STRATEGY

         define a family of algorithms , encapsulats each ones , and make them interchangeable . STRATEGY let the algorithm vary independently from client that use it .
         
         定义一系列算法并逐个封装为单独的类 ，并且使之能互相交换。策略模式能使这些各种各样的算法独立的被客户端调用。（呵呵   翻译的可能不标准）

           separating what changes from what stay the same .

           identify the aspects of your application taht vary ang separate them from what stay the same.

           Has-a can be better than Is-a.