.out.println(n + " bottle" + plural
+				+ " of " + stuff + ";");
+			System.out.println("You take one down "
+				+ "and pass it around:");
+			--n;
+			plural = (n == 1) ? "" : "s";
+			if (n == 0)
+				break loop;
+			System.out.println(n + " bottle" + plural
+				+ " of " + stuff + " on the wall!");
+			System.out.println();
+		}
+		System.out.println("No bottles of " +
+								stuff + " on the wall!");
+	}
+}
+
+the method printSong will print a version of a children's song. Popular values for stuff include "pop" and "beer"; the most popular value for n is 100. Here is the output that results from Bottles.printSong("slime", 3):

+

3 bottles of slime on the wall,
+3 bottles of slime;
+You take one down and pass it around:
+2 bottles of slime on the wall!
+
+2 bottles of slime on the wall,
+2 bottles of slime;
+You take one down and pass it around:
+1 bottle of slime on the wall!
+
+1 bottle of slime on the wall,
+1 bottle of slime;
+You take one down and pass it around:
+No bottles of slime on the wall!
+
+ +In the code, note the careful conditional generation of the singular "bottle" when appropriate rather than the plural "bottles"; note also how the string concatenation operator was used to break the long constant string:

+

+"You take one down and pass it around:"
+
+ +into two pieces to avoid an inconveniently long line in the source code.

+ +

15.18.2 Additive Operators (+ and -) for Numeric Types

+ +The binary + operator performs addition when applied to two operands of numeric type, producing the sum of the operands. The binary - operator performs subtraction, producing the difference of two numeric operands.

+ +Binary numeric promotion is performed on the operands (§5.6.2). The type of an additive expression on numeric operands is the promoted type of its operands. If this promoted type is int or long, then integer arithmetic is performed; if this promoted type is float or double, then floating-point arithmetic is performed.

+ +Note that binary numeric promotion performs value set conversion (§5.1.8).

+ +Addition is a commutative operation if the operand expressions have no side effects. Integer addition is associative when the operands are all of the same type, but floating-point addition is not associative.

+ +If an integer addition overflows, then the result is the low-order bits of the mathematical sum as represented in some sufficiently large two's-complement format. If overflow occurs, then the sign of the result is not the same as the sign of the mathematical sum of the two operand values.

+ +The result of a floating-point addition is determined using the following rules of IEEE arithmetic:

+

    +
  • If either operand is NaN, the result is NaN. + +
  • The sum of two infinities of opposite sign is NaN. + +
  • The sum of two infinities of the same sign is the infinity of that sign. + +
  • The sum of an infinity and a finite value is equal to the infinite operand. + +
  • The sum of two zeros of opposite sign is positive zero. + +
  • The sum of two zeros of the same sign is the zero of that sign. + +
  • The sum of a zero and a nonzero finite value is equal to the nonzero operand. + +
  • The sum of two nonzero finite values of the same magnitude and opposite sign is positive zero. + +
  • In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, and the operands have the same sign or have different magnitudes, the exact mathematical sum is computed. A floating-point value set is then chosen: +
      + +
    • If the addition expression is FP-strict (§15.4): +
        + +
      • If the type of the addition expression is float, then the float value set must be chosen. + +
      • If the type of the addition expression is double, then the double value set must be chosen. +
      + +
    • If the addition expression is not FP-strict: +
        + +
      • If the type of the addition expression is float, then either the float value set or the float-extended-exponent value set may be chosen, at the whim of the implementation. + +
      • If the type of the addition expression is double, then either the double value set or the double-extended-exponent value set may be chosen, at the whim of the implementation. +
      +
    +
    +Next, a value must be chosen from the chosen value set to represent the sum. If the magnitude of the sum is too large to represent, we say the operation overflows; the result is then an infinity of appropriate sign. Otherwise, the sum is rounded to the nearest value in the chosen value set using IEEE 754 round-to-nearest mode. The Java programming language requires support of gradual underflow as defined by IEEE 754 (§4.2.4). +
+The binary - operator performs subtraction when applied to two operands of numeric type producing the difference of its operands; the left-hand operand is the minuend and the right-hand operand is the subtrahend. For both integer and floating-point subtraction, it is always the case that a-b produces the same result as a+(-b).

+ +Note that, for integer values, subtraction from zero is the same as negation. However, for floating-point operands, subtraction from zero is not the same as negation, because if x is +0.0, then 0.0-x is +0.0, but -x is -0.0.

+ +Despite the fact that overflow, underflow, or loss of information may occur, evaluation of a numeric additive operator never throws a run-time exception.

+ +

15.19 Shift Operators

+ +The shift operators include left shift <<, signed right shift >>, and unsigned right shift >>>; they are syntactically left-associative (they group left-to-right). The left-hand operand of a shift operator is the value to be shifted; the right-hand operand specifies the shift distance.

+

    +ShiftExpression:
+	AdditiveExpression
+	ShiftExpression << AdditiveExpression
+	ShiftExpression >> AdditiveExpression
+	ShiftExpression >>> AdditiveExpression
+
+The type of each of the operands of a shift operator must be a primitive integral type, or a compile-time error occurs. Binary numeric promotion (§5.6.2) is not performed on the operands; rather, unary numeric promotion (§5.6.1) is performed on each operand separately. The type of the shift expression is the promoted type of the left-hand operand.

+ +If the promoted type of the left-hand operand is int, only the five lowest-order bits of the right-hand operand are used as the shift distance. It is as if the right-hand operand were subjected to a bitwise logical AND operator & (§15.22.1) with the mask value 0x1f. The shift distance actually used is therefore always in the range 0 to 31, inclusive.

