Question

I am not quite sure why i am getting an exception in this method to begin with and why at this location? (No modification happens there)

   Iterator<EventGroup> groupIterator = eventGroups.iterator();
   while (groupIterator.hasNext()) {
        EventGroup eventGroup = groupIterator.next();

        Iterator<ScheduledEvent> eventIterator = eventGroup.getEvents().iterator();
        while (eventIterator.hasNext()) {

            ScheduledEvent event = eventIterator.next();   // <------------- ConcurrentModificationException

            boolean found = ((SomeObject) event).getSomeId() == someId;
            if (found) {
                unschedule++;
                unscheduleEvent(event);
                eventGroup.remove(event);

                if (eventGroup.isEmpty()) {
                    eventGroups.remove(eventGroup);
                }
            }
        }
    }

It seems that the only thing being modified here is eventGroup, but i am using an iterator to walk over it.

Please help me understand what’s happening here?

Answer 1

eventIterator is an iterator on eventGroup and you call eventGroup.remove(event); while iterating. You should use the iterator instead to remove the item:

eventIterator.remove();

This behaviour is explained in the javadoc of ArrayList:

The iterators returned by this class’s iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove or add methods, the iterator will throw a ConcurrentModificationException.

Note: you have the same issue with eventsGroup and you should replace eventGroups.remove(eventGroup); with groupIterator.remove();.