@Override
    public LongWritable add(LongWritable t1, LongWritable t2) {
      if (t1 == null && t2 == null) {
        return null;
      }
      return new LongWritable((t1 == null ? 0 : t1.get()) + (t2 == null ? 0 : t2.get()));
    }
Define some type specific operation to used in the subclass
ql/src/java/org/apache/hadoop/hive/ql/udf/ptf/BasePartitionEvaluator.java