code: @SuppressWarnings("unchecked")
    public Class<? extends Reducer<?,?,?,?>> getCombinerClass() 
       throws ClassNotFoundException {
      return (Class<? extends Reducer<?,?,?,?>>) 
        conf.getClass(COMBINE_CLASS_ATTR, null);
    }
docs: Get the combiner class for the job.
  
  @return the combiner class for the job.