命令模式就是将请求封装为一个对象，从而使你可用不同的请求对客户进行参数化; 对请求排队或记录请求日志，以及支持可撤销的操作。

在软件系统中，行为请求者与行为实现者通常是一种紧耦合的关系，但某些场合，比如需要对行为进行记录、撤销或重做、事务等处理时，这种无法抵御变化的紧耦合的设计就不太合适。

这是一个遥控器的例子
一个灯类，有开灯、关灯、昏暗灯光之类的方法。
一个接口命令类：定义了执行和撤销两个方法。
下面有四个类实现了命令接口，分别是：开灯命令、关灯命令、昏暗开灯命令、昏暗关灯命令。
一个遥控器类，在模式中叫做调度者，依赖命令接口，调用命令的方法。
客户类，上图没有画，初始化命令类，传入遥控器类，调用遥控器类相应方法。

代码：
Light.java:

package headfirst.command.undo;

public class Light {
	String location;
	int level;

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

	public void on() {
		level = 100;
		System.out.println("Light is on");
	}

	public void off() {
		level = 0;
		System.out.println("Light is off");
	}

	public void dim(int level) {
		this.level = level;
		if (level == 0) {
			off();
		}
		else {
			System.out.println("Light is dimmed to " + level + "%");
		}
	}

	public int getLevel() {
		return level;
	}
}

命令接口 Command.java:

package headfirst.command.undo;

public interface Command {
	public void execute();
	public void undo();
}

开灯命令 LightOnCommand.java：

package headfirst.command.undo;

public class LightOnCommand implements Command {
	Light light;
 
	public LightOnCommand(Light light) {
		this.light = light;
	}
 
	public void execute() {
		light.on();
	}
 
	public void undo() {
		light.off();
	}
}

关灯命令 LightOffCommand.java：

package headfirst.command.undo;

public class LightOffCommand implements Command {
	Light light;
 
	public LightOffCommand(Light light) {
		this.light = light;
	}
 
	public void execute() {
		light.off();
	}
 
	public void undo() {
		light.on();
	}
}

昏暗开灯命令 DimmerLightOnCommand.java:

package headfirst.command.undo;

public class DimmerLightOnCommand implements Command {
	Light light;
	int prevLevel;

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

	public void execute() {
		prevLevel = light.getLevel();
		light.dim(75);
	}

	public void undo() {
		light.dim(prevLevel);
	}
}

昏暗关灯 DimmerLightOffCommand.java:

package headfirst.command.undo;

public class DimmerLightOffCommand implements Command {
	Light light;
	int prevLevel;

	public DimmerLightOffCommand(Light light) {
		this.light = light;
		prevLevel = 100;
	}

	public void execute() {
		prevLevel = light.getLevel();
		light.off();
	}

	public void undo() {
		light.dim(prevLevel);
	}
}

空命令 NoCommand.java：

package headfirst.command.undo;

public class NoCommand implements Command {
	public void execute() { }
	public void undo() { }
}

遥控器 RemoteControlWithUndo.java:

package headfirst.command.undo;

import java.util.*;

//
// This is the invoker
//
public class RemoteControlWithUndo {
	Command[] onCommands;
	Command[] offCommands;
	Command undoCommand;
 
	public RemoteControlWithUndo() {
		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;
		}
		undoCommand = noCommand;
	}
  
	public void setCommand(int slot, Command onCommand, Command offCommand) {
		onCommands[slot] = onCommand;
		offCommands[slot] = offCommand;
	}
 
	public void onButtonWasPushed(int slot) {
		onCommands[slot].execute();
		undoCommand = onCommands[slot];
	}
 
	public void offButtonWasPushed(int slot) {
		offCommands[slot].execute();
		undoCommand = offCommands[slot];
	}
 
	public void undoButtonWasPushed() {
		undoCommand.undo();
	}
  
	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");
		}
		stringBuff.append("[undo] " + undoCommand.getClass().getName() + "\n");
		return stringBuff.toString();
	}
}

入口类：

package headfirst.command.undo;

public class RemoteLoader {
 
	public static void main(String[] args) {
		RemoteControlWithUndo remoteControl = new RemoteControlWithUndo();
 
		Light livingRoomLight = new Light("Living Room");
 
		LightOnCommand livingRoomLightOn = 
				new LightOnCommand(livingRoomLight);
		LightOffCommand livingRoomLightOff = 
				new LightOffCommand(livingRoomLight);
 
		remoteControl.setCommand(0, livingRoomLightOn, livingRoomLightOff);
 
		remoteControl.onButtonWasPushed(0);
		remoteControl.offButtonWasPushed(0);
		System.out.println(remoteControl);
		remoteControl.undoButtonWasPushed();
		remoteControl.offButtonWasPushed(0);
		remoteControl.onButtonWasPushed(0);
		System.out.println(remoteControl);
		remoteControl.undoButtonWasPushed();

		CeilingFan ceilingFan = new CeilingFan("Living Room");
   
		CeilingFanMediumCommand ceilingFanMedium = 
				new CeilingFanMediumCommand(ceilingFan);
		CeilingFanHighCommand ceilingFanHigh = 
				new CeilingFanHighCommand(ceilingFan);
		CeilingFanOffCommand ceilingFanOff = 
				new CeilingFanOffCommand(ceilingFan);
  
		remoteControl.setCommand(0, ceilingFanMedium, ceilingFanOff);
		remoteControl.setCommand(1, ceilingFanHigh, ceilingFanOff);
   
		remoteControl.onButtonWasPushed(0);
		remoteControl.offButtonWasPushed(0);
		System.out.println(remoteControl);
		remoteControl.undoButtonWasPushed();
  
		remoteControl.onButtonWasPushed(1);
		System.out.println(remoteControl);
		remoteControl.undoButtonWasPushed();
	}
}