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