0.5 Interface polymorphism - the big picture  (Page 3/9)

A form of multiple inheritance

In ActionScript, a class can inherit from (extend) only one other class. However, a class can inherit from (implement) any number of interfaces. Furthermore, each interface can extend any number of otherinterfaces.

Therefore, an ActionScript class can inherit any number of concrete methods from one superclass and can inherit any number of abstract methods from any number of interfaces.

Concrete methods are required

Any class that inherits an abstract method must provide a concrete definition for the method or the class cannot be compiled.

ActionScript classes cannot be declared abstract. Therefore, the abstract method cannot be passed down the class hierarchy for definition by asubclass as is the case in Java. The concrete version must be defined in the class in which it is inherited. This means that it must be defined inthe class that implements the interface.

What is a concrete method?

As a minimum, a concrete method is a method signature followed by a pair of matching curly braces (possibly containing a return statement) . Normally the body of the method is coded between the curly braces defining thebehavior of the method.

If the return type is void, the matching curly braces may be empty. In that case, the concrete method exhibits no observable behavior. It returnsimmediately without doing anything when it is called.

If the return type is not void, there must be a return statement that returns a value of the correct type.

Do empty methods have any value?

It is not uncommon for a class to implement an interface for which some of the interface methods are of no interest with regard to an object of the newclass. However, the implementing class must provide concrete definitions for all of the methods inherited from the interface.

In that case, it is common practice to simply define the uninteresting methods as empty methods in the new class. If they ever do get called,they will simply return immediately without doing anything.

Preview

The program named Interface01 that I will explain in this lesson is an update of the program named Polymorph02 that I explained in the earlier lesson titled "Polymorphism - The Big Picture" (see Baldwin's ActionScript programming website ).

The earlier program illustrated runtime polymorphism based on class inheritance and the overriding of inherited concrete methods.

This program will illustrate runtime polymorphism based on interface inheritance and the concrete definition of inherited abstract methods.

The project file structure

The project file structure is shown in Figure 1.

Figure

Project file structure.

Six ActionScript files

This project consists of one MXML file named Interface01 , three class files, and three interface files. The class files are named:

• MyRectangle.as
• MyCircle.as
• Driver.as

The interface files are named:

• IVolume.as
• ICircumference.as
• IArea.as

Interface can declare any number of methods

An interface can declare any number of methods including setter and getter methods. As you will see later, the interface named IArea declares two methods named area and id .