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