设计模式之命令模式
让我们从一个简单的项目（遥控器）了解命令模式，然后再给出他的定义：
此项目地址：
实现命令接口：
首先，让所有的命令对象实现相同的包含一个方法的接口。我们暂且使用一贯的名称execute（）。
这就是命令接口（Command.java）：
1 public interface Command {
2 public void execute(); //简单，只须一个方法 execute()
3 }
实现一个打开电灯的命令：
现在，假设想实现一个打开电灯的命令。有已知我们的Light类有两个方法：on()和off()。下面是如
何将它实现成一个命令（LightOnCommand.java）：
1 //这是一个命令，所以要实现Command接口
2 public class LightOnCommand implements Command{
3 Light light;
4
5 // 构造器被传入某个电灯（比如客厅的电灯），以便让这
6 // 个命令控制。一旦调用execute()，就有这个电灯对象
7 // 成为接收者。负责接收请求。
8 public LightOnCommand(Light light){
9 this.light = light;
10 }
11
12 // 这个execute()方法调用接收对象（我们正在控制的电灯）的on()方法
13 public void execute(){
14 light.on();
15 }
16 }
现在有了LightOnCommand类，接下来让我们看一下如何使用它（SimpleRemoteController.java）：
1 public class SimpleRemoteControl {
2 //有一个插槽持有命令，而这个命令控制着一个装置
3 Command slot;
4
5 public SimpleRemoteControl(){};
6
7 //这个方法用来设置插槽控制的命令。如果想改变遥
8 // 控器按钮的行为，可以多次调用这个方法
9 public void setCommand(Command command){
10 slot = command;
11 }
12
13 //当按下按钮时，这个方法就会被调用，使得当前命
14 // 令衔接插槽，并调用方法execute()
15 public void buttonWasPressed(){
16 slot.execute();
17 }
18 }
现在让我们测试一下（Main.java）：
1 public class Main {
2
3 public static void main(String[] args) {
4 SimpleRemoteControl remote = new SimpleRemoteControl();
5 Light light = new Light();
6 LightOnCommand lightOn = new LightOnCommand(light);
7
8 remote.setCommand(lightOn);
9 remote.buttonWasPressed();
10 }
11 }
结果：
好啦！通过如上例子，现在我们该给出命令模式的定义了：
命令模式：将“请求”封装成对象，以便使用不同的请求、队列或者日志来参数化其他对象。命令
模式也支持撤销的操作。
来看看他的类图：
上面完成了简单遥控，但是要知道，一个遥控器可不止有一个插槽（每个插槽都具备了“开”和“关”
按钮），所以我们现在就要用到数组，作如下工作：
1 onCommand[0] = onCommand; 2 offCommand[0] = offCommand;
实现遥控器(RemoteController.java）:
1 public class RemoteControl {
2 //这时候，遥控器要处理7个开宇关的命令，使用相应的数组记录这些命令
3 Command[] onCommands;
4 Command[] offCommands;
5
6 public RemoteControl() {
7 //在构造器中，只需实例化并初始化这两个开与关的数组
8 onCommands = new Command[7];
9 offCommands = new Command[7];
10
11 Command noCommand = new NoCommand();
12 for (int i = 0; i < 7; i++) {
13 onCommands[i] = noCommand;
14 offCommands[i] = noCommand;
15 }
16 }
17
18 //这个方法需要有3个参数，分别是插槽的位置，开的命令、关的命令。这些命令将记录
19 // 在开关数组中对应的插槽位置、以供稍后使用
20 public void setCommand(int slot, Command onCommand, Command offCommand) {
21 onCommands[slot] = onCommand;
22 offCommands[slot] = offCommand;
23 }
24
25 //当按下开或关的按钮，硬件就会负责调用相应的方法，也就是onButtonWasPushed
26 // 或offButtonWasPushed
27 public void onButtonWasPushed(int slot) {
28 onCommands[slot].execute();
29 }
30
31 public void offButtonWasPushed(int slot) {
32 offCommands[slot].execute();
33 }
34
35 public String toString() {
36 StringBuffer stringBuff = new StringBuffer();
37 stringBuff.append("\n------ Remote Control -------\n");
38 for (int i = 0; i < onCommands.length; i++) {
39 stringBuff.append("[slot " + i + "] " + onCommands[i].getClass().getName()
40 + " " + offCommands[i].getClass().getName() + "\n");
41 }
42 return stringBuff.toString();
43 }
44 }
实现命令：
LightOffCommand.java:
1 public class LightOffCommand implements Command {
2 Light light;
3
4 public LightOffCommand(Light light) {
5 this.light = light;
6 }
7
8 public void execute() {
9 light.off();
10 }
11 }
还有很多命令就不一一列举了，原理都一样，相当于很多机械操作。
开始测试遥控器：
1 public class RemoteLoader {