+ +If the promoted type of the left-hand operand is long, then only the six lowest-order bits of the right-hand operand are used as the shift distance. It is as if the right-hand operand were subjected to a bitwise logical AND operator & (§15.22.1) with the mask value 0x3f. The shift distance actually used is therefore always in the range 0 to 63, inclusive.

+ +At run time, shift operations are performed on the two's complement integer representation of the value of the left operand.

+ +The value of n<<s is n left-shifted s bit positions; this is equivalent (even if overflow occurs) to multiplication by two to the power s.

+ +The value of n>>s is n right-shifted s bit positions with sign-extension. The resulting value is . For nonnegative values of n, this is equivalent to truncating integer division, as computed by the integer division operator /, by two to the power s.

+ +The value of n>>>s is n right-shifted s bit positions with zero-extension. If n is positive, then the result is the same as that of n>>s; if n is negative, the result is equal to that of the expression (n>>s)+(2<<~s) if the type of the left-hand operand is int, and to the result of the expression (n>>s)+(2L<<~s) if the type of the left-hand operand is long. The added term (2<<~s) or (2L<<~s) cancels out the propagated sign bit. (Note that, because of the implicit masking of the right-hand operand of a shift operator, ~s as a shift distance is equivalent to 31-s when shifting an int value and to 63-s when shifting a long value.)

+ +

15.20 Relational Operators

+ +The relational operators are syntactically left-associative (they group left-to-right), but this fact is not useful; for example, a<b<c parses as (a<b)<c, which is always a compile-time error, because the type of a<b is always boolean and < is not an operator on boolean values.

+

    +RelationalExpression:
+	ShiftExpression
+	RelationalExpression < ShiftExpression
+	RelationalExpression > ShiftExpression
+	RelationalExpression <= ShiftExpression
+	RelationalExpression >= ShiftExpression
+	RelationalExpression instanceof ReferenceType
+
+The type of a relational expression is always boolean.

+ +

15.20.1 Numerical Comparison Operators <, <=, >, and >=

+ +The type of each of the operands of a numerical comparison operator must be a primitive numeric type, or a compile-time error occurs. Binary numeric promotion is performed on the operands (§5.6.2). If the promoted type of the operands is int or long, then signed integer comparison is performed; if this promoted type is float or double, then floating-point comparison is performed.

+ +Note that binary numeric promotion performs value set conversion (§5.1.8). Comparison is carried out accurately on floating-point values, no matter what value sets their representing values were drawn from.

+ +The result of a floating-point comparison, as determined by the specification of the IEEE 754 standard, is:

+

    +
  • If either operand is NaN, then the result is false. + +
  • All values other than NaN are ordered, with negative infinity less than all finite values, and positive infinity greater than all finite values. + +
  • Positive zero and negative zero are considered equal. Therefore, -0.0<0.0 is false, for example, but -0.0<=0.0 is true. (Note, however, that the methods Math.min and Math.max treat negative zero as being strictly smaller than positive zero.) +
+Subject to these considerations for floating-point numbers, the following rules then hold for integer operands or for floating-point operands other than NaN:

+

    +
  • The value produced by the < operator is true if the value of the left-hand operand is less than the value of the right-hand operand, and otherwise is false. + +
  • The value produced by the <= operator is true if the value of the left-hand operand is less than or equal to the value of the right-hand operand, and otherwise is false. + +
  • The value produced by the > operator is true if the value of the left-hand operand is greater than the value of the right-hand operand, and otherwise is false. + +
  • The value produced by the >= operator is true if the value of the left-hand operand is greater than or equal to the value of the right-hand operand, and otherwise is false. +
+

15.20.2 Type Comparison Operator instanceof

+ +The type of a RelationalExpression operand of the instanceof operator must be a reference type or the null type; otherwise, a compile-time error occurs. The ReferenceType mentioned after the instanceof operator must denote a reference type; otherwise, a compile-time error occurs.

+ +At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast (§15.16) to the ReferenceType without raising a ClassCastException. Otherwise the result is false.

+ +If a cast of the RelationalExpression to the ReferenceType would be rejected as a compile-time error, then the instanceof relational expression likewise produces a compile-time error. In such a situation, the result of the instanceof expression could never be true.

+ +Consider the example program:

+

class Point { int x, y; }
+class Element { int atomicNumber; }
+class Test {
+	public static void main(String[] args) {
+		Point p = new Point();
+		Element e = new Element();
+		if (e instanceof Point) {											// compile-time error
+			System.out.println("I get your point!");
+			p = (Point)e;										// compile-time error
+		}
+	}
+}
+
+This example results in two compile-time errors. The cast (Point)e is incorrect because no instance of Element or any of its possible subclasses (none are shown here) could possibly be an instance of any subclass of Point. The instanceof expression is incorrect for exactly the same reason. If, on the other hand, the class Point were a subclass of Element (an admittedly strange notion in this example):

+

class Point extends Element { int x, y; }
+
+then the cast would be possible, though it would require a run-time check, and the instanceof expression would then be sensible and valid. The cast (Point)e would never raise an exception because it would not be executed if the value of e could not correctly be cast to type Point.

+ +

15.21 Equality Operators

+ +The equality operators are syntactically left-associative (they group left-to-right), but this fact is essentially never useful; for example, a==b==c parses as (a==b)==c. The result type of a==b is always boolean, and c must therefore be of type boolean or a compile-time error occurs. Thus, a==b==c does not test to see whether a, b, and c are all equal.

