Guava MultiMap and ConcurrentModificationException

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++;
    }
}

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.

If another thread could modify your multimap while this logic is running, you'll need to add a synchronized block to MHarris's code:

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

Or, you could omit the iterator as follows,

synchronized (eventMultimap) {
  int oldSize = eventMultimap.size();
  eventMultimap.values().removeAll(Collections.singleton(command));
  nbRemoved = oldSize - eventMultimap.size();
}

The removeAll() call doesn't require synchronization. However, if you omit the synchronized block, the multimap could mutate between the removeAll() call and one of the size() calls, leading to an incorrect value of nbRemoved.

Now, if your code is single-threaded, and you just want to avoid a ConcurrentModificationException call, you can leave out the Multimaps.synchronizedMultimap and synchronized (eventMultimap) logic.