每一个object都含有state和method，因此定义一个object的时候一定要说明他的含有什么data member，还有什么method。
多用design pattern，少用inheritance；using interfaces
1. 设计一个文本编辑器
a）composite pattern来分别管理只读文本和可读写文本
b）singleton 一个文本管理manage
c) observer 可以告诉其他文本一些global状态的改变，比如format
2. Zoo
An interface for Animal，It has several abstract methods: sell,buy,sleep,eat
use composite pattern to implement differentspecies animal， like bird,cat. These species are also abstract class. And the implementation of specific animal,like cat,tiger,puma.
each abstract class has a state: habit,which shows the common habit of a particular species. two methods: sethabit / gethabit. eachconcrete class has more information: living area and so on. override method: sell buy,eat.