+

    +EqualityExpression:
+	RelationalExpression
+	EqualityExpression == RelationalExpression
+	EqualityExpression != RelationalExpression
+
+The == (equal to) and the != (not equal to) operators are analogous to the relational operators except for their lower precedence. Thus, a<b==c<d is true whenever a<b and c<d have the same truth value.

+ +The equality operators may be used to compare two operands of numeric type, or two operands of type boolean, or two operands that are each of either reference type or the null type. All other cases result in a compile-time error. The type of an equality expression is always boolean.

+ +In all cases, a!=b produces the same result as !(a==b). The equality operators are commutative if the operand expressions have no side effects.

+ +

15.21.1 Numerical Equality Operators == and !=

+ +If the operands of an equality operator are both of primitive numeric type, binary numeric promotion is performed on the operands (§5.6.2). If the promoted type of the operands is int or long, then an integer equality test is performed; if the promoted type is float or double, then a floating-point equality test is performed.

+ +Note that binary numeric promotion performs value set conversion (§5.1.8). Comparison is carried out accurately on floating-point values, no matter what value sets their representing values were drawn from.

+ +Floating-point equality testing is performed in accordance with the rules of the IEEE 754 standard:

+

    +
  • If either operand is NaN, then the result of == is false but the result of != is true. Indeed, the test x!=x is true if and only if the value of x is NaN. (The methods Float.isNaN and Double.isNaN may also be used to test whether a value is NaN.) + +
  • Positive zero and negative zero are considered equal. Therefore, -0.0==0.0 is true, for example. + +
  • Otherwise, two distinct floating-point values are considered unequal by the equality operators. In particular, there is one value representing positive infinity and one value representing negative infinity; each compares equal only to itself, and each compares unequal to all other values. +
+Subject to these considerations for floating-point numbers, the following rules then hold for integer operands or for floating-point operands other than NaN:

+

    +
  • The value produced by the == operator is true if the value of the left-hand operand is equal to the value of the right-hand operand; otherwise, the result is false. + +
  • The value produced by the != operator is true if the value of the left-hand operand is not equal to the value of the right-hand operand; otherwise, the result is false. +
+

15.21.2 Boolean Equality Operators == and !=

+ +If the operands of an equality operator are both of type boolean, then the operation is boolean equality. The boolean equality operators are associative.

+ +The result of == is true if the operands are both true or both false; otherwise, the result is false.

+ +The result of != is false if the operands are both true or both false; otherwise, the result is true. Thus != behaves the same as ^ (§15.22.2) when applied to boolean operands.

+ +

15.21.3 Reference Equality Operators == and !=

+ +If the operands of an equality operator are both of either reference type or the null type, then the operation is object equality.

+ +A compile-time error occurs if it is impossible to convert the type of either operand to the type of the other by a casting conversion (§5.5). The run-time values of the two operands would necessarily be unequal.

+ +At run time, the result of == is true if the operand values are both null or both refer to the same object or array; otherwise, the result is false.

+ +The result of != is false if the operand values are both null or both refer to the same object or array; otherwise, the result is true.

+ +While == may be used to compare references of type String, such an equality test determines whether or not the two operands refer to the same String object. The result is false if the operands are distinct String objects, even if they contain the same sequence of characters. The contents of two strings s and t can be tested for equality by the method invocation s.equals(t). See also §3.10.5.

+ +

15.22 Bitwise and Logical Operators

+ +The bitwise operators and logical operators include the AND operator &, exclusive OR operator ^, and inclusive OR operator |. These operators have different precedence, with & having the highest precedence and | the lowest precedence. Each of these operators is syntactically left-associative (each groups left-to-right). Each operator is commutative if the operand expressions have no side effects. Each operator is associative.

+

    +AndExpression:
+	EqualityExpression
+	AndExpression & EqualityExpression
+
+ExclusiveOrExpression:
+	AndExpression
+	ExclusiveOrExpression ^ AndExpression
+
+InclusiveOrExpression:
+	ExclusiveOrExpression
+	InclusiveOrExpression | ExclusiveOrExpression
+
+The bitwise and logical operators may be used to compare two operands of numeric type or two operands of type boolean. All other cases result in a compile-time error.

+ +

15.22.1 Integer Bitwise Operators &, ^, and |

+ +When both operands of an operator &, ^, or | are of primitive integral type, binary numeric promotion is first performed on the operands (§5.6.2). The type of the bitwise operator expression is the promoted type of the operands.

+ +For &, the result value is the bitwise AND of the operand values.

+ +For ^, the result value is the bitwise exclusive OR of the operand values.

+ +For |, the result value is the bitwise inclusive OR of the operand values.

+ +For example, the result of the expression 0xff00 & 0xf0f0 is 0xf000. The result of 0xff00 ^ 0xf0f0 is 0x0ff0.The result of 0xff00 | 0xf0f0 is 0xfff0.

+ +

15.22.2 Boolean Logical Operators &, ^, and |

+ +When both operands of a &, ^, or | operator are of type boolean, then the type of the bitwise operator expression is boolean.

+ +For &, the result value is true if both operand values are true; otherwise, the result is false.

+ +For ^, the result value is true if the operand values are different; otherwise, the result is false.

+ +For |, the result value is false if both operand values are false; otherwise, the result is true.

+ +

15.23 Conditional-And Operator &&

+ +The && operator is like & (§15.22.2), but evaluates its right-hand operand only if the value of its left-hand operand is true. It is syntactically left-associative (it groups left-to-right). It is fully associative with respect to both side effects and result value; that is, for any expressions a, b, and c, evaluation of the expression ((a)&&(b))&&(c) produces the same result, with the same side effects occurring in the same order, as evaluation of the expression (a)&&((b)&&(c)).

