code: public Collection<Registration> getRegistrations(String topic, boolean forceCreate) {
          Collection<Registration> listenerList = registrations.get(topic);
          if (listenerList == null && forceCreate) {
              ConstructorFunction<String, Collection<Registration>> func
                      = key -> newSetFromMap(new ConcurrentHashMap<Registration, Boolean>());
              return ConcurrencyUtil.getOrPutIfAbsent(registrations, topic, func);
          }
          return listenerList;
      }
docs: Returns the {@link Registration}s for the event {@code topic}. If there are no
  registrations and {@code forceCreate}, it will create a concurrent set and put it in the registration map.
  
  @param topic       the event topic
  @param forceCreate whether to create the registration set if none exists or to return null
  @return the collection of registrations for the topic or null if none exists and {@code forceCreate} is {@code false}