Week1Day2_Chapter7_8.md

Week 1 Day 2

Review of Chapter 7 - 8

Review of Chapter 7 User-Defined Methods

A method is a named list of statements. It prevents the program from being large and confusing.

Example:

c1 = (f1 - 32.0) * (5.0 / 9.0);
c2 = (f2 - 32.0) * (5.0 / 9.0);
c3 = (f3 - 32.0) * (5.0 / 9.0);
c4 = (f4 - 32.0) * (5.0 / 9.0);
c5 = (f5 - 32.0) * (5.0 / 9.0);

public double F2C(f) {
  c = (f - 32.0) * (5.0 / 9.0);
  return c;
}

c1 = F2C(f1);
c2 = F2C(f2);
c3 = F2C(f3);
c4 = F2C(f4);
c5 = F2C(f5);

Method definition

• A method definition consists of the new method's name and a block of statements. Ex: public static double calcPizzaArea() { /* block of statements */ }
• A method call is an invocation of a method's name, causing the method's statements to execute.
• The method's name can be any valid identifier. A block is a list of statements surrounded by braces.
• Methods must be defined within a class.
• The access modifier public indicates the method may be called from any class in the program.
• The access modifier static indicates the method only uses values that are passed to the method.

Exercise: p7.1.4 Method basics

Return Statement

A method may return one value using a return statement.

• A method can only return one item, not two or more.
• The type of the returned value must be the same as the return type in the declaration.
• A return type of void indicates that a method does not return any value.

Parameters

A programmer can influence a method's behavior via an input.

• A parameter is a method input specified in a method definition. Ex: A pizza area method might have diameter as an input.
• An argument is a value provided to a method's parameter during a method call. Ex: A pizza area method might be called as calcPizzaArea(12.0) or as calcPizzaArea(16.0).
• A parameter is like a variable declaration. Upon a call, the parameter's memory location is allocated, and the parameter is assigned with the argument's value. Upon returning to the original call location, the parameter is deleted from memory.
• A method definition may have multiple parameters, separated by commas.

Exercise: c7.1.1 Basic method call, c7.1.3 Volume of a pyramid.

Methods with Branches / Loops

A method's block of statements may include branches, loops, and other statements.

Exercise: c7.5.2 Popcorn

Exercise: c7.6.2 Shampoo

Scope of Variable / Method Definitions

The name of a defined variable or method item is only visible to part of a program, known as the item's scope.

• A variable declared in a method has scope limited to inside that method.
• A variable declared within a class but outside any method is called a class member variable or field. A field is visible from anywhere in the class.
• If a method's local variable (including a parameter) has the same name as a field, then in that method the name refers to the local item and the field is inaccessible.
• A method is visible to any other methods in the same class.
• Public variables and methods are visible to other classes in the program.

Method Name Overloading

Sometimes it is convenient for a program to have two methods with the same name but differing in the number or types of parameters. This is known as method overloading. The compiler determines which method to call based on the argument types.

Example:

public class DatePrinter {
   public static void printDate(int currDay, int currMonth, int currYear) {    
      System.out.print(currMonth + "/" + currDay + "/" + currYear);
   }

   public static void printDate(int currDay, String currMonth, int currYear) {
      System.out.print(currMonth + " " + currDay + ", " + currYear);
   }

   public static void main(String[] args) {
      
      printDate(30, 7, 2012);
      System.out.println();
      
      printDate(30, "July", 2012);
      System.out.println();
   }
}

Exercise: p7.11.1 Method name overloading

Review of Chapter 8 Arrays

An array is a special variable having one name, but storing a list of data items, with each item being directly accessible. In java, an array is an ordered list of items of a given data type. Each item in an array is called an element.

Array Declaration

dataType[] arrayName = new dataType[numElements];

• The [ ](brackets) symbol after the data type indicates that the variable is an array reference.
• The new keyword creates space in memory to store the array with the specific number of elements.

Separate array declaration and array allocation A programmer can declare an array reference variable without allocating the array at that time and later assign the variable with an allocated array.

int[] gameScores;
...
gameScores = new int[4];

Exercise: p8.2.3 Array basics

Iterating Through Arrays

for (i = 0; i < myArray.length; i++) {
   // Loop body accessing myArray[i]
}

Exercise: c8.4.5 Printing array elements separated by commas

Modifying an Array with a Loop

A program may need to modify elements while iterating through an array.

Exercise: c8.7.1 Decrement array elements

Exercise: c8.7.2 Copy and modify array elements

Array Parameters

Passing an array to a method is an example of passing a reference type variable rather than a primitive type variable

Passing a primitive type variable to a method

• Example: int, double, boolean, char
• A copy of the variable is passed to the method.
• Modifying the variable inside the method does not affect the original variable.
• The copy is removed from computer memory after the method call.

Passing a reference type variable to a method

• Example: String, Scanner, arrays
• A copy of the reference (address) is passed to the method.
• Modifying the variable inside the method does affect the original variable.

Example: A method that swaps the first and last element of its array parameter.

Oversized Arrays

An oversize array is an array where the number of elements used is less than or equal to the memory allocated. Since the number of elements used in an oversize array is usually less than the array's length, a separate integer variable is used to keep track of how many array elements are currently used. Oversize arrays are useful when the number of elements stored in the array is not known in advance, or when the number of elements stored in an array varies over time.

Format:

int[] salesTransactions = new int[1000];
int salesTransactionsSize = 0;

Example: Add an element to an oversized array

Write a addElement() method that adds a new string to the end of an array of strings.

• The method should first check if there is room for the extra element.
• If the array is already full, create a new array of twice the size and copy all existing elements to it.
• The method should return the new array and the updated size.