+

    +ConditionalAndExpression:
+	InclusiveOrExpression
+	ConditionalAndExpression && InclusiveOrExpression
+
+Each operand of && must be of type boolean, or a compile-time error occurs. The type of a conditional-and expression is always boolean.

+ +At run time, the left-hand operand expression is evaluated first; if its value is false, the value of the conditional-and expression is false and the right-hand operand expression is not evaluated. If the value of the left-hand operand is true, then the right-hand expression is evaluated and its value becomes the value of the conditional-and expression. Thus, && computes the same result as & on boolean operands. It differs only in that the right-hand operand expression is evaluated conditionally rather than always.

+ +

15.24 Conditional-Or Operator ||

+ +The || operator is like | (§15.22.2), but evaluates its right-hand operand only if the value of its left-hand operand is false. It is syntactically left-associative (it groups left-to-right). It is fully associative with respect to both side effects and result value; that is, for any expressions a, b, and c, evaluation of the expression ((a)||(b))||(c) produces the same result, with the same side effects occurring in the same order, as evaluation of the expression (a)||((b)||(c)).

+

    +ConditionalOrExpression:
+	ConditionalAndExpression
+	ConditionalOrExpression || ConditionalAndExpression
+
+Each operand of || must be of type boolean, or a compile-time error occurs. The type of a conditional-or expression is always boolean.

+ +At run time, the left-hand operand expression is evaluated first; if its value is true, the value of the conditional-or expression is true and the right-hand operand expression is not evaluated. If the value of the left-hand operand is false, then the right-hand expression is evaluated and its value becomes the value of the conditional-or expression.

+ +Thus, || computes the same result as | on boolean operands. It differs only in that the right-hand operand expression is evaluated conditionally rather than always.

+ +

15.25 Conditional Operator ? :

+ +The conditional operator ? : uses the boolean value of one expression to decide which of two other expressions should be evaluated.

+ +The conditional operator is syntactically right-associative (it groups right-to-left), so that a?b:c?d:e?f:g means the same as a?b:(c?d:(e?f:g)).

+

    +ConditionalExpression:
+	ConditionalOrExpression
+	ConditionalOrExpression ? Expression : ConditionalExpression
+
+The conditional operator has three operand expressions; ? appears between the first and second expressions, and : appears between the second and third expressions.

+ +The first expression must be of type boolean, or a compile-time error occurs.

+ +The conditional operator may be used to choose between second and third operands of numeric type, or second and third operands of type boolean, or second and third operands that are each of either reference type or the null type. All other cases result in a compile-time error.

+ +Note that it is not permitted for either the second or the third operand expression to be an invocation of a void method. In fact, it is not permitted for a conditional expression to appear in any context where an invocation of a void method could appear (§14.8).

+ +The type of a conditional expression is determined as follows:

+

    +
  • If the second and third operands have the same type (which may be the null type), then that is the type of the conditional expression. + +
  • Otherwise, if the second and third operands have numeric type, then there are several cases: +
      + +
    • If one of the operands is of type byte and the other is of type short, then the type of the conditional expression is short. + +
    • If one of the operands is of type T where T is byte, short, or char, and the other operand is a constant expression of type int whose value is representable in type T, then the type of the conditional expression is T. + +
    • Otherwise, binary numeric promotion (§5.6.2) is applied to the operand types, and the type of the conditional expression is the promoted type of the second and third operands. Note that binary numeric promotion performs value set conversion (§5.1.8). +
    + +
  • If one of the second and third operands is of the null type and the type of the other is a reference type, then the type of the conditional expression is that reference type. + +
  • If the second and third operands are of different reference types, then it must be possible to convert one of the types to the other type (call this latter type T) by assignment conversion (§5.2); the type of the conditional expression is T. It is a compile-time error if neither type is assignment compatible with the other type. +
+At run time, the first operand expression of the conditional expression is evaluated first; its boolean value is then used to choose either the second or the third operand expression:

+

    +
  • If the value of the first operand is true, then the second operand expression is chosen. + +
  • If the value of the first operand is false, then the third operand expression is chosen. +
+The chosen operand expression is then evaluated and the resulting value is converted to the type of the conditional expression as determined by the rules stated above. The operand expression not chosen is not evaluated for that particular evaluation of the conditional expression.

+ +

15.26 Assignment Operators

+ +There are 12 assignment operators; all are syntactically right-associative (they group right-to-left). Thus, a=b=c means a=(b=c), which assigns the value of c to b and then assigns the value of b to a.

+

    +AssignmentExpression:
+	ConditionalExpression
+	Assignment
+
+Assignment:
+	LeftHandSide AssignmentOperator AssignmentExpression
+
+LeftHandSide:
+	ExpressionName
+	FieldAccess
+	ArrayAccess
+
+AssignmentOperator: one of
+	= *= /= %= += -= <<= >>= >>>= &= ^= |=
+
+The result of the first operand of an assignment operator must be a variable, or a compile-time error occurs. This operand may be a named variable, such as a local variable or a field of the current object or class, or it may be a computed variable, as can result from a field access (§15.11) or an array access (§15.13). The type of the assignment expression is the type of the variable.

+ +At run time, the result of the assignment expression is the value of the variable after the assignment has occurred. The result of an assignment expression is not itself a variable.

+ +A variable that is declared final cannot be assigned to (unless it is a blank final variable (§4.5.4)), because when an access of a final variable is used as an expression, the result is a value, not a variable, and so it cannot be used as the first operand of an assignment operator.

