public void testRegister() {
    assertEquals(0, registry.getSubscribersForTesting(String.class).size());

    registry.register(new StringSubscriber());
    assertEquals(1, registry.getSubscribersForTesting(String.class).size());

    registry.register(new StringSubscriber());
    assertEquals(2, registry.getSubscribersForTesting(String.class).size());

    registry.register(new ObjectSubscriber());

      // anyway, if the set is empty it'll just be wrapping an array of length 0
    }
  }

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

  /**