用個大家熟悉的東西來做例子---------------微信公眾號

當你訂閱一個公眾號時，每當公眾號釋出新的資訊時會及時通知訂閱者，訂閱者就是觀察者，微信公眾號就是被觀察者---------這就是觀察者模式。

接下來使用JAVA程式碼實現，一共五個類，被觀察者介面類(Observerable.java)、被觀察者實現類(WeChatServer.java)、觀察者介面類(Observer.java)、觀察者實現類(User.java)、測試類(Test.java)

public interface Observerable {
   
   // 註冊觀察者
   void registerObserver(Observer observer);
   // 移除觀察者
   void removeObserver(Observer observer);
   // 通知觀察者
   void notifyObserver();
}

==================================================================

public class WeChatServer implements Observerable {
   
   private List<Observer> list;
   private String message;
   
   public WeChatServer() {
      this.list = new ArrayList<>();
   }
   
   @Override
   public void registerObserver(Observer observer) {
      list.add(observer);
   }
   
   @Override
   public void removeObserver(Observer observer) {
      if(!list.isEmpty()) {
         list.remove(observer);
      }
   }
   
   @Override
   public void notifyObserver() {
      for(int i = 0;i < list.size();i++) {
         Observer observer = list.get(i);
         observer.update(message);
      }
   }
   
   public void sendInfo(String message) {
      this.message = message;
      System.out.println("被觀察者更改資訊:" + message);
      notifyObserver();
   }
}

==================================================================

public interface Observer {
   
   void update(String message);
}

==================================================================

public class User implements Observer {
   
   private String name;
   private String message;
   
   public User(String name) {
      this.name = name;
   }
   
   @Override
   public void update(String message) {
      this.message = message;
      read();
   }
   
   public void read() {
      System.out.println("通知觀察者" + name +",資訊為" + message);
   }
}

==================================================================

public class Test {
   
   public static void main(String[] args) {
      
      WeChatServer server = new WeChatServer();
      
      Observer userOne = new User("zhangsan");
      Observer userTwo = new User("lisi");
      Observer userThree = new User("wangwu");
      
      server.registerObserver(userOne);
      server.registerObserver(userTwo);
      server.registerObserver(userThree);
      
      server.sendInfo("第一條資訊");
      
      server.removeObserver(userTwo);
      server.sendInfo("第二條資訊");
   }
}