[Design Pattern] Command Pattern

在 97 Things Every Programmer Should Know 一書中提到：

雖然在一般遇到的案例裡，使用 if - then - else 比多型更實用些，但絕大多數用 polymorphism 的寫作風格會讓程式碼更精簡、更易讀，且更少脆弱的程式碼。所以在我們的程式碼裡，要是錯過使用 polymorphism 的機會，便會逐漸增加  if - then - else 敘述句的數量。試試 command  pattern 吧

假設我有一個機器人物件，他會執行前進、後退、左轉與右轉，這四種指令

package albert.practice.designpattern.command;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.extern.slf4j.Slf4j;

@AllArgsConstructor
@Data
@Slf4j
public class Robot {

    private String robotName;

    public void goAhead() {
        log.debug(robotName + " 前進 !");
    }

    public void turnLeft() {
        log.debug(robotName + " 左轉 !");
    }

    public void turnRight() {
        log.debug(robotName + " 右轉 !");
    }

    public void turnBack() {
        log.debug(robotName + " 後退 !");
    }
}

另外建立一個 enumeration 來判斷，此次呼叫要執行哪一個指令

package albert.practice.designpattern.command;

public enum CommandEnum {
    GO_HEAD, TURN_BACK, TURN_RIGHT, TURN_LEFT;
}

建立機器人物件與執行指令如下：

package albert.practice.designpattern.command;

public class RobotTestClient {

    public static void main(String[] args) {
        Robot robot = new Robot("瓦力");

        RobotTestClient client = new RobotTestClient();
        client.executeRobot(robot, CommandEnum.GO_HEAD);
        client.executeRobot(robot, CommandEnum.TURN_LEFT);
        client.executeRobot(robot, CommandEnum.TURN_RIGHT);        
    }

    public void executeRobot(Robot robot, CommandEnum command) {
        if (CommandEnum.GO_HEAD == command) {
            robot.goAhead();
        } else if (CommandEnum.TURN_BACK == command) {
            robot.turnBack();
        } else if (CommandEnum.TURN_LEFT == command) {
            robot.turnLeft();
        } else if (CommandEnum.TURN_RIGHT == command) {
            robot.turnRight();
        }
    }

}

在上述的程式碼就會發現，如果指令越來越多，就會有越來越多的 if-else 判斷式，此時就可以套用 command pattern

首先，先建立 Command interface

package albert.practice.designpattern.command;

public interface Command {
    void execute();
}

然後，分別建立四個指令的 classes，並實作 Command interface
前進指令：

package albert.practice.designpattern.command;

public class GoAheadCommand implements Command {

    private Robot robot;

    public GoAheadCommand(Robot robot) {
        super();
        this.robot = robot;
    }

    @Override
    public void execute() {
        this.robot.goAhead();
    }

}

後退指令：

package albert.practice.designpattern.command;

public class TurnBackCommand implements Command {
    private Robot robot;

    public TurnBackCommand(Robot robot) {
        super();
        this.robot = robot;
    }

    @Override
    public void execute() {
        this.robot.turnBack();
    }

}

向左轉指令：

package albert.practice.designpattern.command;

public class TurnLeftCommand implements Command {
    private Robot robot;

    public TurnLeftCommand(Robot robot) {
        super();
        this.robot = robot;
    }

    @Override
    public void execute() {
        this.robot.turnLeft();
    }

}

向右轉指令：

package albert.practice.designpattern.command;

public class TurnRightCommand implements Command {
    private Robot robot;

    public TurnRightCommand(Robot robot) {
        super();
        this.robot = robot;
    }

    @Override
    public void execute() {
        this.robot.turnRight();
    }

}

建立機器人物件與操作指令，程式修改如下：

package albert.practice.designpattern.command;

public class RobotTestClient {

    public static void main(String[] args) {
        Robot robot = new Robot("瓦力");
        new GoAheadCommand(robot).execute();
        new TurnLeftCommand(robot).execute();
        new TurnRightCommand(robot).execute();
    }

}