ActionScript 3.0 Design Patterns

32 | Chapter 1: Object-Oriented Programming, Design Patterns, and ActionScript 3.0

When the application finally implements the methods originating in the abstract

class, it does so by programming to the interface (AbstractClass) but instantiates

through the subclass (Subclass). So the instance,doDemo, is typed asAbstractClass

but instantiated asSubclass.

The following shows what appears in the Output window when you test the program:

This is the overidden abstract method

I'm a concrete method from an abstract class

At this point you may be scratching your head wondering why you should go

through this kind of convolution to do what you could do with a non-abstract class.

Instead of subclassing the abstract class, overriding one of the methods, and then

implementing the subclass, we could have just written both methods the way we

wanted them in the first place and not have to override anything. The next section

attempts to answer that question.

Example 1-22. Subclass.as

package
{
//Subclass of Abstract class
public class Subclass extends AbstractClass
{
override function abstractMethod( ):void
{
trace("This is the overidden abstract method");
}
}
}

Example 1-23. ImplementSub.as

package
{
//Implement Subclass of Abstract class
import flash.display.Sprite;

public class ImplementSub extends Sprite
{
private var doDemo:AbstractClass;

public function ImplementSub( )
{
doDemo=new Subclass( );
doDemo.abstractMethod( );
doDemo.concreteMethod( );
}
}
}