code: public LongWritable getGroupingLongWritable(String label) {
          HadoopObject o = getHadoopObject(KEY, label, ObjectUtil.LONG, "Long");
          if (o == null) {
              return null;
          }
          return (LongWritable) o.getObject();
      }
docs: Get grouping {@link LongWritable} value
  @param label target label
  @return {@link LongWritable} value of the label. If it is not null.