A type hierarchy is composed of subtypes and supertypes.The intuitive idea of a subtype is one whose objects provide all the behavior of objects of another type (the supertype) plus something extra.

• 意思是说，类型层次由子类型和超类型组成，直觉告诉我们，子类型的含义就是该类型的对象提供了另外一个类型（超类型）的对象的所有行为功能，并有所扩充。