}
  }

  @VisibleForTesting
  Set<Subscriber> getSubscribersForTesting(Class<?> eventType) {
    return MoreObjects.firstNonNull(subscribers.get(eventType), ImmutableSet.<Subscriber>of());
  }

  /**
   * Gets an iterator representing an immutable snapshot of all subscribers to the given event at
   * the time this method is called.
   */
  Iterator<Subscriber> getSubscribers(Object event) {