目的

将命令封装成对象中，具有以下作用：

  • 使用命令来参数化其他对象
  • 将命令放入队列中进行排队
  • 将命令的操作记录到日志中
  • 支持可撤销的操作

类图

  • Command: 命令
  • Receiver: 命令接收者，也就是命令真正的执行者
  • Invoker: 通过它来调用命令
  • Client: 可以设置命令与命令的接收者

命令类图.png

实现

设计一个遥控器，可以控制电灯开关。
遥控器.jpg

  1. public interface Command{
  2.     void execute();
  3. }
  6. public class LightOnCommand implements Command{
  7.     Light light;
  9.     public LightOnCommand(Light light){
  10.         this.light = light;
  11.     }
  13.     @Override
  14.     public void execute(){
  15.         light.on();
  16.     }
  17. }
  20. public class LightOffCommand implements Command{
  21.     Light light;
  23.     public LightOffCommand(Light light){
  24.         this.light = light;
  25.     }
  27.     @Override
  28.     public void execute(){
  29.         light.off();
  30.     }
  31. }
  34. public class Light{
  35.     public void on(){
  36.         System.out.println("Light is on!");
  37.     }
  39.     public void off(){
  40.         System.out.println("Light is off!");
  41.     }
  42. }
  45. //遥控器
  46. public class Invoker{
  47.     private Command[] onCommands;
  48.     private Command[] offCommands;
  49.     private final int slotNum = 7;
  51.     public Invoker(){
  52.         this.onCommand = new Command[slotNum];
  53.         this.offCommand = new Command[slotNum];
  54.     }
  56.     public void setOnCommand(Command command, int slot){
  57.         onCommands[slot] = command;
  58.     }
  60.     public void setOffCommand(Command command, int slot){
  61.         offCommands[slot] = command;
  62.     }
  64.     public void onButtonWasPushed(int slot){
  65.         onCommands[slot].execute();
  66.     }
  68.     public void offButtonWasPushed(int slot){
  69.         offCommands[slot].execute();
  70.     }
  71. }
  74. public class Client{
  75.     public static void main(String[] args){
  76.         Invoker invoker = new Invoker();
  77.         Light light = new Light();
  78.         Command lightOnCommand = new LightOnCommand(light);
  79.         Command lightOffCommand = new LightOffCommand(light);
  80.         invoker.setOnCommand(lightOnCommand, 0);
  81.         invoker.setOffCommand(lightOffCommand, 0);
  82.         invoker.onButtonWasPushed(0);
  83.         invoker.offButtonWasPushed(0);
  84.     }
  85. }

JDK中的体现