Observer design pattern example

 https://howtodoinjava.com/design-patterns/behavioral/observer-design-pattern/

 

public interface Subject 
{
    public void attach(Observer o);
    public void detach(Observer o);
    public void notifyUpdate(Message m);
} 

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

public class MessagePublisher implements Subject {
    
    private List<Observer> observers = new ArrayList<>();

    @Override
    public void attach(Observer o) {
        observers.add(o);
    }

    @Override
    public void detach(Observer o) {
        observers.remove(o);
    }

    @Override
    public void notifyUpdate(Message m) {
        for(Observer o: observers) {
            o.update(m);
        }
    }
}

public interface Observer 
{
    public void update(Message m);
}

public class MessageSubscriberOne implements Observer 
{
    @Override
    public void update(Message m) {
        System.out.println("MessageSubscriberOne :: " + m.getMessageContent());
    }
}

public class MessageSubscriberTwo implements Observer 
{
    @Override
    public void update(Message m) {
        System.out.println("MessageSubscriberTwo :: " + m.getMessageContent());
    }
}

public class MessageSubscriberThree implements Observer 
{
    @Override
    public void update(Message m) {
        System.out.println("MessageSubscriberThree :: " + m.getMessageContent());
    }
}

public class Message 
{
    final String messageContent;
    
    public Message (String m) {
        this.messageContent = m;
    }

    public String getMessageContent() {
        return messageContent;
    }
}

public class Main 
{
    public static void main(String[] args) 
    {
        MessageSubscriberOne s1 = new MessageSubscriberOne();
        MessageSubscriberTwo s2 = new MessageSubscriberTwo();
        MessageSubscriberThree s3 = new MessageSubscriberThree();
        
        MessagePublisher p = new MessagePublisher();
        
        p.attach(s1);
        p.attach(s2);
        
        p.notifyUpdate(new Message("First Message"));   //s1 and s2 will receive the update
        
        p.detach(s1);
        p.attach(s3);
        
        p.notifyUpdate(new Message("Second Message")); //s2 and s3 will receive the update
    }
}