code: public void put(CallbackType callbackType, CallbackMetadata callbackMetadata) {
      if (callbacks == null) {
        callbacks = new EnumMap<>(CallbackType.class);
      }
      List<CallbackMetadata> callbackMetadataList = callbacks.get(callbackType);
      if (callbackMetadataList == null) {
        callbackMetadataList = new ArrayList<>();
        callbacks.put(callbackType, callbackMetadataList);
      }
      callbackMetadataList.add(callbackMetadata);
    }
docs: Adds the given CallbackEventMetadata.
  
  @param callbackType
  the callback type
  @param callbackMetadata
  the metadata of the callback