里氏替换原则

Liskov Subsitution Principle, LSP :

• 　　If for object o1 of type S there is an abject o2 of type T such that for all programs P defined in terms of T,

the behavior of P is unchanged when o1 is subsituted for o2 then S is a subtype  of  T.

• 　　Functions that use pointers or reference to base classes must be able to use objects of derived classes without 

knowing it.

1. 　　子类必须完全实现父类的方法
2. 　　子类可以有自己的个性
3. 　　覆盖或实现父类的方法时输入参数可以被放大
4. 　　覆写或实现父类的方法时输出结果可以被缩小