The observer pattern java questions

When they were informed of the changes occurring observer observer , the observer implementation of relevant methods. With the observed changes occur after calling the method yourself what is different. Where is the advantage . If two lines and comment out the following two lines of what's different .
For example:
import java.util.Observable;
import java.util.Observer;

public class Test {
public static void main (String [] args) {
NumObservable number = new NumObservable ();
number.addObserver (new NumObserver ());
number.setData (1);
number.setData (2);
number.setData (3);
}
}

class NumObservable extends Observable {
private int data = 0;

public int getData () {
return data;
}

public void setData (int i) {
data = i;
/ / setChanged ();
/ / notifyObservers ();
NumObserver no = new NumObserver ();
no.update (this, 1);
}
}

class NumObserver implements Observer {
public void update (Observable o, Object arg) {
NumObservable myObserable = (NumObservable) o;
System.out.println ("Data has changed to" + myObserable.getData ());
}
}
------ Solution ----------------------------------- ---------
Qibushibai spent inside new design patterns, problem-solving nature of design patterns is decoupled , new on the inside is written dead

If at this time adds another observer , according to what you write is no way to inform the viewer
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