import javax.annotation.CheckForNull;

/**
 * Registry of subscribers to a single event bus.
 *
 * @author Colin Decker
 */
@ElementTypesAreNonnullByDefault
final class SubscriberRegistry {

  /**
   * All registered subscribers, indexed by event type.
   *
   * <p>The {@link CopyOnWriteArraySet} values make it easy and relatively lightweight to get an