Classes and Objects

Here we will learn how functions can create and encapsulate data structures.

Classes

A class in scala can be defined with its constructor as below:

class Foo(x: Int, y: Int) {
   def numer = x
   def denom = y
}

This definition has 2 entities:

1. A "type" named Foo
2. A Constructor to create elements of this type.

Scala keeps types and values in different namespaces, so there is no conflict between the definitions of Foo.

Implicit class variables

In the above examples, instead of taking constructor params and assigning them to class variables/constants, we can directly generate implicit class variables/constants in the constructor:

class Foo2(var numer: Int, val denom: Int) {

}

Here numer is a variable and denom is a constant. No getters/setters needed. These implicit variables can also have access specifiers eg. private.

So here, you cannot do:

println(Foo.x) // since its just a parameter and not a class variables

but we can do this:

println(Foo2.numer)

Note: As seen above, in case of classes, using a keyword like var or val before the param name makes it a class variable. Leaving out the keyword makes it a parameter. However, in case of functions, parameters are assumed to be val.Eg.

def foo(x: Int) = {}

is the same as

def foo(val x: Int) = {}

Objects

Objects are elements of a class type, created by using the new keyword.

val foo = new Foo(1,2)

Methods

Functions can be packaged in the class:

class Foo(x: Int, y: Int) {
   def numer = x
   def denom = y

   def multiplyBy(that: Foo) {
       new Foo( numer * that.numer, denom * that.denom )
   }
}

// call
val foo1 = new Foo(1,2)
val foo2 = new Foo(5,6)
foo1.multiply(foo2)

Some pointers

• String Representation: Overwrite the toString() method of a class to provide a custom string representation of a class, just like in JAVA.
• Access Specifiers: Variables and function definitions in a class that are only for use inside the class can be marked with the private access specifier like in JAVA.
• this: The this keyword can be used inside a class to refer to current object.

require and assert

The require keyword be used to define requirements of a particular class. It takes in a condition and an optional message.

Example:

class Foo(x: Int) {
   require( x! = 0, "Integer must be non-zero")

   def numer = x

   def divideBy(that: Foo) {
       new Foo( this.numer / that.numer)
   }
}

So if we call:

val foo1 = new Foo(4)
val foo2 = new Foo(0)
foo1.divideBy(foo2)  // Fails with "IllegalArgumentException: requirement failed: Integer must be non-zero"

Similarly assert also takes in a condition and an optional message.

val x = sqrt(y)
assert(x >= 0)

// Fails with AssertionError instead of IllegalArgumentException

The difference between require and assert is the "intent" of use.

• require is used to enforce a precondition on the caller of a function
• assert is used to check the code of a function

Constructor

Classes are defined with an implicit constructor which is the primary constructor. Additional constructors can be defined by using the this keyword.

class Foo(x: Int, y: Int) {
   def numer = x
   def denom = y

   // additional constructor which uses the primary constructor
   def this(x: Int) = {
      this(x, 1)
   }

   def multiplyBy(that: Foo) {
       new Foo( numer * that.numer, denom * that.denom )
   }
}