More on polymorphism

When dealing with type hierarchies, you often want to treat an object not as the specific type that it is, but instead as its bast type. This allows you to write code that doesn't depend on specific type. This is called polymorphism.

The power of Polymorphism

In the Shape example, methods manipulate generic shapes, unconcerned about whether they're circles, squares, triangles, or some shape that hasn't even been defined yet. All shapes can be drawn, erased, and moved, so these methods simply send a message to a Shape object; they don't worry about how the object copes with the message.

Such code is unaffected by the addition of new types, and adding new types is the most common way to extend an object-oriented program to handle new situations.

This ability to easily extend a design by deriving new subtypes is one of the essential ways to encapsulate change. This greatly improves designs while reducing the cost of software maintenance.