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

    registry.unregister(s2);
    assertTrue(registry.getSubscribersForTesting(String.class).isEmpty());
  }

  public void testUnregister_notRegistered() {
    assertThrows(IllegalArgumentException.class, () -> registry.unregister(new StringSubscriber()));

    StringSubscriber s1 = new StringSubscriber();
    registry.register(s1);