Java 设计模式之命令模式

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;
    }

    @Override
    public void execute() {
        light.turnOff();
    }
}

public class LightOnCommand implements Command {
    private Light light;

    public LightOnCommand(Light light) {
        this.light = light;
    }

    @Override
    public 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);
    }

    @Override
    void execute() {
        receiver.bakeFish();
    }
}

public class RoastMeatCommand extends Command {
    public RoastMeatCommand(RoastCook receiver) {
        super(receiver);
    }

    @Override
    void 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