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Java Observer

Posted on 1997-04-03
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Last Modified: 2008-03-04
I am having difficulties using Java Observer and Observable.

This is how I tried, but it didn't work. Please email
me with the mistakes I am making.

class x_class extends Observable
{

    LinkedList TL;
--
--
--
   notifyObserver(TL)
--
--
--
}

class y_class extends Object implements Observer
{
--
--
--
   void update(x_class x, LinkedList L)
{
--
--
--
}

in the init function in the applet

void init()
{
    y_class y = new y_class;
    x_class x = new x_class;
    x.addObserver(y);
}

I redd the answer urgently.


Thank you,
0
Comment
Question by:rparamal
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2 Comments
 

Accepted Solution

by:
koehn earned 100 total points
ID: 1219405
From the source code (Observable.java):
    /**
     * Notifies all observers of the specified observable change
     * which occurred.
     * @param arg what is being notified
     */
    public synchronized void notifyObservers(Object arg) {
      if (hasChanged()) {
          if (obs != null) {
            if (obs instanceof ObserverList) {
                ((ObserverList) obs).notifyObservers(this, arg);
            } else {
                ((Observer) obs).update(this, arg);
            }
          }
          clearChanged();
      }
    }

So you have two options:
- override hasChanged() to always return true

- call setChanged() before calling notifyObservers(arg) {recommended!}

0
 
LVL 1

Expert Comment

by:gerry99
ID: 1219406
The answer given is correct.  'setChanged' must be called before 'notifyObserver'.  Note: 'notifyObserver' will automatically clear the changed state of the Observable class.
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