code: public static Class<? extends WritableComparable> getNamedOutputKeyClass(JobConf conf,
                                                  String namedOutput) {
      checkNamedOutput(conf, namedOutput, false);
      return conf.getClass(MO_PREFIX + namedOutput + KEY, null,
  	WritableComparable.class);
    }
docs: Returns the key class for a named output.
  
  @param conf        job conf
  @param namedOutput named output
  @return class for the named output key