code: @CheckForNull
      public String getNextNodeId( @Nonnull final String nodeId ) throws IllegalArgumentException {
          return _nodeIds.getNextNodeId( nodeId );
      }
docs: Gets the next node id for the given one from the list of all node ids.
  If there's only a single node known, conceptionally there's no next node
  and therefore <code>null</code> is returned.
  @param nodeId the node id for that the next one is determined.
  @return the next node id or <code>null</code>.
  
  @throws IllegalArgumentException thrown if the given nodeId is not part of this list.
  
  @see NodeIdList#getNextNodeId(String)