命令模式

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。命令模式也支持可撤销的操作。

一个命令对象通过在特定接受者上绑定一组动作来封装一个请求，要达到这一点，命令对象将动作和接受者包装进对象中。这个对象只暴露一个execute()，当此方法被调用的时候，接受者就会进行这些动作。

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[]命令模式的实现

命令接口

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

命令实现类

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

命令实现类

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

命令实现类

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

命令实现类

public class StereoOnWithCDCommand implements Command {private Stereo stereo;public StereoOnWithCDCommand(Stereo stereo) {    this.stereo = stereo;}@Overridepublic void execute() {    stereo.on();    stereo.setCD();    stereo.setVolume();}@Overridepublic void undo() {    stereo.off();}}

命令实现类

public class StereoOffWithCDCommand implements Command {private Stereo stereo;public StereoOffWithCDCommand(Stereo stereo) {    this.stereo = stereo;}@Overridepublic void execute() {    stereo.off();}@Overridepublic void undo() {    stereo.on();    stereo.setCD();    stereo.setVolume();}}

命令实现类

public class MacroCommand implements Command {private Command[] commands;public MacroCommand(Command[] commands) {    this.commands = commands;}@Overridepublic void execute() {    for (int i = 0; i < commands.length; i++) {        commands[i].execute();    }}@Overridepublic void undo() {}}

接受者类

public class Light {public void on() {    System.out.println("light on");}public void off() {    System.out.println("light off");}}

接受者类

public class Stereo {public void on() {    System.out.println("stereo on");}public void setCD() {    System.out.println("stereo setCD");}public void setVolume() {    System.out.println("stereo setVolume");}public void off() {    System.out.println("stereo off");}}

调用者类

public class RemoteControl {private Command[] onCommands;private Command[] offCommands;private Command undoCommand;public RemoteControl() {    onCommands = new Command[7];    offCommands = new Command[7];    NoCommand 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 onButtonWasPressed(int slot) {    onCommands[slot].execute();    undoCommand = onCommands[slot];}public void offButtonWasPressed(int slot) {    offCommands[slot].execute();    undoCommand = offCommands[slot];}public void undoButtonWasPressed() {    undoCommand.undo();}}

测试

public static void main(String args[]) {    RemoteControl control = new RemoteControl();    Light light = new Light();    Stereo stereo = new Stereo();    Command lightOnCommand = new LightOnCommand(light);    Command lightOffCommand = new LightOffCommand(light);    Command stereoOnCommand = new StereoOnWithCDCommand(stereo);    Command stereoOffCommand = new StereoOffWithCDCommand(stereo);    control.setCommand(0, lightOnCommand, lightOffCommand);    control.setCommand(1, stereoOnCommand, stereoOffCommand);    control.onButtonWasPressed(0);    System.out.println("****************");    control.offButtonWasPressed(1);    System.out.println("****************");    control.undoButtonWasPressed();    System.out.println("****************");    Command[] partyOn = {lightOnCommand, stereoOnCommand};    Command[] partyOff = {lightOffCommand, stereoOffCommand};    MacroCommand partyOnMacro = new MacroCommand(partyOn);    MacroCommand partyOffMacro = new MacroCommand(partyOff);    control.setCommand(2, partyOnMacro, partyOffMacro);    control.onButtonWasPressed(2);    System.out.println("****************");    control.offButtonWasPressed(2);}

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()命令模式的用途：队列请求，日程安排，线程池，工作队列等。
()命令模式将发送请求的对象和执行请求的对象解耦。
()在被解耦的两者之间是通过命令对象进行沟通的，命令对象封装了接收者和一个或者一组动作。
()调用者通过调用命令对象的execute()发出请求，这会使得接收者的动作被调用。
()命令可以支持撤销，做法是实现一个undo()来回到execute()被执行前的状态。
()宏命令是命令的一种简单延伸，允许调用多个命令，宏方法也可以支持撤销。