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