code: public Class<?> getMapOutputValueClass() {
      Class<?> retv = getClass("mapred.mapoutput.value.class", null,
          Object.class);
      if (retv == null) {
        retv = getOutputValueClass();
      }
      return retv;
    }
docs: Get the value class for the map output data. If it is not set, use the
  (final) output value class This allows the map output value class to be
  different than the final output value class.
  
  @return the map output value class.