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