code: public Optional<OPFItem> getItemById(String id)
    {
      return Optional.fromNullable(itemsById.get(id));
    }
docs: Search the item with the given ID.
  
  @param id
  the ID of the item to search, can be <code>null</code>.
  @return An {@link Optional} containing the item if found, or
  {@link Optional#absent()} if not found.