code: @Override
      public Node getParentNode(String parentNodeId)
      {
          NodeLink link = new NodeLink(parentNodeId, getNodeId());
  
          if (this.parents == null)
          {
              return null;
          }
          else
          {
              return this.parents.get(link);
          }
      }
docs: Retrieves parent node of this node for a given parent node ID