Observable and Observer interface

1. Observable is a class.
2. Observer is an interface.
3. Observable class maintains a list of observers.
4. If something changes in Observable then it will notify all its observers.

 
import java.util.Observable;
import java.util.Observer;

class NewPosts extends Observable {
  private String post;

  public String getPost() {
    return post;
  }

  public void updatePost(String post) {
    this.post = post;
    setChanged();
    notifyObservers(post);
  }

  public static void main(String[] args) {
    NewPosts posts = new NewPosts();
    User user1 = new User();
    User user2 = new User();
    posts.addObserver(user1);
    posts.addObserver(user2);
    board.updatePost("New Post is available");
  }
}

class User implements Observer {
  public void update(Observable o, Object arg) {
    System.out.println("New Post available: " + arg);
  }
}

In the above example, a user registers by adding himself to the list of observers. When a new post is available, the user will be notified. Similarly, when a new post is available the observable updates all the users in the observers list.

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