ConcurrentModificationException in List

Question

In the following code I am getting a ConcurrentModificationException that I can't seem to rectify.
I have tried
1. sync'ing the 2 public methods
2. sync'ing within the methods (currently shown in code) this was most often recommended in forums that I have read on this problem.
3. sync'ing on the current object ("this", as shown in the code) and sync'ing on the list object itself.
4. Using Collections.synchronizedList(new Vector ...)
After chasing the problem I am pretty sure that the .add method is being called while the Iteraor loop is running, but shouldn't sync'ing fix that? The list is private and is not accessed from any other class outside the current one shown. Is there something wrong with my sync'ing?
Any recommendations will be appreciated.
Thanks,
sseric
Error:
Exception in thread "Thread-2" java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:449)
at java.util.AbstractList$Itr.next(AbstractList.java:420)
at genetviz.utils.BroadcastGraphAction.broadcast(BroadcastGraphAction.java:80)
at genetviz.utils.Graph.init(Graph.java:106)
at genetviz.io.FileOpener$FileLoader.run(FileOpener.java:312)
at java.lang.Thread.run(Thread.java:595)
Code:
import java.util.*;
import javax.swing.event.ChangeEvent;
import java.awt.Component;
import java.awt.Dimension;
public abstract class BroadcastGraphAction extends GraphRenderer
     implements GraphListener {
public abstract void setGraph(Graph graph);
private Vector<GraphListener> graphListenerList = new Vector<GraphListener>();
public void addGraphListener(GraphListener gl)
synchronized(this)
graphListenerList.add(gl);
public void broadcast(Graph gr)
if(gr == null) return;
synchronized(this)
for (Iterator iter = graphListenerList.iterator(); iter.hasNext();)
Object obj = iter.next();
if (obj instanceof GraphListener) ((GraphListener)obj).setGraph(gr);
}

Answer 1

takeshi10,
please see below for response,
when posting code, remember to use code tags...thank you. I'm new to the forums
except operations with the iterator, you shouldnt add / remove items on the list while iterating over it...yep, that's the problem.
i believe the setGraph method is adding the Graph to the list (tough, because of its abstractness i cant really tell)...no. the "Graph" is independent of the "GraphListener" and is not being added to the graphListenerList. The problem appears to be that .add is being called after .iterator is called on the graphListenerList. This is what I need to prevent through synchronization, or other techniques.
sync'inc will only prevent acess by other threads... right, which is why I used it in my code. So one of 2 things is happening then,
1. my synchronization is correct and it is not another thread causing the exception
2. Something is wrong with my synchronization
Though I'm not familiar with this, I tried something on the first note. I created a thread in the current class keeping it alive with a while loop and Thread.sleep, and I tried locking on it to ensure that an independent thread controlled the locking, for example,
public void addGraphListener(GraphListener gl)
     if(lock==null)
     { lock = new Thread(new LockRunner());}
     synchronized(lock)
        graphListenerList.add(gl);
  private class LockRunner extends Object implements Runnable
     public void run()
        try {
          while(true) Thread.sleep(1000);
           } catch (InterruptedException e) {
            System.out.println("BroadcastGraphAction sleep exception");
public void broadcast(Graph gr)
     if(gr == null) return;
     if(lock==null)
       lock = new Thread(new LockRunner());
     synchronized(lock)
       for (Iterator iter = graphListenerList.iterator(); iter.hasNext();)
           Object obj = iter.next();
           if (obj instanceof GraphListener) ((GraphListener)obj).setGraph(gr);
  }This did not work either. Unfortunately, I am only marginally experienced with threads and thread locking so I'm not quite sure I'm getting this right (well, obvioudly I'm not getting this right). Other suggestions?
sseric