+ +

15.26.1 Simple Assignment Operator =

+ +A compile-time error occurs if the type of the right-hand operand cannot be converted to the type of the variable by assignment conversion (§5.2).

+ +At run time, the expression is evaluated in one of two ways. If the left-hand operand expression is not an array access expression, then three steps are required:

+

    +
  • First, the left-hand operand is evaluated to produce a variable. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason; the right-hand operand is not evaluated and no assignment occurs. + +
  • Otherwise, the right-hand operand is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs. + +
  • Otherwise, the value of the right-hand operand is converted to the type of the left-hand variable, is subjected to value set conversion (§5.1.8) to the appropriate standard value set (not an extended-exponent value set), and the result of the conversion is stored into the variable. +
+If the left-hand operand expression is an array access expression (§15.13), then many steps are required:

+

    +
  • First, the array reference subexpression of the left-hand operand array access expression is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason; the index subexpression (of the left-hand operand array access expression) and the right-hand operand are not evaluated and no assignment occurs. + +
  • Otherwise, the index subexpression of the left-hand operand array access expression is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and the right-hand operand is not evaluated and no assignment occurs. + +
  • Otherwise, the right-hand operand is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs. + +
  • Otherwise, if the value of the array reference subexpression is null, then no assignment occurs and a NullPointerException is thrown. + +
  • Otherwise, the value of the array reference subexpression indeed refers to an array. If the value of the index subexpression is less than zero, or greater than or equal to the length of the array, then no assignment occurs and an ArrayIndexOutOfBoundsException is thrown. + +
  • Otherwise, the value of the index subexpression is used to select a component of the array referred to by the value of the array reference subexpression. This component is a variable; call its type SC. Also, let TC be the type of the left-hand operand of the assignment operator as determined at compile time. +
      + +
    • If TC is a primitive type, then SC is necessarily the same as TC. The value of the right-hand operand is converted to the type of the selected array component, is subjected to value set conversion (§5.1.8) to the appropriate standard value set (not an extended-exponent value set), and the result of the conversion is stored into the array component. + +
    • If TC is a reference type, then SC may not be the same as TC, but rather a type that extends or implements TC. Let RC be the class of the object referred to by the value of the right-hand operand at run time. +
      +The compiler may be able to prove at compile time that the array component will be of type TC exactly (for example, TC might be final). But if the compiler cannot prove at compile time that the array component will be of type TC exactly, then a check must be performed at run time to ensure that the class RC is assignment compatible (§5.2) with the actual type SC of the array component. This check is similar to a narrowing cast (§5.5, §15.16), except that if the check fails, an ArrayStoreException is thrown rather than a ClassCastException. Therefore:

      +

        + +
      • If class RC is not assignable to type SC, then no assignment occurs and an ArrayStoreException is thrown. +
      +
+Otherwise, the reference value of the right-hand operand is stored into the selected array component.

+ +The rules for assignment to an array component are illustrated by the following example program:

+

class ArrayReferenceThrow extends RuntimeException { }
+class IndexThrow extends RuntimeException { }
+class RightHandSideThrow extends RuntimeException { }
+class IllustrateSimpleArrayAssignment {
+	static Object[] objects = { new Object(), new Object() };
+	static Thread[] threads = { new Thread(), new Thread() };
+	static Object[] arrayThrow() {
+		throw new ArrayReferenceThrow();
+	}
+	static int indexThrow() { throw new IndexThrow(); }
+	static Thread rightThrow() {
+		throw new RightHandSideThrow();
+	}
+	static String name(Object q) {
+		String sq = q.getClass().getName();
+		int k = sq.lastIndexOf('.');
+		return (k < 0) ? sq : sq.substring(k+1);
+	}
+	static void testFour(Object[] x, int j, Object y) {
+		String sx = x == null ? "null" : name(x[0]) + "s";
+		String sy = name(y);
+		System.out.println();
+		try {
+			System.out.print(sx + "[throw]=throw => ");
+			x[indexThrow()] = rightThrow();
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print(sx + "[throw]=" + sy + " => ");
+			x[indexThrow()] = y;
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print(sx + "[" + j + "]=throw => ");
+			x[j] = rightThrow();
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print(sx + "[" + j + "]=" + sy + " => ");
+			x[j] = y;
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+	}
+	public static void main(String[] args) {
+		try {
+			System.out.print("throw[throw]=throw => ");
+			arrayThrow()[indexThrow()] = rightThrow();
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print("throw[throw]=Thread => ");
+			arrayThrow()[indexThrow()] = new Thread();
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print("throw[1]=throw => ");
+			arrayThrow()[1] = rightThrow();
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print("throw[1]=Thread => ");
+			arrayThrow()[1] = new Thread();
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		testFour(null, 1, new StringBuffer());
+		testFour(null, 1, new StringBuffer());
+		testFour(null, 9, new Thread());
+		testFour(null, 9, new Thread());
+		testFour(objects, 1, new StringBuffer());
+		testFour(objects, 1, new Thread());
+		testFour(objects, 9, new StringBuffer());
+		testFour(objects, 9, new Thread());
+		testFour(threads, 1, new StringBuffer());
+		testFour(threads, 1, new Thread());
+		testFour(threads, 9, new StringBuffer());
+		testFour(threads, 9, new Thread());
+	}
+}
+
+This program prints:

