定義命令模式

　　命令模式將“請求”封裝成物件，以便使用不同的請求、佇列或者日誌來引數化其他物件。命令模式也支援可撤銷的操作。命令物件將動作和接收者包進物件中。

應用場景

命令模式的類圖

　遙控器問題：

可以把遙控器的按鈕看做是一個命令，按下命令的時候不需要知道命令是怎麼執行的，我們可以將遙控器每個按鍵看做是一個命令物件，實現的時候用陣列來表示遙控器的按鈕位置，然後為陣列賦值具體對應的命令物件

class Light {
	String location = "";

	public Light(String location) {
		this.location = location;
	}

	public void on() {
		System.out.println(location + " light is on");
	}

	public void off() {
		System.out.println(location + " light is off");
	}
}
//命令類實現
class LivingroomLightOffCommand implements Command {
	Light light;

	public LivingroomLightOffCommand(Light light) {
		this.light = light;
	}

	public void execute() {
		light.off();
	}
}
 class LivingroomLightOnCommand implements Command {
	Light light;

	public LivingroomLightOnCommand(Light light) {
		this.light = light;
	}

	public void execute() {
		light.on();
	}
}
class Stereo {
    String location;

    public Stereo(String location) {
        this.location = location;
    }

    public void on() {
        System.out.println(location + " stereo is on");
    }

    public void off() {
        System.out.println(location + " stereo is off");
    }

    public void setCD() {
        System.out.println(location + " stereo is set for CD input");
    }

    public void setDVD() {
        System.out.println(location + " stereo is set for DVD input");
    }

    public void setRadio() {
        System.out.println(location + " stereo is set for Radio");
    }

    public void setVolume(int volume) {
        // code to set the volume
        // valid range: 1-11 (after all 11 is better than 10, right?)
        System.out.println(location + " Stereo volume set to " + volume);
    }
}
 class StereoOffCommand implements Command {
    Stereo stereo;
 
    public StereoOffCommand(Stereo stereo) {
        this.stereo = stereo;
    }
 
    public void execute() {
        stereo.off();
    }
}
 class StereoOnWithCDCommand implements Command {
    Stereo stereo;
 
    public StereoOnWithCDCommand(Stereo stereo) {
        this.stereo = stereo;
    }
 
    public void execute() {
        stereo.on();
        stereo.setCD();
        stereo.setVolume(11);
    }
}

 class NoCommand implements Command {
    public void execute() { }
    public void undo() { }
}
 // This is the invoker
//
//invoker相當於訂單。它可以呼叫execute方法
 class RemoteControl {
	Command[] onCommands;
	Command[] offCommands;
 
	public RemoteControl() {     //在構造器例項化並初始化兩個開與關的陣列
		onCommands = new Command[7];
		offCommands = new Command[7];
 
		Command noCommand = new NoCommand();
		for (int i = 0; i < 7; i++) {
			onCommands[i] = noCommand;
			offCommands[i] = noCommand; //在構造器中將每個插槽都預先指定成noCommand物件，以便確定每個插槽永遠都有命令物件
		}
	}
  //將命令記錄在開關陣列中對應的插槽位置插槽
	public void setCommand(int slot, Command onCommand, Command offCommand) {
		onCommands[slot] = onCommand;
		offCommands[slot] = offCommand;
	}
  //按下按鍵 呼叫execute
	public void onButtonWasPushed(int slot) {
		onCommands[slot].execute();
	}

	public void offButtonWasPushed(int slot) {
		offCommands[slot].execute();
	}
  
	public String toString() {
		StringBuffer stringBuff = new StringBuffer();
		stringBuff.append("\n------ Remote Control -------\n");
		for (int i = 0; i < onCommands.length; i++) {
			stringBuff.append("[slot " + i + "] " + onCommands[i].getClass().getName()
				+ "    " + offCommands[i].getClass().getName() + "\n");
		}
		return stringBuff.toString();
	}
}
//測試
public class RemoteLoader {
 
	public static void main(String[] args) {
		RemoteControl remoteControl = new RemoteControl();
 
