外观模式给多个复杂子系统提供一个统一的界面，使得用户对子系统的使用更简便。
用户故事：家庭影院中有一套智能设备，每个设备都有若干个操作方法，用户希望通过一个手机APP经过简单的几个按键就对所有设备执行启动、关闭等操作。

一、UML类图

在这里插入图片描述

二、代码示例

三个具体类

1. DVD

package behaviour.facade;

public class DVDPlayer {
    public static DVDPlayer getInstance() {
        return Holder.player;
    }
    private DVDPlayer() {}
    private static class Holder {
        private static DVDPlayer player = new DVDPlayer();
    }

    public void on() {
        System.out.println("DVD on");
    }

    public void off() {
        System.out.println("DVD off");
    }

    public void play() {
        System.out.println("DVD playing");
    }

    public void pause() {
        System.out.println("DVD paused");
    }

}

2. Projector

package behaviour.facade;

public class Projector {
    public static Projector getInstance() {
        return Holder.projector;
    }
    private Projector() {}
    private static class Holder {
        private static Projector projector = new Projector();
    }

    public void on() {
        System.out.println("Projector on");
    }

    public void off() {
        System.out.println("Projector off");
    }

    public void focus() {
        System.out.println("Popcorn is focusing");
    }


}

3. Popcorn Machine

package behaviour.facade;

public class Popcorn {
    public static Popcorn getInstance() {
        return Holder.popMachine;
    }
    private Popcorn() {}
    private static class Holder {
        private static Popcorn popMachine = new Popcorn();
    }

    public void on() {
        System.out.println("Popcorn on");
    }

    public void off() {
        System.out.println("Popcorn off");
    }

    public void pop() {
        System.out.println("Popcorn making popcorn");
    }


}

外观类

package behaviour.facade;

public class ApplianceFacade {
    private DVDPlayer player;
    private Popcorn popMachine;
    private Projector projector;

    public ApplianceFacade() {
        this.player = DVDPlayer.getInstance();
        this.popMachine = Popcorn.getInstance();
        this.projector = Projector.getInstance();
    }

    public void on() {
        player.on();
        popMachine.on();
        projector.on();
    }

    public void play() {
        player.play();
    }

    public void pop() {
        popMachine.pop();
    }

    public void off() {
        player.off();
        popMachine.off();
        projector.off();
    }
}

客户类

package behaviour.facade;

public class Client {
    public static void main(String[] args) {
        ApplianceFacade facade = new ApplianceFacade();
        facade.on();
        facade.pop();
        facade.play();
        facade.off();
    }
}

运行结果

DVD on
Popcorn on
Projector on
---------------
Popcorn making popcorn
---------------
DVD playing
---------------
DVD off
Popcorn off
Projector off

Process finished with exit code 0