+

throw[throw]=throw => ArrayReferenceThrow
+throw[throw]=Thread => ArrayReferenceThrow
+throw[1]=throw => ArrayReferenceThrow
+throw[1]=Thread => ArrayReferenceThrow
+null[throw]=throw => IndexThrow
+null[throw]=StringBuffer => IndexThrow
+null[1]=throw => RightHandSideThrow
+null[1]=StringBuffer => NullPointerException
+null[throw]=throw => IndexThrow
+null[throw]=StringBuffer => IndexThrow
+null[1]=throw => RightHandSideThrow
+null[1]=StringBuffer => NullPointerException
+null[throw]=throw => IndexThrow
+null[throw]=Thread => IndexThrow
+null[9]=throw => RightHandSideThrow
+null[9]=Thread => NullPointerException
+null[throw]=throw => IndexThrow
+null[throw]=Thread => IndexThrow
+null[9]=throw => RightHandSideThrow
+null[9]=Thread => NullPointerException
+Objects[throw]=throw => IndexThrow
+Objects[throw]=StringBuffer => IndexThrow
+Objects[1]=throw => RightHandSideThrow
+Objects[1]=StringBuffer => Okay!
+Objects[throw]=throw => IndexThrow
+Objects[throw]=Thread => IndexThrow
+Objects[1]=throw => RightHandSideThrow
+Objects[1]=Thread => Okay!
+Objects[throw]=throw => IndexThrow
+Objects[throw]=StringBuffer => IndexThrow
+Objects[9]=throw => RightHandSideThrow
+Objects[9]=StringBuffer => ArrayIndexOutOfBoundsException
+Objects[throw]=throw => IndexThrow
+Objects[throw]=Thread => IndexThrow
+Objects[9]=throw => RightHandSideThrow
+Objects[9]=Thread => ArrayIndexOutOfBoundsException
+Threads[throw]=throw => IndexThrow
+Threads[throw]=StringBuffer => IndexThrow
+Threads[1]=throw => RightHandSideThrow
+Threads[1]=StringBuffer => ArrayStoreException
+Threads[throw]=throw => IndexThrow
+Threads[throw]=Thread => IndexThrow
+Threads[1]=throw => RightHandSideThrow
+Threads[1]=Thread => Okay!
+Threads[throw]=throw => IndexThrow
+Threads[throw]=StringBuffer => IndexThrow
+Threads[9]=throw => RightHandSideThrow
+Threads[9]=StringBuffer => ArrayIndexOutOfBoundsException
+Threads[throw]=throw => IndexThrow
+Threads[throw]=Thread => IndexThrow
+Threads[9]=throw => RightHandSideThrow
+Threads[9]=Thread => ArrayIndexOutOfBoundsException
+
+The most interesting case of the lot is the one thirteenth from the end:

+

Threads[1]=StringBuffer => ArrayStoreException
+
+which indicates that the attempt to store a reference to a StringBuffer into an array whose components are of type Thread throws an ArrayStoreException. The code is type-correct at compile time: the assignment has a left-hand side of type Object[] and a right-hand side of type Object. At run time, the first actual argument to method testFour is a reference to an instance of "array of Thread" and the third actual argument is a reference to an instance of class StringBuffer.

+ +

15.26.2 Compound Assignment Operators

+ +All compound assignment operators require both operands to be of primitive type, except for +=, which allows the right-hand operand to be of any type if the left-hand operand is of type String.

+ +A compound assignment expression of the form E1 op= E2 is equivalent to E1 = (T)((E1) op (E2)), where T is the type of E1, except that E1 is evaluated only once. Note that the implied cast to type T may be either an identity conversion (§5.1.1) or a narrowing primitive conversion (§5.1.3). For example, the following code is correct:

+


+short x = 3;
+x += 4.6;
+
+and results in x having the value 7 because it is equivalent to:

+


+short x = 3;
+x = (short)(x + 4.6);
+
+At run time, the expression is evaluated in one of two ways. If the left-hand operand expression is not an array access expression, then four steps are required:

+

    +
  • First, the left-hand operand is evaluated to produce a variable. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason; the right-hand operand is not evaluated and no assignment occurs. + +
  • Otherwise, the value of the left-hand operand is saved and then the right-hand operand is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs. + +
  • Otherwise, the saved value of the left-hand variable and the value of the right-hand operand are used to perform the binary operation indicated by the compound assignment operator. If this operation completes abruptly (the only possibility is an integer division by zero-see §15.17.2), then the assignment expression completes abruptly for the same reason and no assignment occurs. + +
  • Otherwise, the result of the binary operation is converted to the type of the left-hand variable, subjected to value set conversion (§5.1.8) to the appropriate standard value set (not an extended-exponent value set), and the result of the conversion is stored into the variable. +
+If the left-hand operand expression is an array access expression (§15.13), then many steps are required:

+

    +
  • First, the array reference subexpression of the left-hand operand array access expression is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason; the index subexpression (of the left-hand operand array access expression) and the right-hand operand are not evaluated and no assignment occurs. + +
  • Otherwise, the index subexpression of the left-hand operand array access expression is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and the right-hand operand is not evaluated and no assignment occurs. + +
  • Otherwise, if the value of the array reference subexpression is null, then no assignment occurs and a NullPointerException is thrown. + +
  • Otherwise, the value of the array reference subexpression indeed refers to an array. If the value of the index subexpression is less than zero, or greater than or equal to the length of the array, then no assignment occurs and an ArrayIndexOutOfBoundsException is thrown. + +
  • Otherwise, the value of the index subexpression is used to select a component of the array referred to by the value of the array reference subexpression. The value of this component is saved and then the right-hand operand is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs. (For a simple assignment operator, the evaluation of the right-hand operand occurs before the checks of the array reference subexpression and the index subexpression, but for a compound assignment operator, the evaluation of the right-hand operand occurs after these checks.) + +
  • Otherwise, consider the array component selected in the previous step, whose value was saved. This component is a variable; call its type S. Also, let T be the type of the left-hand operand of the assignment operator as determined at compile time. +
      + +
    • If T is a primitive type, then S is necessarily the same as T. +
        + +
      • The saved value of the array component and the value of the right-hand operand are used to perform the binary operation indicated by the compound assignment operator. If this operation completes abruptly (the only possibility is an integer division by zero-see §15.17.2), then the assignment expression completes abruptly for the same reason and no assignment occurs. + +
      • Otherwise, the result of the binary operation is converted to the type of the selected array component, subjected to value set conversion (§5.1.8) to the appropriate standard value set (not an extended-exponent value set), and the result of the conversion is stored into the array component. +
      + +
    • If T is a reference type, then it must be String. Because class String is a final class, S must also be String. Therefore the run-time check that is sometimes required for the simple assignment operator is never required for a compound assignment operator. +
        + +
      • The saved value of the array component and the value of the right-hand operand are used to perform the binary operation (string concatenation) indicated by the compound assignment operator (which is necessarily +=). If this operation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs. +
      +
    +
+Otherwise, the String result of the binary operation is stored into the array component.

+ +The rules for compound assignment to an array component are illustrated by the following example program:

+

class ArrayReferenceThrow extends RuntimeException { }
+class IndexThrow extends RuntimeException { }
+class RightHandSideThrow extends RuntimeException { }
+class IllustrateCompoundArrayAssignment {
+	static String[] strings = { "Simon", "Garfunkel" };
+	static double[] doubles = { Math.E, Math.PI };
+	static String[] stringsThrow() {
+		throw new ArrayReferenceThrow();
+	}
+	static double[] doublesThrow() {
+		throw new ArrayReferenceThrow();
+	}
+	static int indexThrow() { throw new IndexThrow(); }
+	static String stringThrow() {
+		throw new RightHandSideThrow();
+	}
+	static double doubleThrow() {
+		throw new RightHandSideThrow();
+	}
+	static String name(Object q) {
+		String sq = q.getClass().getName();
+		int k = sq.lastIndexOf('.');
+		return (k < 0) ? sq : sq.substring(k+1);
+	}
+	static void testEight(String[] x, double[] z, int j) {
+		String sx = (x == null) ? "null" : "Strings";
+		String sz = (z == null) ? "null" : "doubles";
+		System.out.println();
+		try {
+			System.out.print(sx + "[throw]+=throw => ");
+			x[indexThrow()] += stringThrow();
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print(sz + "[throw]+=throw => ");
+			z[indexThrow()] += doubleThrow();
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print(sx + "[throw]+=\"heh\" => ");
+			x[indexThrow()] += "heh";
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print(sz + "[throw]+=12345 => ");
+			z[indexThrow()] += 12345;
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print(sx + "[" + j + "]+=throw => ");
+			x[j] += stringThrow();
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print(sz + "[" + j + "]+=throw => ");
+			z[j] += doubleThrow();
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print(sx + "[" + j + "]+=\"heh\" => ");
+			x[j] += "heh";
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print(sz + "[" + j + "]+=12345 => ");
+			z[j] += 12345;
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+	}
+	public static void main(String[] args) {
+		try {
+			System.out.print("throw[throw]+=throw => ");
+			stringsThrow()[indexThrow()] += stringThrow();
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print("throw[throw]+=throw => ");
+			doublesThrow()[indexThrow()] += doubleThrow();
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print("throw[throw]+=\"heh\" => ");
+			stringsThrow()[indexThrow()] += "heh";
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print("throw[throw]+=12345 => ");
+			doublesThrow()[indexThrow()] += 12345;
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print("throw[1]+=throw => ");
+			stringsThrow()[1] += stringThrow();
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print("throw[1]+=throw => ");
+			doublesThrow()[1] += doubleThrow();
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print("throw[1]+=\"heh\" => ");
+			stringsThrow()[1] += "heh";
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		try {
+			System.out.print("throw[1]+=12345 => ");
+			doublesThrow()[1] += 12345;
+			System.out.println("Okay!");
+		} catch (Throwable e) { System.out.println(name(e)); }
+		testEight(null, null, 1);
+		testEight(null, null, 9);
+		testEight(strings, doubles, 1);
+		testEight(strings, doubles, 9);
+	}
+}
+
+This program prints:

+

throw[throw]+=throw => ArrayReferenceThrow
+throw[throw]+=throw => ArrayReferenceThrow
+throw[throw]+="heh" => ArrayReferenceThrow
+throw[throw]+=12345 => ArrayReferenceThrow
+throw[1]+=throw => ArrayReferenceThrow
+throw[1]+=throw => ArrayReferenceThrow
+throw[1]+="heh" => ArrayReferenceThrow
+throw[1]+=12345 => ArrayReferenceThrow
+null[throw]+=throw => IndexThrow
+null[throw]+=throw => IndexThrow
+null[throw]+="heh" => IndexThrow
+null[throw]+=12345 => IndexThrow
+null[1]+=throw => NullPointerException
+null[1]+=throw => NullPointerException
+null[1]+="heh" => NullPointerException
+null[1]+=12345 => NullPointerException
+null[throw]+=throw => IndexThrow
+null[throw]+=throw => IndexThrow
+null[throw]+="heh" => IndexThrow
+null[throw]+=12345 => IndexThrow
+null[9]+=throw => NullPointerException
+null[9]+=throw => NullPointerException
+null[9]+="heh" => NullPointerException
+null[9]+=12345 => NullPointerException
+Strings[throw]+=throw => IndexThrow
+doubles[throw]+=throw => IndexThrow
+Strings[throw]+="heh" => IndexThrow
+doubles[throw]+=12345 => IndexThrow
+Strings[1]+=throw => RightHandSideThrow
+doubles[1]+=throw => RightHandSideThrow
+Strings[1]+="heh" => Okay!
+doubles[1]+=12345 => Okay!
+Strings[throw]+=throw => IndexThrow
+doubles[throw]+=throw => IndexThrow
+Strings[throw]+="heh" => IndexThrow
+doubles[throw]+=12345 => IndexThrow
+Strings[9]+=throw => ArrayIndexOutOfBoundsException
+doubles[9]+=throw => ArrayIndexOutOfBoundsException
+Strings[9]+="heh" => ArrayIndexOutOfBoundsException
+doubles[9]+=12345 => ArrayIndexOutOfBoundsException
+
+The most interesting cases of the lot are tenth and eleventh from the end:

