@Override
    public void reset(AggregationBuffer agg) throws HiveException {
      SumLongAgg myagg = (SumLongAgg) agg;
      myagg.empty = true;
      myagg.sum = 0;
    }
class for storing double sum value.
itests/util/src/main/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFSumList.java