说明：  设计面向对象软件比较困难，而设计可复用的面向对象软件就更加困难。你必须找到相 关的对象，以适当的粒度将它们归类，再定义类的接口和继承层次，建立对象之间的基本关 系。你的设计应该对手头的问题有针对性，同时对将来的问题和需求也要有足够的通用性。 你也希望避免重复设计或尽可能少做重复设计
(It is difficult to design object-oriented software, but it is more difficult to design reusable object-oriented software. You have to find the face Objects of relation, classify them with proper granularity, define class interface and inheritance level, and establish basic relation between objects Department. Your design should be specific to the problem at hand, and at the same time, it should be general enough to the future problems and requirements. You also want to avoid repetitive design or do it as little as possible.)