code: public void setReducerClass(Class<? extends Reducer> cls
                                ) throws IllegalStateException {
      ensureState(JobState.DEFINE);
      conf.setClass(REDUCE_CLASS_ATTR, cls, Reducer.class);
    }
docs: Set the {@link Reducer} for the job.
  @param cls the <code>Reducer</code> to use
  @throws IllegalStateException if the job is submitted