大话设计模式——16.命令模式（Command Pattern）

简介

请求以命令的形式包裹在对象中，并传给调用对象。调用对象寻找可以处理该命令的对象进行执行。命令模式是一种特殊的策略模式，体现多个策略执行的问题，而不是选择的问题

UML图

应用场景

• 界面选择、键盘、按钮、事件操作都类似命令模式

示例

电视遥控器、电视、观看电视的人

1. 接收者：电视机

public class TVReceiver {

    public void on() {
        System.out.println("打开电视");
    }

    public void off() {
        System.out.println("关闭电视");
    }

    public void changeStation() {
        System.out.println("换台");
    }
}

2. 命令（指令）：

public interface Command {

    void excute();
}

public class OnCommand implements Command {


    private TVReceiver tvReceiver;

    public OnCommand(TVReceiver tvReceiver) {
        this.tvReceiver = tvReceiver;
    }

    @Override
    public void excute() {
        System.out.println("onCommand----execute");
        tvReceiver.on();
    }
}

public class OffCommand implements Command {


    private TVReceiver tvReceiver;

    public OffCommand(TVReceiver tvReceiver) {
        this.tvReceiver = tvReceiver;
    }

    @Override
    public void excute() {
        System.out.println("OffCommand----execute");
        tvReceiver.off();
    }
}

public class ChangeStationCommand implements Command {


    private TVReceiver tvReceiver;

    public ChangeStationCommand(TVReceiver tvReceiver) {
        this.tvReceiver = tvReceiver;
    }

    @Override
    public void excute() {
        System.out.println("ChangeStationCommand----execute");
        tvReceiver.changeStation();
    }
}

3. 调用者：观看者

public class Invoker {

    private OnCommand onCommand;

    private OffCommand offCommand;

    private ChangeStationCommand changeStationCommand;

    public OnCommand getOnCommand() {
        return onCommand;
    }

    public void setOnCommand(OnCommand onCommand) {
        this.onCommand = onCommand;
    }

    public OffCommand getOffCommand() {
        return offCommand;
    }

    public void setOffCommand(OffCommand offCommand) {
        this.offCommand = offCommand;
    }

    public ChangeStationCommand getChangeStationCommand() {
        return changeStationCommand;
    }

    public void setChangeStationCommand(ChangeStationCommand changeStationCommand) {
        this.changeStationCommand = changeStationCommand;
    }


    /**
     * 开
     */
    public void on() {
        onCommand.excute();
    }

    /**
     * 关
     */
    public void off() {
        offCommand.excute();
    }

    /**
     * 换台
     */
    public void changeStation() {
        changeStationCommand.excute();
    }
}

4. 运行

public class Main {

    public static void main(String[] args) {
        // 创建接收者（物品）
        TVReceiver tvReceiver = new TVReceiver();

        // 创建指令（遥控器）
        OnCommand onCommand = new OnCommand(tvReceiver);
        OffCommand offCommand = new OffCommand(tvReceiver);
        ChangeStationCommand changeStationCommand = new ChangeStationCommand(tvReceiver);

        // 创建使用者
        Invoker invoker = new Invoker();
        invoker.setOffCommand(offCommand);
        invoker.setOnCommand(onCommand);
        invoker.setChangeStationCommand(changeStationCommand);

        // 使用者作出指示
        invoker.on();
        System.out.println("------------");
        invoker.changeStation();
        System.out.println("------------");
        invoker.off();

    }
}

总结

• 优点

  • 调用者对象与接受者对象没有任何依赖关系，降低了系统的耦合度
  • 方便新的命令添加至系统中，扩展性强

• 缺点

  • 过多的命令会导致系统中存在过多的具体命令类