public interface Subject {

 //methods to register and unregister observers
 public void register(Observer obj);
 public void unregister(Observer obj);

 //method to notify observers of change
 public void notifyObservers();

 //method to get updates from subject
 public Object getUpdate(Observer obj);

}

public interface Observer {

 //method to update the observer, used by subject
 public void update();

 //attach with subject to observe
 public void setSubject(Subject sub);
}

import java.util.ArrayList;
import java.util.List;

public class MyTopic implements Subject {

 private List<Observer> observers;
 private String message;
 private boolean changed;
 private final Object MUTEX= new Object();

 public MyTopic(){
  this.observers=new ArrayList<>();
 }
 @Override
 public void register(Observer obj) {
  if(obj == null) throw new NullPointerException("Null Observer");
  if(!observers.contains(obj)) observers.add(obj);
 }

 @Override
 public void unregister(Observer obj) {
  observers.remove(obj);
 }

 @Override
 public void notifyObservers() {
  List<Observer> observersLocal = null;
  //synchronization is used to make sure any observer registered after message is received is not notified
  synchronized (MUTEX) {
   if (!changed)
    return;
   observersLocal = new ArrayList<>(this.observers);
   this.changed=false;
  }
  for (Observer obj : observersLocal) {
   obj.update();
  }

 }

 @Override
 public Object getUpdate(Observer obj) {
  return this.message;
 }

 //method to post message to the topic
 public void postMessage(String msg){
  System.out.println("Message Posted to Topic:"+msg);
  this.message=msg;
  this.changed=true;
  notifyObservers();
 }

}

public class MyTopicSubscriber implements Observer {

 private String name;
 private Subject topic;

 public MyTopicSubscriber(String nm){
  this.name=nm;
 }
 @Override
 public void update() {
  String msg = (String) topic.getUpdate(this);
  if(msg == null){
   System.out.println(name+":: No new message");
  }else
  System.out.println(name+":: Consuming message::"+msg);
 }

 @Override
 public void setSubject(Subject sub) {
  this.topic=sub;
 }

}

public class ObserverPatternTest {

 public static void main(String[] args) {
  //create subject
  MyTopic topic = new MyTopic();

  //create observers
  Observer obj1 = new MyTopicSubscriber("Obj1");
  Observer obj2 = new MyTopicSubscriber("Obj2");
  Observer obj3 = new MyTopicSubscriber("Obj3");

  //register observers to the subject
  topic.register(obj1);
  topic.register(obj2);
  topic.register(obj3);

  //attach observer to subject
  obj1.setSubject(topic);
  obj2.setSubject(topic);
  obj3.setSubject(topic);

  //check if any update is available
  obj1.update();

  //now send message to subject
  topic.postMessage("New Message");
 }

}