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Inheritance

This class defines a private property named value, which can be initialized using the class constructor. The

last function defines what happens when a plus is used with a

MyClass

object. This particular function

defines the behavior when the second operand is also a

MyClass

object: The values of the two objects are

added and a new

MyClass

object is returned. With this definition, it is possible to use the following line:

var obj1 = new MyClass(20);

var obj2 = new MyClass(30);

var obj3 = obj1 + obj2;

alert(obj3.getValue()); //outputs “50”

Finally, classes can have true static properties and methods by using the

static

keyword:

class MyClass {

public static var value : Integer = 25;

}

The static members of a class can then be accessed as you might expect:

var value : Integer = MyClass.value;

Inheritance

Inheritance in ECMAScript Edition 4 is done in the same way as in Java, by using the

extends

keyword.

For example:

class ClassA {

private var value : Integer = 0;

public function ClassA(value : Integer) {

this.value = value;

}

public function getValue() : Integer {

return this.value;

}

class ClassB extends ClassA {

public function ClassB(value : Integer) {

super(value);

}

public function sayValue() : Integer {

alert(this.getValue());

}

First, note that the

extends

keyword is used to inherit from

ClassA

to create

ClassB

. This is a much

simpler and straightforward way to establish inheritance than the traditional ECMAScript method

(although object masquerading and prototype chaining is still available for backwards compatibility).

The second thing to note is the use of the

super()

method to use

ClassA

’s constructor in

ClassB

. Once

again, this is identical to Java.

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The Evolution of JavaScript

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