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

public class AddObserver extends Observer{
 
   public AddObserver(Subject subject){
      this.subject = subject;
      this.subject.attach(this);
   }
 
   @Override
   public void update() {
      System.out.println( "add value is: " 
      + (subject.getState() + subject.getState())); 
   }
}

public class SubObserver extends Observer{
 
   public SubObserver(Subject subject){
      this.subject = subject;
      this.subject.attach(this);
   }
 
   @Override
   public void update() {
      System.out.println( "sub value is: " 
      + (subject.getState() - 5)); 
   }
}

import java.util.ArrayList;
import java.util.List;
 
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 class Test {
   public static void main(String[] args) {
      Subject subject = new Subject();
 
      new AddObserver(subject);
      new SubObserver(subject);
 
      System.out.println("add value state to 5");   
      subject.setState(5);
      System.out.println("add value state to 8");  
      subject.setState(10);
   }
}