大话设计模式——17.状态模式（State Pattern）

简介

对象的行为依赖于它的状态（属性），可以根据状态的改变而改变相关行为。

UML图：

在这里插入图片描述

应用场景：

对象的行为取决于其状态，并且必须要在运行时刻根据状态而改变行为
代码中包含大量与对象状态有关的条件语句

示例

上午、下午、晚上工作的状态

上下文对象：

public class Work {

    /**
     * 工作状态
     */
    private State state;

    /**
     * 当前时间
     */
    private int hour;


    /**
     * 工作是否完成
     */
    private boolean isFinished;


    public Work() {
        // 初始化操作
        this.state = new MorningState();
    }

    public void writeProgram() {
        state.writeProgram(this);
    }


    public State getState() {
        return state;
    }

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

    public int getHour() {
        return hour;
    }

    public void setHour(int hour) {
        this.hour = hour;
    }

    public boolean isFinished() {
        return isFinished;
    }

    public void setFinished(boolean finished) {
        isFinished = finished;
    }
}

状态

public interface State {

    void writeProgram(Work work);
}

public class MorningState implements State {

    @Override
    public void writeProgram(Work work) {
        if (work.getHour() < 12) {
            System.out.println("当前时间：" + work.getHour() + "，干劲十足！");
        } else {
            work.setState(new NoonState());
            work.writeProgram();
        }
    }
}

public class NoonState implements State {

    @Override
    public void writeProgram(Work work) {
        if (work.getHour() < 13) {
            System.out.println("当前时间：" + work.getHour() + "，好饿，开始干饭!");
        } else {
            work.setState(new AfternoonState());
            work.writeProgram();
        }
    }
}

public class AfternoonState implements State {

    @Override
    public void writeProgram(Work work) {
        if (work.getHour() < 18) {
            System.out.println("当前时间：" + work.getHour() + "，开始上班，有点疲惫！");
        } else {
            work.setState(new EveningState());
            work.writeProgram();
        }
    }
}

public class EveningState implements State {

    @Override
    public void writeProgram(Work work) {
        if (!work.isFinished()) {
            System.out.println("当前时间：" + work.getHour() + "，又要加班");
        } else {
            System.out.println("当前时间：" + work.getHour() + "，下班啦！");
        }
    }
}

运行

public class Main {

    public static void main(String[] args) {
        Work work = new Work();
        // 上午
        work.setHour(9);
        work.writeProgram();

        // 中午
        work.setHour(12);
        work.writeProgram();

        // 下午
        work.setHour(16);
        work.writeProgram();

        // 晚上
        work.setHour(18);
//        work.setFinished(true);
        work.writeProgram();
    }
}