轻松掌握Java观察者模式

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();  //订阅消息 subject.addObserver(o1); subject.addObserver(o2); subject.addObserver(o3);  subject.setNews("今天天气真好");  }}/** * 观察者接口 */interface Observer{ public void update(Subject subject);}/** * 目标对象，广播对象  */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("我是广播，我发出了一个消息"); 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("我是观察者，我接收到一个我订阅的广播发出的消息，接下来我要做出响应的操作"); System.out.println(getNews()); } public String getNews() { return news; } public void setNews(String news) { this.news = news; }}