code: public V putIfAbsent (K putKey, V putValue) {
          V existingValue = this.store.putIfAbsent(putKey, putValue);
  
          if ( existingValue == null ) {
              this.notificationExecutor.firePutNotification(this.listeners.iterator(), putKey, putValue);
          }
  
          return  existingValue;
      }
docs: Add the given entry into the registry under the specified key, but only if the key is not already registered.
  
  @param putKey key identifying the entry in the registry to add, if it does not already exist.
  @param putValue value to add to the registry.
  @return existing value in the registry if already defined; null if the new value is added to the registry.