Guava MultiMap and ConcurrentModificationException [duplicate]

Question

I don't understand why I get a ConcurrentModificationException when I iterate through this multimap. I read the following entry, but I am not sure if I understood the whole thing. I tried to add a synchronized block. But my doubt is what to synchronize with, and when.

The multimap is a field and created like this :

private Multimap<GenericEvent, Command> eventMultiMap =   
   Multimaps.synchronizedMultimap(HashMultimap.<GenericEvent, Command> create());

and used like this :

eventMultiMap.put(event, command);

and like this ( I tried to synchronize this part on the map, but without success )

for (Entry<GenericEvent, Command> entry : eventMultiMap.entries()) {
    if (entry.getValue().equals(command)) {
        eventMultiMap.remove(entry.getKey(), entry.getValue());
        nbRemoved++;
    }
}

Answer 1

You may wish to see this blogpost for another pitfall yielding a ConcurrentModificationException when traversing a multimap, with no other thread interfering. In short, if you traverse multimap's keys, accessing the respective collection of values associated with each key and remove some element from such a collection, if that element happens to be the last of the collection you are going to have ConcurrentModificationException when you try to access the next key - because emptying a collection triggers the removal of the key, thus structurally modifying the multimap's keyset.

Answer 2

Calling remove on a collection while you're iterating through it will cause a ConcurrentModificationException every time, even if it's all done in the same thread - the Right Thing to do is get an explicit iterator and call .remove() on that.

Edit: Modifying your example:

Iterator<Map.Entry<GenericEvent, Command>> i = eventMultiMap.entries().iterator();
while (i.hasNext()) {
    if (i.next().getValue().equals(command)) {
        i.remove();
        nbRemoved++;
    }
}