User Guide

Creating and using classes 55
return "Hello world";
}
}
This rule applies only to instance variables (variables that are copied into each instance of a class),
not class variables (variables that belong to the class). For more information about these kinds of
variables, see “Instance and class members” on page 60.
When you initialize arrays inline, only one array is created for all instances of the class:
class Bar {
var foo:Array = new Array();
}
Creating subclasses
In object-oriented programming, a subclass can inherit the properties and methods of another
class, called the superclass. To create this kind of relationship between two classes, you use the
class statement’s extends clause. To specify a superclass, use the following syntax:
class SubClass extends SuperClass {}
The class you specify in SubClass inherits all the properties and methods defined by the
superclass. For example, you might create a Mammal class that defines properties and methods
common to all mammals. To create a variation of the Mammal class, such as a Marsupial class,
you would extend the Mammal class—that is, create a subclass of the Mammal class.
class Marsupial extends Mammal {}
The subclass inherits all the properties and methods of the superclass, including any properties or
methods that you have declared to be private using the
private keyword. (For more information
on private variables, see “Controlling member access” on page 53.)
You can extend your own custom classes as well as many of the built-in ActionScript classes. (You
cannot extend the TextField class or static classes, such as the Math, Key, and Mouse classes.)
When you extend a built-in ActionScript class, your custom class inherits all the methods and
properties of the built-in class.
For example, the following code defines the class JukeBox, which extends the built-in Sound class.
It defines an array called
songList and a method called playSong() that plays a song and
invokes the
loadSound() method, which it inherits from the Sound class.
class JukeBox extends Sound {
var songList:Array = new Array("beethoven.mp3", "bach.mp3", "mozart.mp3");
function playSong(songID:Number):Void {
this.loadSound(songList[songID]);
}
}
If you don’t place a call to super() in the constructor function of a subclass, the compiler
automatically generates a call to the constructor of its immediate superclass with no parameters as
the first statement of the function. If the superclass doesn’t have a constructor, the compiler
creates an empty constructor function and then generates a call to it from the subclass. However,
if the superclass takes parameters in its definition, you must create a constructor in the subclass
and call the superclass with the required parameters.