ConcurrentModificationException when operating on an ArrayList

Question

Hi,
I've got a strange problem. I'm working with a heavily multithreaded program. I have the following code:
public abstract class GHAgentTaskThread implements Runnable, GenericObserverSubject {
protected List observingResultDispatchers = new ArrayList();
public final void addObserver(GenericObserverObserver subscriber) {
synchronized (observingResultDispatchers) {
observingResultDispatchers.add(subscriber);
public final void removeObserver(GenericObserverObserver subscriber) {
synchronized (observingResultDispatchers) {
observingResultDispatchers.remove(subscriber);
protected final void notifyObservingDispatchers() {
synchronized (observingResultDispatchers) {
Iterator i = observingResultDispatchers.iterator();
GenericObserverObserver currentObserver;
while (i.hasNext()) {
currentObserver = (GenericObserverObserver) i.next(); //****
currentObserver.notify(this);
observersIterator = null;
//Other stuff
public class TokenryServerRequestTokenTask
extends GHAgentTaskThread
implements Runnable {
public void run() {
//do some stuff
notifyObservingDispatchers();
I have an external class which creates a new instance of the thread:
GHAgentTaskThread myThread = new TokenryServerRequestTokenTask();
It then adds itself as a listener:
myThread.addObserver(this);
This all works ok. I then start then thread and it goes and does the work I've specified. However, at the end of the run method when I try and notify the observers that the work is done with notifyObservingDispatchers(), I'm getting a:
java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:444)
at java.util.AbstractList$Itr.next(AbstractList.java:421)
The line this is happeneing at is the :
currentObserver = (GenericObserverObserver) i.next();
line, which I've marked //**** in the code above.
Why is this happening? I thought, that given I've synchronized all of the code that deals with the List (which is private) on the List itself that only one thing could be operating on it at one time, and hence this shouldn't occur. Strangely enough too, it doesn't seem to be affecting my program either - it works. However, exceptions are not thrown for no reason!
If anyone can suggest how this could be sorted I'd be very grateful.
Many thanks,
Stu

Answer 1

To englarge slightly on what _JN said, I speculate
that when your GenericObserverObserver.notify() method
is called, it calls one of your other methods in a way
that modifies the list.
cut
Got it in one - I've just spotted that. I was attempting to remove the observer from the subject from within the GenericObserverObserver.notify() method... whoops!
Cheers for the suggestion about the LinkedList - that's also a good idea.
Thank you both for your help.
Cheers,
Stu