code: private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
          Class<?> eventType = subscriberMethod.eventType;
          Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
          CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
          if (subscriptions == null) {
              subscriptions = new CopyOnWriteArrayList<>();
              subscriptionsByEventType.put(eventType, subscriptions);
          } else {
              if (subscriptions.contains(newSubscription)) {
                  throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                          + eventType);
              }
          }
  
          int size = subscriptions.size();
          for (int i = 0; i <= size; i++) {
              if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
                  subscriptions.add(i, newSubscription);
                  break;
              }
          }
  
          List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
          if (subscribedEvents == null) {
              subscribedEvents = new ArrayList<>();
              typesBySubscriber.put(subscriber, subscribedEvents);
          }
          subscribedEvents.add(eventType);
  
          if (subscriberMethod.sticky) {
              if (eventInheritance) {
                  // Existing sticky events of all subclasses of eventType have to be considered.
                  // Note: Iterating over all events may be inefficient with lots of sticky events,
                  // thus data structure should be changed to allow a more efficient lookup
                  // (e.g. an additional map storing sub classes of super classes: Class -> List<Class>).
                  Set<Map.Entry<Class<?>, Object>> entries = stickyEvents.entrySet();
                  for (Map.Entry<Class<?>, Object> entry : entries) {
                      Class<?> candidateEventType = entry.getKey();
                      if (eventType.isAssignableFrom(candidateEventType)) {
                          Object stickyEvent = entry.getValue();
                          checkPostStickyEventToSubscription(newSubscription, stickyEvent);
                      }
                  }
              } else {
                  Object stickyEvent = stickyEvents.get(eventType);
                  checkPostStickyEventToSubscription(newSubscription, stickyEvent);
              }
          }
      }
docs: Must be called in synchronized block