Observer design pattern with Java

ConcreteSubject.java

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

public class ConcreteSubject implements Subject {

	private List<Observer> observers = new ArrayList<>();
	private String message;

	public ConcreteSubject() {
	}

	@Override
	public void update(String message) {
		this.message = message;
		notifyObservers();
	}

	@Override
	public void registerObserver(Observer observer) {
		observers.add(observer);
	}

	@Override
	public void notifyObservers() {
		observers.forEach(observer -> observer.update(message));
	}

}

Observer.java

public interface Observer {

	void update(String message);

}

ObserverA.java

public class ObserverA implements Observer {

	@Override
	public void update(String message) {
		System.out.println("+++++ ObserverA : " + message + " ++++++");
	}

}

ObserverB.java

public class ObserverB implements Observer {

	@Override
	public void update(String message) {
		System.out.println("---- ObserverB : " + message + " ----");
	}

}

ObserverDemo.java

public class ObserverDemo {

	public static void main(String[] args) {
		Subject subject = new ConcreteSubject();
		
		Observer observerA = new ObserverA();
		Observer observerB = new ObserverB();
		
		subject.registerObserver(observerA);
		subject.registerObserver(observerB);
		
		subject.update("hello world");
		
		System.out.println("\n+----------------------------------+\n");
		
		subject.update("hello java");
	}

}

Subject.java

public interface Subject {
	
	void update(String message);

	void registerObserver(Observer observer);

	void notifyObservers();

}