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