java设计模式之状态模式

Posted 2022-07-30 快乐崇拜234

正文在下面，先打个广告：

状态模式的定义与特点

对有状态的对象，把复杂的“判断逻辑”提取到不同的状态对象中，允许状态对象在其内部状态发生改变时改变其行为。

模式的结构

状态模式包含以下主要角色。

• 环境（Context）角色：也称为上下文，它定义了客户感兴趣的接口，维护一个当前状态，并将与状态相关的操作委托给当前状态对象来处理。
• 抽象状态（State）角色：定义一个接口，用以封装环境对象中的特定状态所对应的行为。
• 具体状态（Concrete State）角色：实现抽象状态所对应的行为。

典型类图为：

demo

以学生成绩为例，每次考完试，都根据成绩修改状态。
0-59：low
60-79: middle
80-100: heigh

状态类：

public interface State 

    void setScore(int score);

    String getName();

    int getScore();

三个状态：

public class Low implements State 

    private int score;
    private ScoreContext context;

    public Low(int score, ScoreContext context) 
        this.score = score;
        this.context = context;
        context.setState(this);
    

    @Override
    public void setScore(int score) 
        StateUtil.modifyState(score, context);
        System.out.println("current score:" + score + " State: " + context.getState().getName());
    

    @Override
    public String getName() 
        return "low";
    

    @Override
    public int getScore() 
        return score;
    

public class Middle implements State 

    private int score;
    private ScoreContext context;

    public Middle(int score, ScoreContext context) 
        this.score=score;
        this.context = context;
        context.setState(this);
    

    @Override
    public void setScore(int score) 
        StateUtil.modifyState(score, context);
        System.out.println("current score:" + score + " State: " + context.getState().getName());
    

    @Override
    public String getName() 
        return "Middle";
    
    @Override
    public int getScore() 
        return score;
    

public class High implements State 

    private int score;
    private ScoreContext context;

    public High(int score, ScoreContext context) 
        this.score=score;
        this.context = context;
        context.setState(this);
    

    @Override
    public void setScore(int score) 
        StateUtil.modifyState(score, context);
        System.out.println("current score:" + score + " State: " + context.getState().getName());
    

    @Override
    public String getName() 
        return "High";
    


    @Override
    public int getScore() 
        return score;
    

状态转换工具类

public class StateUtil 
    public static void modifyState(int score, ScoreContext context) 
        if (score >= 60 && score < 80) 
            new Middle(score, context);
         else if (score >= 80) 
            new High(score, context);
         else 
            new Low(score, context);
        
    

成绩上下文类

public class ScoreContext 

    private State state;

    public ScoreContext() 
        new Low(0, this);
    

    public State getState() 
        return state;
    

    public void setState(State state) 
        this.state = state;
    

    public void setScore(int score) 
        state.setScore(score);
    

测试方法：

public class Main 
    public static void main(String[] args) 
        ScoreContext context = new ScoreContext();

        context.setScore(20);
        context.setScore(70);
        System.out.println("=======" + context.getState().getScore());
        context.setScore(90);
        context.setScore(20);

    

输出：

current score:20 State: low
current score:70 State: Middle
=======70
current score:90 State: High
current score:20 State: low