private Queue<Event> unprocessedEvents = new LinkedList<>();

@Override
public void onEvent(@NotNull Event event) {
    unprocessedEvents.add(event);
}

Iterator<Event> eventIterator = unprocessedEvents.iterator();
while (eventIterator.hasNext()) {
    Event event = eventIterator.next();

    //Handle the input from the client/server
    Event event = input(event);

    //Remove the event from the unprocessed inputs
    eventIterator.remove(); //ConcurrentModificationException
}