Observer Design Pattern

App.java

package Observer;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
/**
 * Observer design pattern
 */

interface Observer{
    void update(Observable obj);
}

/**
 * Class than defines behaviour for a Observable
 */
class Observable {
    boolean isChanged=false;
    List<Observer> observerList = new ArrayList<Observer>();
    public void setChanged() {
        isChanged = Boolean.TRUE;
    }
    void addObserver(Observer observerObj){
        observerList.add(observerObj);
    }
    void notifyObservers(Object arg){
        Iterator<Observer> itr = observerList.iterator();
        while(isChanged&&itr.hasNext()){
            itr.next().update(this);
        }
    }
}
class EventObserver extends Observable{
     void setChange(){
         setChanged();
         notifyObservers("Time Up");
     }
}

public class App {
    public static void main( String[] args ){
        EventObserver observervable = new EventObserver();
        //Register 3 observers
        observervable.addObserver(obj -> {
            System.out.println("Notified Observer 1");
        });
        observervable.addObserver(obj ->{
            System.out.println("Notified Observer 2");
        });
        observervable.addObserver(obj ->{
            System.out.println("Notified Observer 3");
        });
        observervable.setChange();
    }
}