GWT app getting java.util.ConcurrentModificationException from MVC pattern

Question

I am getting this error everytime my Observers are traversed.

@Override
public void notifyObservers(ModelViewInterface model) {
    for(Observer<ModelViewInterface> o : this.observers)
        o.notify(model);
}

GWT does not have threads, so it is not a synchronization issue.

It seems to happen after I press a button, any ideas of how to avoid this error?

Answer 1

From the javadoc of ConcurrentModificationException:

Note that this exception does not always indicate that an object has been concurrently modified by a different thread. If a single thread issues a sequence of method invocations that violates the contract of an object, the object may throw this exception. For example, if a thread modifies a collection directly while it is iterating over the collection with a fail-fast iterator, the iterator will throw this exception.

So in your case, it seems that o.notify(model) modifies this.observers - directly or indirectly. This is a common phenomenon when modifying the collection you're iterating over.

To avoid concurrent modification, you can operate on a copy of the collection like this:

for(Observer<ModelViewInterface> o : 
           new ArrayList<ModelViewInterface>(this.observers)) {

    o.notify(model);
}

However, sometimes this is not what you want - the current behaviour of o.notify could also indicate a bug.