public void addToDispatchList(Node dispatchedObj){
      if(dispatchedObj != null) {
        retMap.put(dispatchedObj, null);
      }
    }
base class for operator graph walker this class takes list of starting ops
and walks them one by one. it maintains list of walked operators
(dispatchedList) and a list of operators that are discovered but not yet
dispatched
ql/src/java/org/apache/hadoop/hive/ql/lib/TaskGraphWalker.java