Observable Class and Observer Interface

ObserverDemo.java

package com.company;
import java.util.Observable;
import java.util.Observer;

class ObservableObj extends Observable{

    private int watched;

    ObservableObj (int watched){

        this.watched = watched;

    }

    public void setWatched(int value){

        System.out.println("set new value as "+value );

        if(this.watched!=value) {
            this.watched = value;
            setChanged();
            System.out.println(hasChanged());
            notifyObservers(value);
        }
        else
            System.out.println("Nothing has changed");
        //after notification, it will reset the changed state, or else it will keep notify;

    }

}

class ObserverObj implements Observer{

       public void update (Observable o, Object arg)
    {
         System.out.println("observer has been updated to "+ arg);

    }

}

public class ObserverDemo {

    public static void main(String[] args){

          ObservableObj notifier = new ObservableObj(1);
          ObserverObj subscriber = new ObserverObj();

          notifier.addObserver(subscriber);

          notifier.setWatched(1);


    }

}


/*
* 1. create ObservableObj extends java.util.Observable class
*     - Add trigger function: setWatched(value), it does update the state and notify the observer.
*       remember to setChanged() before notifyObservers(), or else it won't work.
* 2. create ObserverObj implements java.util.Observer interface
* 3. add observer to ObservableObj using notifier.addObserver() method
* 4. invoke the trigger function in notifier.
*
* */