2
3 public static void main(String[] args) {
4 RemoteControl remoteControl = new RemoteControl();
5
6 Light livingRoomLight = new Light("Living Room");
7 Light kitchenLight = new Light("Kitchen");
8 CeilingFan ceilingFan= new CeilingFan("Living Room");
9 GarageDoor garageDoor = new GarageDoor("");
10 Stereo stereo = new Stereo("Living Room");
11
12 LightOnCommand livingRoomLightOn = new LightOnCommand(livingRoomLight);
13 LightOffCommand livingRoomLightOff = new LightOffCommand(livingRoomLight);
14 LightOnCommand kitchenLightOn = new LightOnCommand(kitchenLight);
15 LightOffCommand kitchenLightOff = new LightOffCommand(kitchenLight);
16
17 CeilingFanOnCommand ceilingFanOn = new CeilingFanOnCommand(ceilingFan);
18 CeilingFanOffCommand ceilingFanOff = new CeilingFanOffCommand(ceilingFan);
19
20 GarageDoorUpCommand garageDoorUp = new GarageDoorUpCommand(garageDoor);
21 GarageDoorDownCommand garageDoorDown = new GarageDoorDownCommand(garageDoor);
22
23 StereoOnWithCDCommand stereoOnWithCD = new StereoOnWithCDCommand(stereo);
24 StereoOffCommand stereoOff = new StereoOffCommand(stereo);
25
26 remoteControl.setCommand(0, livingRoomLightOn, livingRoomLightOff);
27 remoteControl.setCommand(1, kitchenLightOn, kitchenLightOff);
28 remoteControl.setCommand(2, ceilingFanOn, ceilingFanOff);
29 remoteControl.setCommand(3, stereoOnWithCD, stereoOff);
30
31 System.out.println(remoteControl);
32
33 remoteControl.onButtonWasPushed(0);
34 remoteControl.offButtonWasPushed(0);
35 remoteControl.onButtonWasPushed(1);
36 remoteControl.offButtonWasPushed(1);
37 remoteControl.onButtonWasPushed(2);
38 remoteControl.offButtonWasPushed(2);
39 remoteControl.onButtonWasPushed(3);
40 remoteControl.offButtonWasPushed(3);
41 }
42 }
注：在遥控器中，我们不想每次都检查是否某个插槽都加载了命令。比方说，在这个onButtonWasP-
ushed()方法中，我们可能需要这样的代码：
1 public void onButtonWasPushed(int slot){
2 if(onCommands[slot] != null){
3 onCommands[slot].execute();
4 }
5 }
So，为了避免上述做法，我们实现一个不做事前的命令：
1 public class NoCommand implements Command {
2 public void execute() { }
3 }
这么一来，在RemoteControl构造器中，我们将个每个插槽都预先制定或NoCommand对象，以便
确定每个插槽永远都有名命令对象。
1 Command noCommand = new NoCommand();
2 for (int i = 0; i < 7; i++) {
3 onCommands[i] = noCommand;
4 offCommands[i] = noCommand;
5 }
所以在测试的输出中，没有被明确指定命令的插槽，其命令将是默认的NoCommand对象。
第二个项目代码地址：
https://github.com/Stray-Kite/Design-Pattern/tree/master/src/headfirst/designpatterns/command/remote
还记得我们之前说的undo操作嘛？现在我们来实现它：
代码地址：https://github.com/Stray-Kite/Design-Pattern/tree/master/src/headfirst/designpatterns/command/undo
我们只需要在Command.java，我们只需加上第三行代码就好:
1 public interface Command {
2 public void execute();
3 public void undo();
4 }
然后再从LightOnCommand开始下手：
1 public class LightOnCommand implements Command {
2 Light light;
3 int level;
4 public LightOnCommand(Light light) {
5 this.light = light;
6 }
7
8 public void execute() {
9 level = light.getLevel();
10 light.on();
11 }
12
13 //加上
14 public void undo() {
15 light.dim(level);
16 }
17 }
最后是RemoteControllerWithUndo.java:
1 public class RemoteControlWithUndo {
2 Command[] onCommands;
3 Command[] offCommands;
4 Command undoCommand;
5
6 public RemoteControlWithUndo() {
