Observer Design Pattern

It’s a behavioral design pattern, that enables a Subscriber/Observer to register with Observable/Provider/Subject in order to receive notifications whenever the state of the provider/subject changes.

• Terminology
  • Observable/Provider/Subject - The data source which maintains a collection of subscribers and notifies them.
  • Observer/Subscriber - These are the listeners that’ll get notified by Provider

A known use case for observer pattern is subscribing to blogs or newsletters to get notified whenever a new post is published.

One data source many subscribers.

Here is the UML diagram depicting the scenario.

Here is an example implementation of Observer design pattern.

public class BlogPost {

  private String author;
  private String title;
  private String content;

  public BlogPost(String author, String title, String content) {
    this.author = author;
    this.title = title;
    this.content = content;
  }

  public String getAuthor() {
    return author;
  }

  public String getTitle() {
    return title;
  }

  public String getContent() {
    return content;
  }

  @Override
  public String toString() {
    return "author=" + getAuthor() + ";" + "title=" + getTitle() + ";" + "content=" + getContent();
  }
}

// Observable/subject/provider
public class Blog {

  private BlogPost blogPost;

  public void addBlogPost(BlogPost blogPost) {
    this.blogPost = blogPost;
    /*
    * Assume the follwing logic in place
    * 1. Call publish blog post
    * 2. Post published confirmation
    */
    notifySubscribers(blogPost);
  }

  private List<Subscriber> subscribers = new ArrayList<>();

  public void addSubsriber(Subscriber subscriber) {
    subscribers.add(subscriber);
  }

  public void removeSubscriber(Subscriber subscriber) {
    subscribers.remove(subscriber);
  }

  private void notifySubscribers(BlogPost blogPost) {
    for (Subscriber subscriber : subscribers) {
      subscriber.update(blogPost);
    }
  }
}

// Observer/Subscriber
public interface Subscriber { 
  void update(Object value);
}

public class Subscriber1 implements Subscriber {

  public void update(Object value) {
    BlogPost blogPost = (BlogPost) value;
    System.out.println("Subscriber1: Received new blog post notification: " + blogPost);
  }
}

public class Subscriber2 implements Subscriber {

  public void update(Object value) {
    BlogPost blogPost = (BlogPost) value;
    System.out.println("Subscriber2: Received new blog post notification: " + blogPost);
  }
}

public class ProgramDriver {

  public static void main(String[] args) {
    // Many subscribers
    Subscriber subscriber1 = new Subscriber1();
    Subscriber subscriber2 = new Subscriber2();

    // One Observable/Provider/Subject/DataSource
    Blog blog = new Blog();
    blog.addSubsriber(subscriber1); // Registers subscriber1
    blog.addSubsriber(subscriber2); // Registers subscriber2

    // Publishing a new BlogPost should notify suscbribers
    BlogPost blogPost = new BlogPost("sairaghavak", "Observer Pattern", "....");
    blog.addBlogPost(blogPost); // This triggers the notification
  }
}

---

Another way to implement observer pattern using the Java beans API

java.beans.PropertyChangeEvent

java.beans.PropertyChangeListener

java.beans.PropertyChangeSupport

BlogPost - Refer above snippet

// Observable/subject/provider
public class Blog {

  private final PropertyChangeSupport pcs = new PropertyChangeSupport(this);

  public void addPropertyChangeListener(PropertyChangeListener listener) {
    this.pcs.addPropertyChangeListener(listener);
  }

  public void removePropertyChangeListener(PropertyChangeListener listener) {
    this.pcs.removePropertyChangeListener(listener);
  }

  private BlogPost blogPost;

  public void addBlogPost(BlogPost newBlogPost) {
    BlogPost oldBlogPost = this.blogPost;
    this.blogPost = newBlogPost;
    /*
    * Assume the follwing logic in place
    * 1. Call publish blog post
    * 2. Post published confirmation
    */
    this.pcs.firePropertyChange("blogPost", oldBlogPost, blogPost);
  }
}

// Observer: Gets notified when a new post is created
public class Subscriber1 implements PropertyChangeListener {

  @Override
  public void propertyChange(PropertyChangeEvent evt) {
    BlogPost blogPost = (BlogPost) evt.getNewValue();
    System.out.println("Subscriber1: Received new blog post notification: " + blogPost);
  }
}

// Observer: Gets notified when a new post is created
public class Subscriber2 implements PropertyChangeListener {
  @Override
  public void propertyChange(PropertyChangeEvent evt) {
    BlogPost blogPost = (BlogPost) evt.getNewValue();
    System.out.println("Subscriber2: Received new blog post notification: " + blogPost);
  }
}

ProgramDriver - Refer above snippet

Both the approaches to implement this pattern results in same output as shown below

Subscriber1: Received new blog post notification: author=sairaghavak;title=Observer Pattern;content=....
Subscriber2: Received new blog post notification: author=sairaghavak;title=Observer Pattern;content=....

References:

1. Observer Pattern
2. Observer design Pattern in .NET
3. SO thread on Observer Pattern in Java