设计模式整理_状态模式

　　状态模式允许对象在内部状态改变的时候,改变它的行为,对象看起来好像修改了它的类.因为这个模式将状态封装成为独立的类,并将动作委托到代表当前状态的对象,而行为会随着内部状态而改变.

　　在状态模式中,Context内部持有状态,State接口定义了一个所有具体状态的共同接口,任何状态都实现这个相同的接口,这样一来,各个状态可以互相替换.具体状态实现状态接口,所以当Context改变状态的时候,行为也跟着改变.而不管在什么时候,只要有人调用Context的具体方法,它就会用来委托状态处理,下面用具体事例来说明,Person类代表Context.

　　

public class Person {
    //给定的三种状态
    State happyState=new HappyState(this);
    State angryState=new AngryState(this);
    State sadState=new SadState(this);
    State state=happyState;    //初始状态
    //返回状态,以便于状态的设置
    public State getHappyState() {
        return happyState;
    }
    public State getAngryState() {
        return angryState;
    }
    public State getSadState() {
        return sadState;
    }
    public void setState(State state) {
        this.state = state;    //在条件变化时,设置状态
    }
    //下面三个方法将调用状态自己的方法
    public void listen() {
        state.listen();
    }
    public void doHomeWork() {
        state.doHomeWork();
    }
    public void study() {
        state.study();
    }
}

import java.util.Scanner;
//状态接口,所有状态类均需实现此接口
public interface State {
    public void listen();
    public void doHomeWork();
    public void study();
}
//三个实现类
class HappyState implements State{
    private Person p;    //需要将Person对象传入,以便于改变状态
    
    public HappyState(Person p) {
        this.p = p;
    }

    @Override
    public void listen() {
        System.out.println("I don't know the lesson");
        System.out.println("Do you want to listen again");
        Scanner sc=new Scanner(System.in);
        String str=sc.nextLine();
        if(sc.equals("no"))
        p.setState(p.getSadState());    //通过客户的选择动态的改变状态
    }

    @Override
    public void doHomeWork() {
        System.out.println("I don't know how to do it.");
        System.out.println("review?");
        Scanner sc=new Scanner(System.in);
        String str=sc.nextLine();
        if(str.equals("no")) {
            p.setState(p.getAngryState());
        }
    }

    @Override
    public void study() {
        System.out.println("Studying Happy");
    }
    
}
class AngryState implements State{
    private Person p;
    
    public AngryState(Person p) {
        this.p = p;
    }

    @Override
    public void listen() {
        System.out.println("I can't listen more!");
    }

    @Override
    public void doHomeWork() {
        System.out.println("It is too Hard!!!");
    }

    @Override
    public void study() {
        System.out.println("Just want to sleep!!");
    }
}
class SadState implements State{
    private Person p;
    
    public SadState(Person p) {
        this.p = p;
    }

    @Override
    public void listen() {
        System.out.println("why I can't understand it!");
    }

    @Override
    public void doHomeWork() {
        System.out.println("I can't do the homework,");
        System.out.println("Just listen again?");
        Scanner sc=new Scanner(System.in);
        String str=sc.nextLine();
        if(str.equals("yes")) {
            p.setState(p.getHappyState());
            p.listen();
        }
        else 
            p.setState(p.getAngryState());
    }

    @Override
    public void study() {
        System.out.println("I can't study!");
    }
}

public class Test {
    public static void main(String[] args) {
        Person p=new Person();
        /*客户调用Context的方法,而不知道其内部具体的状态实现*/
        p.listen();
        p.doHomeWork();
        p.study();
    }
}

　　UML类图如下: