@Override
    public DoubleWritable div(DoubleWritable sum, long numRows) {
      if (sum == null || numRows == 0) {
        return null;
      }

      return new DoubleWritable(sum.get() / numRows);
    }
Define some type specific operation to used in the subclass
ql/src/java/org/apache/hadoop/hive/ql/udf/ptf/BasePartitionEvaluator.java