code: @Nullable
    @IsLocked (ELockType.READ)
    protected final INTERFACETYPE getAtIndex (@Nonnegative final int nIndex)
    {
      return m_aRWLock.readLocked ( () -> CollectionHelper.getAtIndex (m_aMap.values (), nIndex));
    }
docs: Get the item at the specified index. This method only returns defined
  results if an ordered map is used for data storage.
  
  @param nIndex
  The index to retrieve. Should be &ge; 0.
  @return <code>null</code> if an invalid index was provided.