		Light livingRoomLight = new Light("Living Room");
		Light kitchenLight = new Light("Kitchen");
		CeilingFan ceilingFan= new CeilingFan("Living Room");
		GarageDoor garageDoor = new GarageDoor("");
		Stereo stereo = new Stereo("Living Room");
  
		LightOnCommand livingRoomLightOn = 
				new LightOnCommand(livingRoomLight);
		LightOffCommand livingRoomLightOff = 
				new LightOffCommand(livingRoomLight);
		LightOnCommand kitchenLightOn = 
				new LightOnCommand(kitchenLight);
		LightOffCommand kitchenLightOff = 
				new LightOffCommand(kitchenLight);

		StereoOnWithCDCommand stereoOnWithCD =
				new StereoOnWithCDCommand(stereo);
		StereoOffCommand  stereoOff =
				new StereoOffCommand(stereo);
 
		remoteControl.setCommand(0, livingRoomLightOn, livingRoomLightOff);
		remoteControl.setCommand(１, stereoOnWithCD, stereoOff);
  
		System.out.println(remoteControl)
 
		remoteControl.onButtonWasPushed(0);
		remoteControl.offButtonWasPushed(0);
		remoteControl.onButtonWasPushed(1);
		remoteControl.offButtonWasPushed(1);

	}
}
 

　新加入undo方法　然後每個具體的命令類都實現這個方法：undo方法中呼叫的操作和execute()方法中的操作相反，比如on的反面是off。

　遙控器類也要做一些修改：加入新的例項變數記錄上一個操作，當按下撤銷按鈕，就呼叫該例項變數的undo()方法。

　如果撤銷到某一個狀態需要記錄一些變數值，則在execute()方法中加入儲存操作前狀態數值的語句，然後在undo()的時候恢復。

例如將一個控制電風扇的命令加上undo方法

public class CeilingFanOffCommand implements Command {
	CeilingFan ceilingFan;
	int prevSpeed;
  
	public CeilingFanOffCommand(CeilingFan ceilingFan) {
		this.ceilingFan = ceilingFan;
	}
 
	public void execute() {
		prevSpeed = ceilingFan.getSpeed();
		ceilingFan.off();
	}
 
	public void undo() {
		if (prevSpeed == CeilingFan.HIGH) {
			ceilingFan.high();
		} else if (prevSpeed == CeilingFan.MEDIUM) {
			ceilingFan.medium();
		} else if (prevSpeed == CeilingFan.LOW) {
			ceilingFan.low();
		} else if (prevSpeed == CeilingFan.OFF) {
			ceilingFan.off();
		}
	}
}
 

  　實現Party模式就是按下一個遙控器開關按鈕會有一系列的執行者j接收到命令按一定的次序執行操作。實現得方法是使用巨集命令，用命令陣列儲存一大堆命令，在execute()方法中用一個for迴圈遍歷這個陣列，依次執行陣列各個執行物件的execute

//實現命令陣列
class MacroCommand implements Command {
	Command[] commands;
 
	public MacroCommand(Command[] commands) {
		this.commands = commands;
	}
 
	public void execute() {
		for (int i = 0; i < commands.length; i++) {
			commands[i].execute();
		}
	}
 
	public void undo() {
		for (int i = 0; i < commands.length; i++) {
			commands[i].undo();
		}
	}
}
//使用巨集命令示例
　　　　Command[] partyOn = { lightOn, stereoOn, tvOn, hottubOn};
		Command[] partyOff = { lightOff, stereoOff, tvOff, hottubOff};
  
		MacroCommand partyOnMacro = new MacroCommand(partyOn);
		MacroCommand partyOffMacro = new MacroCommand(partyOff);
 
		remoteControl.setCommand(0, partyOnMacro, partyOffMacro);
  
		System.out.println(remoteControl);
		System.out.println("--- Pushing Macro On---");
		remoteControl.onButtonWasPushed(0);
		System.out.println("--- Pushing Macro Off---");
		remoteControl.offButtonWasPushed(0);