Observable extension example

PanelObservable.java

package Helpers;

import java.util.Observable;
import java.util.Observer;
import java.util.Vector;

/**
 * @author Shane van Wyk [GFN5594]
 * @author Robert McLeod
 * @since 02/10/2011
 */
public class PanelObservable extends Observable {

    protected boolean changed;
    protected Vector obs;
    
    private Observable observable;
    
    public void setObservable( Observable observable ) {
        this.observable = observable;
    }

    public void setChanged() {
        super.setChanged();
        notifyObservers();
    }
    
    public void notifyObservers(Object arg) {
        /*
         * a temporary array buffer, used as a snapshot of the state of
         * current Observers.
         */
        Object[] arrLocal;

        synchronized (this) {
            /* We don't want the Observer doing callbacks into
             * arbitrary code while holding its own Monitor.
             * The code where we extract each Observable from
             * the Vector and store the state of the Observer
             * needs synchronization, but notifying observers
             * does not (should not).  The worst result of any
             * potential race-condition here is that:
             * 1) a newly-added Observer will miss a
             *   notification in progress
             * 2) a recently unregistered Observer will be
             *   wrongly notified when it doesn't care
             */
            if (!changed)
                return;
            arrLocal = obs.toArray();
            clearChanged();
        }

        if ( observable == null )
            observable = this;
        
        for (int i = arrLocal.length-1; i>=0; i--)
            ((Observer)arrLocal[i]).update(observable, arg);
    }
}