設計模式--狀态模式

狀态模式

定義：允許一個對象在其内部狀态改變時，改變它的行為

舉例，在網站上觀看視訊的時候，可以暫停，播放，停止，快進，這都是狀态。

類型：行為型

适用場景：（1）一個對象存在多個狀态（不同狀态下行為不同），且狀态可互相轉換。（不同狀态下的行為不同，舉例，比如在做電梯，隻有在電梯停止的時候門才可以打開，電梯在上升或者下降的過程中，不能停止）

優點：（1）将不同的狀态隔離，（2）把各種狀态的轉換邏輯，分布到State的子類中，減少互相間依賴，（3）增加新的狀态非常簡單

缺點：（1）狀态多的業務場景導緻類數目增加，系統變複雜

狀态模式	兩者經常配合使用
享元模式

代碼

// 每個 class 都是一個 .java 檔案

public abstract class CourseVideoState {
    protected CourseVideoContext courseVideoContext;

    public void setCourseVideoContext(CourseVideoContext courseVideoContext) {
        this.courseVideoContext = courseVideoContext;
    }

    public abstract void play();
    public abstract void speed();
    public abstract void pause();
    public abstract void stop();
}

public class PauseState extends CourseVideoState {

    @Override
    public void play() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.PLAY_STATE);
    }

    @Override
    public void speed() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.SPEED_STATE);
    }

    @Override
    public void pause() {
        System.out.println("暫停播放課程視訊狀态");
    }

    @Override
    public void stop() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.STOP_STATE);
    }
}

public class PlayState extends CourseVideoState {

    @Override
    public void play() {
        System.out.println("正常播放課程視訊狀态");
    }

    @Override
    public void speed() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.SPEED_STATE);
    }

    @Override
    public void pause() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.PAUSE_STATE);
    }

    @Override
    public void stop() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.STOP_STATE);
    }
}

public class SpeedState extends CourseVideoState {
    @Override
    public void play() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.PLAY_STATE);
    }

    @Override
    public void speed() {
        System.out.println("快進播放課程視訊狀态");
    }

    @Override
    public void pause() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.PAUSE_STATE);
    }

    @Override
    public void stop() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.STOP_STATE);
    }
}

public class StopState extends CourseVideoState {
    @Override
    public void play() {
        super.courseVideoContext.setCourseVideoState(CourseVideoContext.PLAY_STATE);
    }

    @Override
    public void speed() {
        System.out.println("ERROR 停止狀态不能快進!!");
    }

    @Override
    public void pause() {
        System.out.println("ERROR 停止狀态不能暫停!!");
    }

    @Override
    public void stop() {
        System.out.println("停止播放課程視訊狀态");
    }
}

// 上下文
public class CourseVideoContext {

    private CourseVideoState courseVideoState;
    public final static PlayState PLAY_STATE = new PlayState();
    public final static StopState STOP_STATE = new StopState();
    public final static PauseState PAUSE_STATE = new PauseState();
    public final static SpeedState SPEED_STATE = new SpeedState();

    public CourseVideoState getCourseVideoState() {
        return courseVideoState;
    }

    public void setCourseVideoState(CourseVideoState courseVideoState) {
        this.courseVideoState = courseVideoState;
        this.courseVideoState.setCourseVideoContext(this);
    }
    public void play(){
        this.courseVideoState.play();
    }

    public void speed(){
        this.courseVideoState.speed();
    }

    public void stop(){
        this.courseVideoState.stop();
    }

    public void pause(){
        this.courseVideoState.pause();
    }
}

// 測試代碼
public class Test {
    public static void main(String[] args) {
        CourseVideoContext courseVideoContext = new CourseVideoContext();
        courseVideoContext.setCourseVideoState(new PlayState());

        System.out.println("目前狀态:"+courseVideoContext.getCourseVideoState().getClass().getSimpleName());
        courseVideoContext.pause();

        System.out.println("目前狀态:"+courseVideoContext.getCourseVideoState().getClass().getSimpleName());

        courseVideoContext.speed();

        System.out.println("目前狀态:"+courseVideoContext.getCourseVideoState().getClass().getSimpleName());

        courseVideoContext.stop();

        System.out.println("目前狀态:"+courseVideoContext.getCourseVideoState().getClass().getSimpleName());

        courseVideoContext.speed();
    }
}
           

UML類圖

源碼示例