Observer pattern

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.

Lets take a blog example, where anyone can register and unregister to this blog. Whenever there is a new post then the registered persons will be notified.

public interface Subject {
    void registerObserver(Observer o);
    void unRegisterObserver(Observer o);
    void notifyObservers();    
}

public interface Observer {
    void update();
}

import java.util.*;
public class MyBlog implements Subject {
    Set<Observer> mySubscribers = new HashSet<> ();
    
    @Override
    public void registerObserver(Observer o){
        mySubscribers.add(o);
    }
    
    @Override
    public void unRegisterObserver(Observer o){
        mySubscribers.remove(o);
    }
    
    @Override
    public void notifyObservers(){
        Iterator<Observer> iter = mySubscribers.iterator();
        
        while(iter.hasNext()){
            Observer obs = iter.next();
            obs.update();
        }
    }
    
    void addPost(){
        notifyObservers();
    }
}

public class Subscribers implements Observer{
    
    String mailId;
    
    Subscribers(String mailId){
        this.mailId = mailId;
    }
    
    @Override
    public String toString(){
        return mailId;
    }
    
    @Override
    public void update(){
        System.out.println(this + " received notification");
    }
}

public class ObserverPattern {
    public static void main(String args[]){
        MyBlog learnJava = new MyBlog();
        
        Subscribers s1 = new Subscribers("bcd@bcd.com");
        Subscribers s2 = new Subscribers("def@def.com");
        Subscribers s3 = new Subscribers("ghi@ghi.com");
        
        learnJava.registerObserver(s1);
        learnJava.registerObserver(s2);
        learnJava.registerObserver(s3);
        
        learnJava.addPost();
        
        learnJava.unRegisterObserver(s2);
        
        learnJava.addPost();
    }
}

Output

bcd@bcd.com received notification
ghi@ghi.com received notification
def@def.com received notification
bcd@bcd.com received notification
ghi@ghi.com received notification

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