7 onCommands = new Command[7];
8 offCommands = new Command[7];
9
10 Command noCommand = new NoCommand();
11 for(int i=0;i<7;i++) {
12 onCommands[i] = noCommand;
13 offCommands[i] = noCommand;
14 }
15 undoCommand = noCommand;
16 }
17
18 public void setCommand(int slot, Command onCommand, Command offCommand) {
19 onCommands[slot] = onCommand;
20 offCommands[slot] = offCommand;
21 }
22
23 public void onButtonWasPushed(int slot) {
24 onCommands[slot].execute();
25 undoCommand = onCommands[slot];
26 }
27
28 public void offButtonWasPushed(int slot) {
29 offCommands[slot].execute();
30 undoCommand = offCommands[slot];
31 }
32
33 public void undoButtonWasPushed() {
34 undoCommand.undo();
35 }
36
37 public String toString() {
38 StringBuffer stringBuff = new StringBuffer();
39 stringBuff.append("\n------ Remote Control -------\n");
40 for (int i = 0; i < onCommands.length; i++) {
41 stringBuff.append("[slot " + i + "] " + onCommands[i].getClass().getName()
42 + " " + offCommands[i].getClass().getName() + "\n");
43 }
44 stringBuff.append("[undo] " + undoCommand.getClass().getName() + "\n");
45 return stringBuff.toString();
46 }
47 }
测试类（RemoteLoader.java）:
1 public class RemoteLoader {
2
3 public static void main(String[] args) {
4 RemoteControlWithUndo remoteControl = new RemoteControlWithUndo();
5
6 Light livingRoomLight = new Light("Living Room");
7
8 LightOnCommand livingRoomLightOn = new LightOnCommand(livingRoomLight);
9 LightOffCommand livingRoomLightOff = new LightOffCommand(livingRoomLight);
10
11 remoteControl.setCommand(0, livingRoomLightOn, livingRoomLightOff);
12
13 remoteControl.onButtonWasPushed(0);
14 remoteControl.offButtonWasPushed(0);
15 System.out.println(remoteControl);
16 remoteControl.undoButtonWasPushed();
17 remoteControl.offButtonWasPushed(0);
18 remoteControl.onButtonWasPushed(0);
19 System.out.println(remoteControl);
20 remoteControl.undoButtonWasPushed();
21 22 }
23 }
好了，实现电灯的撤销很简单。但是，通常，想要实现撤销的功能，需要记录一些状态。让我们是一个更有
趣的例子，比方说厂商类中的天花板的吊扇。吊扇允许有多种转动速度，当然也允许被关闭。
吊扇源码如下：
1 public class CeilingFan {
2 public static final int HIGH = 3;
3 public static final int MEDIUM = 2;
4 public static final int LOW = 1;
5 public static final int OFF = 0;
6 String location;
7 int speed;
8
9 public CeilingFan(String location) {
10 this.location = location;
11 speed = OFF;
12 }
13
14 public void high() {
15 speed = HIGH;
16 System.out.println(location + " ceiling fan is on high");
17 }
18
19 public void medium() {
20 speed = MEDIUM;
21 System.out.println(location + " ceiling fan is on medium");
22 }
23
24 public void low() {
25 speed = LOW;
26 System.out.println(location + " ceiling fan is on low");
27 }
28
29 public void off() {
30 speed = OFF;
31 System.out.println(location + " ceiling fan is off");
32 }
33
34 public int getSpeed() {
35 return speed;
36 }
37 }
加入撤销到吊扇的命令类（CeilingFanHighCommand.java）：
1 public class CeilingFanHighCommand implements Command {
2 CeilingFan ceilingFan;
3 int prevSpeed;
4
5 public CeilingFanHighCommand(CeilingFan ceilingFan) {
6 this.ceilingFan = ceilingFan;
7 }
8
9 public void execute() {
10 prevSpeed = ceilingFan.getSpeed();
11 ceilingFan.high();
12 }
13
