code: public RawComparator getOutputValueGroupingComparator() {
      Class<? extends RawComparator> theClass = getClass("mapred.output.value.groupfn.class", null,
          RawComparator.class);
      if (theClass == null) {
        return getOutputKeyComparator();
      }
  
      return ReflectionUtils.newInstance(theClass, this);
    }
docs: Get the user defined {@link WritableComparable} comparator for
  grouping keys of inputs to the reduce.
  
  @return comparator set by the user for grouping values.
  @see #setOutputValueGroupingComparator(Class) for details.