命令模式

定义：将“请求”封装成对象，以便使用不同的请求

命令模式解决了应用程序中对象的职责以及它们之间的通信方式

类型：行为型

适用场景：（1）请求调用者和请求接受者需要解耦，使得调用者和接收者不直接交互，（2）需要抽象出等待执行的行为

优点：（1）降低耦合，（2）容易扩展新命令或者一组命令

缺点：（1）命令的无限扩展会增加类的数量，提高系统实现复杂度

代码

// 每个 class 都是一个 .java 文件

public interface Command {
    void execute();
}

public class CloseCourseVideoCommand implements Command {
    private CourseVideo courseVideo;

    public CloseCourseVideoCommand(CourseVideo courseVideo) {
        this.courseVideo = courseVideo;
    }

    @Override
    public void execute() {
        courseVideo.close();
    }
}

public class OpenCourseVideoCommand implements Command {
    private CourseVideo courseVideo;

    public OpenCourseVideoCommand(CourseVideo courseVideo) {
        this.courseVideo = courseVideo;
    }

    @Override
    public void execute() {
        courseVideo.open();
    }
}

public class CourseVideo {
    private String name;

    public CourseVideo(String name) {
        this.name = name;
    }
    public void open(){
        System.out.println(this.name+"课程视频开放");
    }
    public void close(){
        System.out.println(this.name+"课程视频关闭");
    }
}

public class Staff {
    private List<Command> commandList = new ArrayList<Command>();
    public void addCommand(Command command){
        commandList.add(command);
    }

    public void executeCommands(){
        for(Command command : commandList){
            command.execute();
        }
        commandList.clear();
    }
}

// 测试类
public class Test {
    public static void main(String[] args) {
        CourseVideo courseVideo = new CourseVideo("Java设计模式精讲 -- By Geely");
        OpenCourseVideoCommand openCourseVideoCommand = new OpenCourseVideoCommand(courseVideo);
        CloseCourseVideoCommand closeCourseVideoCommand = new CloseCourseVideoCommand(courseVideo);

        Staff staff = new Staff();
        staff.addCommand(openCourseVideoCommand);
        staff.addCommand(closeCourseVideoCommand);

        staff.executeCommands();
    }
}

UML类图

源码示例

jdk 中的 Runnable 接口，

junit.framework 包中的 Test 接口