3.8 Polymorphism

Polymorphism , the third pillar of object-oriented programming, is closely related to inheritance. The prefix poly means many; morph means form. Thus, polymorphism refers to the ability of a single type or class to take many forms.

There are times that you will know you have a collection of a general type, for example a collection of Controls, but you do not know (or care) what the specific subtype each of your controls is (one may be a button, another a list box, etc.). The important thing is that you know they all inherit shared abilities (e.g., the draw method) and that you can treat them all as controls. If you write a programming instruction that tells each control to draw itself, this is implemented properly on a per-control basis (i.e., buttons draw as buttons, listboxes draw as listboxes). You do not need to know how each subtype accomplishes this; you only need to know that each type is defined to be able to draw.

Polymorphism allows you to treat a collection of disparate derived types (buttons, list boxes, etc.) as a group. You treat the general group of controls the same way, and each individual control does the right thing according to its specific type. Chapter 11 provides more concrete examples.