ActionScript 3.0 Design Patterns

Inheritance | 27

In addition to invoking themakeSound( )method, theDogandCatinstances invoke their

own methods,bark( ) andmeow( ). Also, when you test the application, you will see:

QuadPets is instantiated

That output is caused by the line:

trace(“QuadPets is instantiated”);

placed in the constructor function of theQuadPetsclass. It fires whenever an instance

of the class is invoked. So in addition to having the capacity to use methods from the

superclass, subclasses inherit any actions in the constructor function of the superclass.

Open a Flash document, and typeTestPetsin the Document class window. When

you test it, you should see the following in the Output window:

QuadPets is instantiated

Superclass:Pet Sound

Dog class: Bow wow

QuadPets is instantiated

Superclass:Pet Sound

Cat class: Meow

Looking at the output, both the dog and cat instances display two superclass mes-

sages (QuadPets is instantiated,Superclass:Pet Sound) and one message unique to

the respective subclasses (Dog class: Bow wow,Cat class: Meow.) These examples

show how inheritance works, but in practical applications, and in design patterns,

inheritance is planned so that they cut down on redundancy and help build a pro-

gram to achieve an overall goal.

Example 1-16. TestPets.as

package
{
import flash.display.Sprite;

public class TestPets extends Sprite
{
public function TestPets( ):void
{
var dog:Dog=new Dog( );
dog.makeSound( );
dog.bark( );
var cat:Cat=new Cat( );
cat.makeSound( );
cat.meow( );
}
}
}