Decouple an abstraction from its implementation so that the two can vary independently.
将抽象和实现解耦，使得两者可以独立地变化。

The classes and objects participating in this pattern are:

• defines the abstraction's interface.
• maintains a reference to an object of type Implementor.

• extends the interface defined by Abstraction.

• defines the interface for implementation classes. This interface doesn't have to correspond exactly to Abstraction's interface; in fact the two interfaces can be quite different. Typically the Implementation interface provides only primitive operations, and Abstraction defines higher-level operations based on these primitives.

• implements the Implementor interface and defines its concrete implementation.