* immutable snapshot of all current subscribers to an event without any locking.
   */
  private final ConcurrentMap<Class<?>, CopyOnWriteArraySet<Subscriber>> subscribers =
      Maps.newConcurrentMap();

  /** The event bus this registry belongs to. */
  @Weak private final EventBus bus;

  SubscriberRegistry(EventBus bus) {
    this.bus = checkNotNull(bus);
  }

   * default implementations and are "new"d upon get().
   */
  private static final ConcurrentMap<Class<?>, Class<?>> implementations = Maps.newConcurrentMap();

  private static <T> void setImplementation(Class<T> type, Class<? extends T> implementation) {
    checkArgument(type != implementation, "Don't register %s to itself!", type);
    checkArgument(

   * default implementations and are "new"d upon get().
   */
  private static final ConcurrentMap<Class<?>, Class<?>> implementations = Maps.newConcurrentMap();

  private static <T> void setImplementation(Class<T> type, Class<? extends T> implementation) {
    checkArgument(type != implementation, "Don't register %s to itself!", type);
    checkArgument(

    return new LinkedHashMap<>(capacity(expectedSize));
  }

  /**
   * Creates a new empty {@link ConcurrentHashMap} instance.
   *
   * @since 3.0
   */
  public static <K, V> ConcurrentMap<K, V> newConcurrentMap() {
    return new ConcurrentHashMap<>();
  }

  /**
   * Creates a <i>mutable</i>, empty {@code TreeMap} instance using the natural ordering of its
   * elements.
   *

    return new LinkedHashMap<>(capacity(expectedSize));
  }

  /**
   * Creates a new empty {@link ConcurrentHashMap} instance.
   *
   * @since 3.0
   */
  public static <K, V> ConcurrentMap<K, V> newConcurrentMap() {
    return new ConcurrentHashMap<>();
  }

  /**
   * Creates a <i>mutable</i>, empty {@code TreeMap} instance using the natural ordering of its
   * elements.
   *