0.3 Inheritance - the big picture  (Page 2/9)

Listings

• Listing 1 . Complete listing for Skins01.mxml.
• Listing 2 . Beginning of the class named Driver.
• Listing 3 . The constructor for the Driver class.
• Listing 4 . The method named buttonHandler.
• Listing 5 . Beginning of the FancyButton class.
• Listing 6 . Constructor for the FancyButton class.
• Listing 7 . The toggleSkin method.
• Listing 8 . Source code for the file named Skins01.mxml.
• Listing 9 . Source code for the class named Driver.
• Listing 10 . Source code for the class named FancyButton.
• Listing 11 . Source code for the class named ButterflyButton.
• Listing 12 . Source code for the class named FrogButton.

Supplemental material

I recommend that you also study the other lessons in my extensive collection of online programming tutorials. You will find a consolidated index at www.DickBaldwin.com .

General background information

The first major characteristic of an object-oriented program is encapsulation, which I explained in the previous lesson. The second of the threemajor characteristics is inheritance , followed by polymorphism . I will explain inheritance from a big-picture viewpoint in this lesson and will explain polymorphism in the next lesson.

A new class can extend an existing class

A class can be defined to inherit the properties, events, and methods of another class. From a syntax viewpoint, this is accomplished using the extends keyword.

The class being extended is often called the base class or the superclass , and the new class is often called the derived class or the subclass .

What is inherited?

The subclass inherits the data representation and behavior of the superclass (and all of its superclasses) . However, the subclass can modify the behavior of inherited methods by overriding them. (That will be one of the topics in the next lesson on polymorphism.) The subclass can also add new data representation and behavior that is unique to itsown purposes.

The superclass remains viable

A program can instantiate objects of a superclass as well as from its subclass. From a practical viewpoint, the superclass doesn't even knowthat it has been extended.

A hierarchy of classes

Inheritance is hierarchical. In other words, a class may be the subclass of one (and only one) other class and may be the superclass of one or more other classes.

For example, the ActionScript 3 Button class is a subclass of the UIComponent class, and is the superclass of the following classes:

• AccordianHeader
• CheckBox
• LinkButton
• PopUpButton
• RadioButton
• ScrollThumb
• SliderThumb

An orderly hierarchy

Inheritance mechanisms allow you build an orderly hierarchy of classes.

When several of your abstract data types have characteristics in common, you can design their commonalities into a single superclass and separate theirunique characteristics into unique subclasses. This is one of the purposes of inheritance. (This purpose is illustrated by the program that I will explain later in this lesson.)

The airship hierarchy

For example, suppose you are building a program dealing with airships. All airships have altitude and range parameters in common. Therefore, you couldbuild a base Airship class containing data and methods having to do with range and altitude.