14 public void undo() {
15 if (prevSpeed == CeilingFan.HIGH) {
16 ceilingFan.high();
17 } else if (prevSpeed == CeilingFan.MEDIUM) {
18 ceilingFan.medium();
19 } else if (prevSpeed == CeilingFan.LOW) {
20 ceilingFan.low();
21 } else if (prevSpeed == CeilingFan.OFF) {
22 ceilingFan.off();
23 }
24 }
25 }
添加测试类（RemoteLoader.java）：
1 public class RemoteLoader {
2
3 public static void main(String[] args) {
4 CeilingFan ceilingFan = new CeilingFan("Living Room");
5
6 CeilingFanMediumCommand ceilingFanMedium = new CeilingFanMediumCommand(ceilingFan);
7 CeilingFanHighCommand ceilingFanHigh = new CeilingFanHighCommand(ceilingFan);
8 CeilingFanOffCommand ceilingFanOff = new CeilingFanOffCommand(ceilingFan);
9
10 remoteControl.setCommand(0, ceilingFanMedium, ceilingFanOff);
11 remoteControl.setCommand(1, ceilingFanHigh, ceilingFanOff);
12
13 remoteControl.onButtonWasPushed(0);
14 remoteControl.offButtonWasPushed(0);
15 System.out.println(remoteControl);
16 remoteControl.undoButtonWasPushed();
17
18 remoteControl.onButtonWasPushed(1);
19 System.out.println(remoteControl);
20 remoteControl.undoButtonWasPushed();
21 }
22 }
最后，我们相同时弄暗灯光、打开音响和电视、设置好DVD，并让热水器开始加温：
代码地址： https://github.com/Stray-Kite/Design-Pattern/tree/master/src/headfirst/designpatterns/command/party
MacroCommand.java:
1 public class MacroCommand implements Command {
2 Command[] commands;
3
4 public MacroCommand(Command[] commands) {
5 this.commands = commands;
6 }
7
8 public void execute() {
9 for (int i = 0; i < commands.length; i++) {
10 commands[i].execute();
11 }
12 }
13
14 public void undo() {
15 for (int i = commands.length -1; i >= 0; i--) {
16 commands[i].undo();
17 }
18 }
19 }
使用宏命令（RemoteLoader.java）：
1 public class RemoteLoader {
2
3 public static void main(String[] args) {
4
5 RemoteControl remoteControl = new RemoteControl();
6
7 //首先创建想要进入宏的命令集合
8 Light light = new Light("Living Room");
9 TV tv = new TV("Living Room");
10 Stereo stereo = new Stereo("Living Room");
11 Hottub hottub = new Hottub();
12
13 LightOnCommand lightOn = new LightOnCommand(light);
14 StereoOnCommand stereoOn = new StereoOnCommand(stereo);
15 TVOnCommand tvOn = new TVOnCommand(tv);
16 HottubOnCommand hottubOn = new HottubOnCommand(hottub);
17 LightOffCommand lightOff = new LightOffCommand(light);
18 StereoOffCommand stereoOff = new StereoOffCommand(stereo);
19 TVOffCommand tvOff = new TVOffCommand(tv);
20 HottubOffCommand hottubOff = new HottubOffCommand(hottub);
21
22 //接下来创建两个数组，其中一个用来记录开启命令，另一个用来记录关闭命令，并在数组内放入对应的命令
23 Command[] partyOn = { lightOn, stereoOn, tvOn, hottubOn};
24 Command[] partyOff = { lightOff, stereoOff, tvOff, hottubOff};
25
26 MacroCommand partyOnMacro = new MacroCommand(partyOn);
27 MacroCommand partyOffMacro = new MacroCommand(partyOff);
28
29 //然后将宏命令指定给我们所希望的按钮
30 remoteControl.setCommand(0, partyOnMacro, partyOffMacro);
31
32 //最后，按一按按钮，测试一下
33 System.out.println(remoteControl);
34 System.out.println("--- Pushing Macro On---");
35 remoteControl.onButtonWasPushed(0);
36 System.out.println("--- Pushing Macro Off---");
37 remoteControl.offButtonWasPushed(0);
38 }
39 }
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