public class Demo {

 public static void main(String[] args) {
 ActualSubject subject = new ActualSubject();
 
 ObserverA o1 = new ObserverA();
 ObserverA o2 = new ObserverA();
 ObserverA o3 = new ObserverA();
 
 //訂閱消息
 subject.addObserver(o1);
 subject.addObserver(o2);
 subject.addObserver(o3);
 
 subject.setNews("今天天氣真好");
 
 }
}

/**
 * 觀察者接口
 */
interface Observer{
 public void update(Subject subject);
}

/**
 * 目標對象，廣播對象 
 */
class Subject{
 private List<Observer> list = new ArrayList<>();
 public void addObserver(Observer o){
 list.add(o);
 }
 public void removeObserver(Observer o){
 list.remove(o);
 }
 public void notifyAllObserver(){
 for(Observer o:list){
  o.update(this);
 }
 }
}

class ActualSubject extends Subject{
 private String news;
 public void setNews(String news) {
 this.news = news;
 System.out.println("我是廣播，我發(fā)出了一個消息");
 notifyAllObserver();
 }
 public String getNews() {
 return news;
 }
}

class ObserverA implements Observer{
 private String news;
 public void update(Subject subject) {
 setNews(((ActualSubject)subject).getNews());
 System.out.println("我是觀察者，我接收到一個我訂閱的廣播發(fā)出的消息，接下來我要做出響應(yīng)的操作");
 System.out.println(getNews());
 }
 public String getNews() {
 return news;
 }
 public void setNews(String news) {
 this.news = news;
 }
}