code: public void subscribe(final Class<? extends Notification> notificationClass, final Subscription subscription) {
          if (!SUBSCRIPTION_MAP.containsKey(notificationClass)) {
              SUBSCRIPTION_MAP.put(notificationClass, new ArrayList<>());
          }
          final ArrayList<Subscription> subscriptions = SUBSCRIPTION_MAP.get(notificationClass);
          if (!subscriptions.contains(subscription)) {
              subscriptions.add(subscription);
          }
      }
docs: {@inheritDoc}
  @since 1.3.0