// Observer
public interface Observer {
    void update();
}

// Subject
public interface Subject {
    void register(Observer obj);
    void unregister(Observer obj);
    void notifyObservers();
    Object getUpdate(Observer obj);
}

// Topic
import java.util.ArrayList;
import java.util.List;

public class Topic implements Subject {
    private List<Observer> observers;
    private String message;

    public Topic() {
        this.observers = new ArrayList<>();
        this.message = "";
    }
    @Override
    public void register(Observer obj) {
        if (!observers.contains(obj)) {
            observers.add(obj);
        }
    }

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

    @Override
    public void notifyObservers() {
        this.observers.forEach(Observer::update);
    }

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

    public void postMessage(String msg) {
        System.out.println("Message sended to Topic: " + msg);
        this.message = msg;
        notifyObservers();
    }
}

// TopicSubscriber
public class TopicSubscriber implements Observer {

	private String name;
	private Subject topic;

	public TopicSubscriber(String name, Subject topic) {
		this.name = name;
		this.topic = topic;
	}

	@Override
	public void update() {
		String msg = (String) topic.getUpdate(this);
		System.out.println(name + ":: got message >> " + msg);
	}
}

// Main
public class Main {

	public static void main(String[] args) {
		Topic topic = new Topic();
		Observer a = new TopicSubscriber("a", topic);
		Observer b = new TopicSubscriber("b", topic);
		Observer c = new TopicSubscriber("c", topic);
		topic.register(a);
		topic.register(b);
		topic.register(c);

		topic.postMessage("nice to meet you");
	}
}