A pattern that a subject maintains a list of observers, and automatically notifies them of any state changes.

Example of Uses

• implement distributed event handling systems
• in an MVC, basically the view portion

Intent

• define a one-to-many dependency, so that when one changes states, it notifies the dependants

Implementation

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

   public int getState() {
      return state;
   }

   public void setState(int state) {
      this.state = state;
      notifyAllObservers();
   }

   public void attach(Observer observer){
      observers.add(observer);		
   }

   public void notifyAllObservers(){
      for (Observer observer : observers) {
         observer.update();
      }
   } 	
}

public abstract class Observer {
   protected Subject subject;
   public abstract void update();
}

public class FirstObserver extends Observer {

   public FirstObserver (Subject subject) {
      this.subject = subject;
      this.subject.attach(this);
   }

   @Override
   public void update() {
      System.out.println( "First String: " + Integer.toBinaryString( subject.getState() ) ); 
   }
}

public class SecondObserver extends Observer {

   public SecondObserver (Subject subject) {
      this.subject = subject;
      this.subject.attach(this);
   }

   @Override
   public void update() {
      System.out.println( "Second String: " + Integer.toHexString( subject.getState() ) ); 
   }
}

public class ObserverPatternDemo {
   public static void main(String[] args) {
      Subject subject = new Subject();

      new FirstObserver(subject);
      new SecondObserver(subject);

      System.out.println("First state change: 15");	
      subject.setState(15);
      System.out.println("Second state change: 10");	
      subject.setState(10);
   }
}