Design Pattern(3)-State Pattern

学过编译原理应该知道有限状态机，状态模式就是适用于那种希望 一个对象在不同状态下呈现不一样的行为的一种design pattern。

我们以一个实例来诠释，有一个Door对象，Door对象有两个状态：OpenState，ClosedState，当我们提交一个请求的时候，它若在OpenState状态，希望它能执行关闭操作，它若在ClosedState状态，希望它能执行打开操作。

官方类图：

State：

public abstract class State { public abstract void handle(Door door); }

OpenState：

public class OpenState extends State{ @Override public void handle(Door door) { System.out.println("the door has closed"); door.changeState(new ClosedState()); } }

ClosedState：

public class ClosedState extends State { @Override public void handle(Door door) { System.out.println("the door has opened"); door.changeState(new OpenState()); } }

Door：

public class Door { State state; public Door(State state){ this.state=state; } public void request() { state.handle(this); } protected void changeState(State state) { this.state=state; } }

Client：

public class Client { public static void main(String[] args){ Door door =new Door(new OpenState()); door.request(); door.request(); door.request(); } }

输出为： the door has closed the door has opened the door has closed