Programming Visual Basic 2005

Polymorphism

There are two powerful aspects to inheritance. One is code reuse. When you create a ListBox class, you're able to reuse some of the logic in the base (Window) class.

What is arguably more powerful, however, is the second aspect of inheritance: polymorphism .

When the phone company sends your phone a ring signal, it does not know what type of phone is on the other end of the line. You might have an old-fashioned Western Electric phone which energizes a motor to ring a bell, or you might have an electronic phone which plays digital music.

As far as the phone company is concerned, it knows only about the "base type" phone and expects that any "instance" of this type knows how to ring. When the phone company tells your phone to ring, it simply expects the phone to "do the right thing." Thus, the phone company treats all the phones it connects to polymorphically.

Creating Polymorphic Types

Because a ListBox is-a Window and a Button is-a Window, we expect to be able to use either of these types in situations that call for a Window. For example, a form might want to keep a collection of all the instances of Window it manages so that when the form is opened, it can tell each Window to draw itself. For this operation, the form does not want to know which windows are actually ListBoxes and which are Buttons; it just wants to tick through its collection and tell each to "Draw." In short, the form wants to treat all its Window objects polymorphically.

Creating Polymorphic Methods ...