Liskov Substitution principle(里氏替換原則)

定義：

1. If for each object o1 of type S there is an object o2 of type T such that for all programs P defined in terms of T,the behavior of P is unchanged when o1 is substituted for o2 then S is a subtype of T.
2. Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it.

簡單的說，就是只要是base class可以使用的地方都能夠使用derived class代替。

但是反過來就不行了，子類別出現的地方，父類別不一定可以使用。