Design pattern – Observer

What is it?

Real world

• Social media
  • When a person updates his status – all his followers gets the notification
  • A follower can follow or unfollow another person at any point of time
  • Once unfollowed, person will not get the notifications from subject in future

Real world diagram

• One-to-many dependencies between objects
• Also known as publish-subscribe pattern
• There are many observers (subscriber objects) that are observing a particular subject (publisher object)
• Observers register themselves to a subject to get a notification when there is a change made inside that subject

Sequence diagram

Architecture

• 

Coding example

Subject and concreteSubject

• Subject – interface or abstract class defining the operations for attaching and de-attaching observers to the subject
• ConcreteSubject – concrete Subject class. It maintain the state of the object and when a change in the state occurs it notifies the attached Observers

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

// MessagePublisher.java
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);
        }
    }
}

Observer and ConcreteObservers

• Observer – interface or abstract class defining the operations to be used to notify this object
• ConcreteObserver – concrete Observer implementations

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

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

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

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

State object

This must be an immutable object so that no class can modify it’s content by mistake

// Message.java
public class Message
{
    final String messageContent;
     
    public Message (String m) {
        this.messageContent = m;
    }
 
    public String getMessageContent() {
        return messageContent;
    }
}

Test communication between publisher and subscribers

// Main.java
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
    }
}

// Console output
MessageSubscriberOne :: First Message
MessageSubscriberTwo :: First Message
 
MessageSubscriberTwo :: Second Message
MessageSubscriberThree :: Second Message