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Java 设计模式之命令模式

概述

• 命令模式(Command)：将一个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。
• 敏捷开发原则告诉我们，不要为代码添加基于猜测的、实际不需要的功能。如果不清楚一个系统是否需要命令模式，一般就不要着急去实现它，事实上，在需要的时候通过重构实现这个模式并不困难，只有在真正需要如撤销／恢复操作等功能时，把原来的代码重构为命令模式才有意义。

UML

• Command：命令接口，定义执行操作的通用接口。
• ConcreteCommand：具体命令类，实现具体的命令操作，持有接受者对象。
• Receiver：接收者累，实际执行操作的对象。
• Invoker：调用者，触发命令的对象。

代码实现

简单实现

定义命令接口：

public interface Command {void execute();
}

定义具体命令类：

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

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

定义接收者类：

public class Light {public void turnOn() {System.out.println("开灯");}public void turnOff() {System.out.println("关灯");}
}

定义调用者类：

public class RemoteControl {private Command command;public void setCommand(Command command) {this.command = command;}public void pressButton() {command.execute();}
}

使用：

public class Client {public static void main(String[] args) {Light light = new Light();Command lightOn = new LightOnCommand(light);Command lightOff = new LightOffCommand(light);RemoteControl control = new RemoteControl();control.setCommand(lightOn);control.pressButton();control.setCommand(lightOff);control.pressButton();}
}

输出：

开灯
关灯

复杂实现

定义抽象命令类：

abstract class Command {protected RoastCook receiver;public Command(RoastCook receiver) {this.receiver = receiver;}abstract void execute();
}

定义接收者类：

public class RoastCook {public void bakeMeet() {System.out.println("烤肉");}public void bakeFish() {System.out.println("烤鱼");}
}

定义具体命令类：

public class RoastFishCommand extends Command {public RoastFishCommand(RoastCook receiver) {super(receiver);}@Overridevoid execute() {receiver.bakeFish();}
}

public class RoastMeatCommand extends Command {public RoastMeatCommand(RoastCook receiver) {super(receiver);}@Overridevoid execute() {receiver.bakeMeet();}
}

定义调用者类：

public class Waiter {private ArrayList<Command> orders = new ArrayList<>();private Command command;// 下单public void setOrder(Command command) {String className = command.getClass().getSimpleName();if (className.equals("RoastFishCommand")) {System.out.println("下单：" + "烤鱼");} else {System.out.println("下单：" + "烤肉");}orders.add(command);}// 取消订单public void cancelOrder(Command command) {String className = command.getClass().getSimpleName();if (className.equals("RoastFishCommand")) {System.out.println("取消：" + "烤鱼");} else {System.out.println("取消：" + "烤肉");}orders.remove(command);}// 通知public void notifyCommand() {for (Command command : orders) {command.execute();}}
}

使用：

public class Client {public static void main(String[] args) {RoastCook receiver = new RoastCook();Command roastMeatCommand = new RoastMeatCommand(receiver);Command roastFishCommand1 = new RoastFishCommand(receiver);Command roastFishCommand2 = new RoastFishCommand(receiver);Command roastFishCommand3 = new RoastFishCommand(receiver);Waiter waiter = new Waiter();waiter.setOrder(roastFishCommand1);waiter.setOrder(roastFishCommand2);waiter.setOrder(roastMeatCommand);waiter.setOrder(roastFishCommand3);waiter.cancelOrder(roastFishCommand2);waiter.notifyCommand();}
}

输出：

下单：烤鱼
下单：烤鱼
下单：烤肉
下单：烤鱼
取消：烤鱼
烤鱼
烤肉
烤鱼