大话设计模式——16.命令模式（Command Pattern）

简介

请求以命令的形式包裹在对象中，并传给调用对象。调用对象寻找可以处理该命令的对象进行执行。命令模式是一种特殊的策略模式，体现多个策略执行的问题，而不是选择的问题

UML图

应用场景

• 界面选择、键盘、按钮、事件操作都类似命令模式

示例

电视遥控器、电视、观看电视的人

1. 接收者：电视机

public class TVReceiver {public void on() {System.out.println("打开电视");}public void off() {System.out.println("关闭电视");}public void changeStation() {System.out.println("换台");}
}

2. 命令（指令）：

public interface Command {void excute();
}

public class OnCommand implements Command {private TVReceiver tvReceiver;public OnCommand(TVReceiver tvReceiver) {this.tvReceiver = tvReceiver;}@Overridepublic void excute() {System.out.println("onCommand----execute");tvReceiver.on();}
}

public class OffCommand implements Command {private TVReceiver tvReceiver;public OffCommand(TVReceiver tvReceiver) {this.tvReceiver = tvReceiver;}@Overridepublic void excute() {System.out.println("OffCommand----execute");tvReceiver.off();}
}

public class ChangeStationCommand implements Command {private TVReceiver tvReceiver;public ChangeStationCommand(TVReceiver tvReceiver) {this.tvReceiver = tvReceiver;}@Overridepublic void excute() {System.out.println("ChangeStationCommand----execute");tvReceiver.changeStation();}
}

3. 调用者：观看者

public class Invoker {private OnCommand onCommand;private OffCommand offCommand;private ChangeStationCommand changeStationCommand;public OnCommand getOnCommand() {return onCommand;}public void setOnCommand(OnCommand onCommand) {this.onCommand = onCommand;}public OffCommand getOffCommand() {return offCommand;}public void setOffCommand(OffCommand offCommand) {this.offCommand = offCommand;}public ChangeStationCommand getChangeStationCommand() {return changeStationCommand;}public void setChangeStationCommand(ChangeStationCommand changeStationCommand) {this.changeStationCommand = changeStationCommand;}/*** 开*/public void on() {onCommand.excute();}/*** 关*/public void off() {offCommand.excute();}/*** 换台*/public void changeStation() {changeStationCommand.excute();}
}

4. 运行

public class Main {public static void main(String[] args) {// 创建接收者（物品）TVReceiver tvReceiver = new TVReceiver();// 创建指令（遥控器）OnCommand onCommand = new OnCommand(tvReceiver);OffCommand offCommand = new OffCommand(tvReceiver);ChangeStationCommand changeStationCommand = new ChangeStationCommand(tvReceiver);// 创建使用者Invoker invoker = new Invoker();invoker.setOffCommand(offCommand);invoker.setOnCommand(onCommand);invoker.setChangeStationCommand(changeStationCommand);// 使用者作出指示invoker.on();System.out.println("------------");invoker.changeStation();System.out.println("------------");invoker.off();}
}

总结

• 优点

  • 调用者对象与接受者对象没有任何依赖关系，降低了系统的耦合度
  • 方便新的命令添加至系统中，扩展性强

• 缺点

  • 过多的命令会导致系统中存在过多的具体命令类