Interfaces(Thinking in Java)

　　The interface keyword takes the concept of abstractness one step further. The abstract keyword allows you to create one or more undefined methods in a class—you provide part of the interface without providing a corresponding implementation. The implementation is provided by inheritors. The interface keyword produces a completely abstract class, one that provides no implementation at all. It allows the creator to determine method names, argument lists, and return types, but no method bodies. An interface provides only a form, but no implementation.

　　However, an interface is more than just an abstract class taken to the extreme, since it allows you to perform a variation of "multiple inheritance" by creating a class that can be upcast to more than one base type.