+

Strings[1]+=throw => RightHandSideThrow
+doubles[1]+=throw => RightHandSideThrow
+
+They are the cases where a right-hand side that throws an exception actually gets to throw the exception; moreover, they are the only such cases in the lot. This demonstrates that the evaluation of the right-hand operand indeed occurs after the checks for a null array reference value and an out-of-bounds index value.

+ +The following program illustrates the fact that the value of the left-hand side of a compound assignment is saved before the right-hand side is evaluated:

+

class Test {
+	public static void main(String[] args) {
+		int k = 1;
+		int[] a = { 1 };
+		k += (k = 4) * (k + 2);
+		a[0] += (a[0] = 4) * (a[0] + 2);
+		System.out.println("k==" + k + " and a[0]==" + a[0]);
+	}
+}
+
+This program prints:

+

k==25 and a[0]==25
+
+The value 1 of k is saved by the compound assignment operator += before its right-hand operand (k = 4) * (k + 2) is evaluated. Evaluation of this right-hand operand then assigns 4 to k, calculates the value 6 for k + 2, and then multiplies 4 by 6 to get 24. This is added to the saved value 1 to get 25, which is then stored into k by the += operator. An identical analysis applies to the case that uses a[0]. In short, the statements

+

k += (k = 4) * (k + 2);
+a[0] += (a[0] = 4) * (a[0] + 2);
+
+behave in exactly the same manner as the statements:

+

k = k + (k = 4) * (k + 2);
+a[0] = a[0] + (a[0] = 4) * (a[0] + 2);
+
+

15.27 Expression

+ +An Expression is any assignment expression:

+

    +Expression:
+	AssignmentExpression
+
+ +Unlike C and C++, the Java programming language has no comma operator.

+ +

15.28 Constant Expression

+
    +ConstantExpression:
+	Expression
+
+A compile-time constant expression is an expression denoting a value of primitive type or a String that is composed using only the following:

+

    +
  • Literals of primitive type and literals of type String + +
  • Casts to primitive types and casts to type String + +
  • The unary operators +, -, ~, and ! (but not ++ or --) + +
  • The multiplicative operators *, /, and % + +
  • The additive operators + and - + +
  • The shift operators <<, >>, and >>> + +
  • The relational operators <, <=, >, and >= (but not instanceof) + +
  • The equality operators == and != + +
  • The bitwise and logical operators &, ^, and | + +
  • The conditional-and operator && and the conditional-or operator || + +
  • The ternary conditional operator ? : + +
  • Simple names that refer to final variables whose initializers are constant expressions + +
  • Qualified names of the form TypeName . Identifier that refer to final variables whose initializers are constant expressions +
+Compile-time constant expressions are used in case labels in switch statements (§14.10) and have a special significance for assignment conversion (§5.2).

+ +A compile-time constant expression is always treated as FP-strict (§15.4), even if it occurs in a context where a non-constant expression would not be considered to be FP-strict.

+ +Examples of constant expressions:

+

true
+(short)(1*2*3*4*5*6)
+Integer.MAX_VALUE / 2
+2.0 * Math.PI
+"The integer " + Long.MAX_VALUE + " is mighty big."
+
+ +
+ + + +
Contents | Prev | Next | IndexJava Language Specification
+Second Edition
+Copyright © 2000 Sun Microsystems, Inc. +All rights reserved +
+Please send any comments or corrections via our feedback form + + diff --git a/topics/java/lci/java.lcf b/topics/java/lci/java.lcf index 4f61f34a..9bbbc5d6 100644 --- a/topics/java/lci/java.lcf +++ b/topics/java/lci/java.lcf @@ -56,10 +56,19 @@ - jls1 + jls1app - tools/jls2bgf java/jls1/syntax.html + tools/html2bgf java/jls1/syntax.kw java/jls1/syntax.html + + + + + + jls1doc + + + tools/html2bgf java/jls1/collect.kw java/jls1/collected.html @@ -83,10 +92,21 @@ - onetwo + one - jls1 + jls1app prepare1 + + + jls1doc + preferLALR + + + + + onetwo + + one refactorStatements structure1 addFeatures1to2 @@ -112,4 +132,4 @@ - \ No newline at end of file + diff --git a/topics/java/lci/xbgf/prepare1.xbgf b/topics/java/lci/xbgf/prepare1.xbgf index b68aba91..ca2bd1a7 100644 --- a/topics/java/lci/xbgf/prepare1.xbgf +++ b/topics/java/lci/xbgf/prepare1.xbgf @@ -16,9 +16,6 @@ Statement - - + Goal