4.12. Manipulating Types

The type system enables you to refer to an object by its own class, by any of its superclasses, by any interface it implements, or by a combination of these. All these potential types for an object are valid because, by being more specific, it must implement a superset of all the more general types' functionality. A Square is still a Rectangle because it is a specific kind of Rectangle, and it is a Shape because it is a specific kind of Shape. It might also be an IPointCollection if it can provide a set of points that comprise its vertices. The point is that these aren't conversions: the class really is all the things that it extends.

Because you can refer to a class by many names, you need ways to get between these types. You should be able to test at runtime if an object is an instance of a given class or implements a particular interface. You should be able to convert a specific type to a general type, and you should be able to attempt to convert general types to specific types. You should even be allowed to attempt to convert types to unrelated types.