命令模式是一种行为型设计模式，它将请求封装成一个对象，从而使您可以使用不同的请求、队列或日志来参数化其他对象。命令模式还支持可撤销操作。
下面是一个简单的Java实现命令模式的示例：
首先，我们定义一个命令接口，它包含一个执行方法和一个撤销方法：
public interface Command {
void execute();
void undo();
}然后，我们实现具体的命令类，这里我们以一个简单的电灯开关为例：
public class LightOnCommand implements Command {
private Light light;
public LightOnCommand(Light light) {
this.light = light;
}
public void execute() {
light.turnOn();
}
public void undo() {
light.turnOff();
}
}
public class LightOffCommand implements Command {
private Light light;
public LightOffCommand(Light light) {
this.light = light;
}
public void execute() {
light.turnOff();
}
public void undo() {
light.turnOn();
}
}接下来，我们定义一个遥控器类，它包含一组命令对象：
public class RemoteControl {
private Command[] onCommands;
private Command[] offCommands;
private Command undoCommand;
public RemoteControl() {
onCommands = new Command[7];
offCommands = new Command[7];
Command 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 onButtonPressed(int slot) {
onCommands[slot].execute();
undoCommand = onCommands[slot];
}
public void offButtonPressed(int slot) {
offCommands[slot].execute();
undoCommand = offCommands[slot];
}
public void undoButtonPressed() {
undoCommand.undo();
}
}最后，我们可以使用这些类来控制电灯的开关：
public class Client {
public static void main(String[] args) {
RemoteControl remoteControl = new RemoteControl();
Light livingRoomLight = new Light();
LightOnCommand livingRoomLightOn = new LightOnCommand(livingRoomLight);
LightOffCommand livingRoomLightOff = new LightOffCommand(livingRoomLight);
remoteControl.setCommand(0, livingRoomLightOn, livingRoomLightOff);
remoteControl.onButtonPressed(0); // turn on the living room light
remoteControl.offButtonPressed(0); // turn off the living room light
remoteControl.undoButtonPressed(); // undo the last command (turn off the living room light)
}
}这个例子中，我们使用命令模式将电灯的开关操作封装成了命令对象，并将这些命令对象存储在遥控器中。当用户按下遥控器上的按钮时，遥控器会调用相应的命令对象的执行方法。如果用户想要撤销上一次操作，遥控器会调用相应的命令对象的撤销方法。
