code: public NodeT getNext() {
          String nextItemKey = queue.poll();
          if (nextItemKey == null) {
              return null;
          }
          return nodeTable.get(nextItemKey);
      }
docs: Gets next node in the DAG which has no dependency or all of it's dependencies are resolved and
  ready to be consumed.
  
  @return next node or null if all the nodes have been explored or no node is available at this moment.