Guava MultiMap and ConcurrentModificationException

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

In java8 you can also use a lambda approach:

eventMultiMap.entries().removeIf(genericEventCommandEntry -> genericEventCommandEntry.getValue().equals(command));