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Java设计模式:观察者模式

23 Sep 2014

注:本文为译文,原文出处java-design-patterns-in-stories

简而言之, 观者模式 = 发布者 + 订阅者.

观察着模式一直被用于图形用户界面的行为监听器中. Swing GUI的例子展示了行为监听器是如何像观察者一样工作的.

下面是一个典型的关于猎头的离职. 在图中有2个角色 - 猎头与应聘者. 应聘者订阅了猎头的信息, 当有新的工作机会时, 猎头会通知应聘者.

类图

Java代码

主题接口.

public interface Subject {
  public void registerObserver(Observer o);
  public void removeObserver(Observer o);
  public void notifyAllObservers();
}

观察者接口.

public interface Observer {
  public void update(Subject s);
}

猎头类实现主题接口

import java.util.ArrayList;

public class HeadHunter implements Subject{

  //define a list of users, such as Mike, Bill, etc.
  private ArrayList<Observer> userList;
  private ArrayList<String> jobs;

  public HeadHunter(){
    userList = new ArrayList<Observer>();
    jobs = new ArrayList<String>();
  }

  @Override
  public void registerObserver(Observer o) {
    userList.add(o);
  }

  @Override
  public void removeObserver(Observer o) {}

  @Override
  public void notifyAllObservers() {
    for(Observer o: userList){
      o.update(this);
    }
  }

  public void addJob(String job) {
    this.jobs.add(job);
    notifyAllObservers();
  }

  public ArrayList<String> getJobs() {
    return jobs;
  }

  public String toString(){
    return jobs.toString();
  }
}

应聘者是一个观察者.

public class JobSeeker implements Observer {

  private String name;

  public JobSeeker(String name){
    this.name = name;
  }
  @Override
  public void update(Subject s) {
    System.out.println(this.name + " got notified!");
    //print job list
    System.out.println(s);
  }
}

开始.

public class Main {

  public static void main(String[] args) {
    HeadHunter hh = new HeadHunter();
    hh.registerObserver(new JobSeeker("Mike"));
    hh.registerObserver(new JobSeeker("Chris"));
    hh.registerObserver(new JobSeeker("Jeff"));

    //Each time, a new job is added, all registered job seekers will get noticed.
    hh.addJob("Google Job");
    hh.addJob("Yahoo Job");
  }
}

JDK中的观察者模式

java.util.EventListener

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