Java 设计模式(6) —— 命令模式

一、命令模式

将请求、命令、动作等封装成对象，这样可以让项目使用这些对象来参数化其他对象。使得命令的请求者和执行者解耦

命令模式

二、示例

家电自动化遥控器API项目：智能遥控器上有多个开关按钮，可是实现不同家电的控制，也可对新家电进行接入拓展

如果用传统面向对象的模式，则是定义一个遥控的对象，在对象内进行每个按钮的监听，分别判断进行相应的处理

1.电灯对象，控制开关


/**
 * 电灯对象
 */
public class Light {
    String loc = "";
    public Light(String loc) {
        this.loc = loc;
    }
    public void On() {
        System.out.println(loc + " On");
    }
    public void Off() {
        System.out.println(loc + " Off");
    }
}

2.收音机对象，控制开关和音量


/**
 * 收音机对象
 */
public class Stereo {
    static int volume = 0;
    public void On() {
        System.out.println("Stereo On");
    }
    public void Off() {
        System.out.println("Stereo Off");
    }
    public void SetCd() {
        System.out.println("Stereo SetCd");
    }
    public void SetVol(int vol) {
        volume = vol;
        System.out.println("Stereo volume=" + volume);
    }
    public int GetVol() {
        return volume;
    }
    public void Start() {
        System.out.println("Stereo Start");
    }
}

3.遥控器对象，进行判断处理


/**
 * 面向对象模式，遥控对象根据按钮的序号进行相应的处理
 */
public class TraditionControl implements Control {
    Light light;
    Stereo stereo;
    public TraditionControl(Light light, Stereo stereo) {
        this.light = light;
        this.stereo = stereo;
    }
    @Override
    public void onButton(int slot) {
        // TODO Auto-generated method stub
        switch (slot) {
        case 0:
            light.On();
            break;
        case 1:
            stereo.On();
            break;
        case 2:
            int vol = stereo.GetVol();
            if (vol < 11) {
                stereo.SetVol(++vol);
            }
            break;
        }
    }
    @Override
    public void offButton(int slot) {
        // TODO Auto-generated method stub
        switch (slot) {
        case 0:
            light.Off();
            break;
        case 1:
            stereo.Off();
            break;
        case 2:
            int vol = stereo.GetVol();
            if (vol > 0) {
                stereo.SetVol(--vol);
            }
            break;
        }
    }
    @Override
    public void undoButton() {
        // TODO Auto-generated method stub
    }
}

缺点：所有操作均在遥控对象内控制，耦合度太高，且如果新增或删减电器则会对整个控制对象进行改动，不利于维护

三、命令模式的实现

1.定义统一的命令方法接口


/**
 * 命令模式，命令方法接口
 */
public interface Command {
    public void execute();
    public void undo();
}

2.创建各自不同的命令对象


/**
 * 命令模式，电灯打开的命令操作
 */
public class LightOnCommand implements Command {
    private Light light;
    public LightOnCommand(Light light) {
        this.light = light;
    }
    @Override
    public void execute() {
        light.On();
    }
    @Override
    public void undo() {
        light.Off();
    }
}


/**
 * 命令模式，电灯关闭的命令操作
 */
public class LightOffCommand implements Command {
    private Light light;
    public LightOffCommand(Light light)
    {
        this.light=light;
    }
    @Override
    public void execute() {
        light.Off();
    }
    @Override
    public void undo() {
        light.On();
    }
}

3.遥控器对象对各命令进行抽象处理


/**
 * 命令模式，所有响应均可抽象成命令，调用时直接调用命令执行方法即可
 */
public class CommandModeControl implements Control {
    private Command[] onCommands;
    private Command[] offCommands;
    public CommandModeControl() {
        onCommands = new Command[5];
        offCommands = new Command[5];
        Command noCommand = new NoCommand();
        for (int i = 0, len = onCommands.length; i < len; i++) {
            onCommands[i] = noCommand;
            offCommands[i] = noCommand;
        }
    }
    public void setCommand(int slot, Command onCommand, Command offCommand) {
        onCommands[slot] = onCommand;
        offCommands[slot] = offCommand;
    }
    // 开
    @Override
    public void onButton(int slot) {
        onCommands[slot].execute();
    }
    // 关
    @Override
    public void offButton(int slot) {
        offCommands[slot].execute();
    }
    // 返回
    @Override
    public void undoButton() {
    }
}

4.新增和删减电器时传入相应的实体类进行命令的初始化


/**
 * 命令模式，便于拓展
 */
public class ControlTest {
    public static void main(String[] args) {
        CommandModeControl control = new CommandModeControl();
        Light bedroomlight = new Light("BedRoom");
        Light kitchlight = new Light("Kitch");
        Stereo stereo = new Stereo();
        LightOnCommand bedroomlighton = new LightOnCommand(bedroomlight);
        LightOffCommand bedroomlightoff = new LightOffCommand(bedroomlight);
        LightOnCommand kitchlighton = new LightOnCommand(kitchlight);
        LightOffCommand kitchlightoff = new LightOffCommand(kitchlight);
        StereoOnCommand stereoOn = new StereoOnCommand(stereo);
        StereoOffCommand stereoOff = new StereoOffCommand(stereo);
        StereoAddVolCommand stereoaddvol = new StereoAddVolCommand(stereo);
        StereoSubVolCommand stereosubvol = new StereoSubVolCommand(stereo);
        control.setCommand(0, bedroomlighton, bedroomlightoff);
        control.setCommand(1, kitchlighton, kitchlightoff);
        control.setCommand(2, stereoOn, stereoOff);
        control.setCommand(3, stereoaddvol, stereosubvol);
        control.onButton(0);
        control.offButton(0);
        control.onButton(1);
        control.offButton(1);
        control.onButton(2);
        control.onButton(3);
        control.offButton(3);
        control.offButton(2);
    }
}

优点：

命令之间相互分离，互不影响，也便于新命令的接入和删除。
即使新增返回按钮，和组合按钮等功能，也易于拓展且不影响其它命令。

Java设计模式所有示例代码，持续更新中