code: @SuppressWarnings("unchecked")
    public static <K1,V1,K2,V2>
    Class<Mapper<K1,V1,K2,V2>> getMapperClass(JobContext job) {
      return (Class<Mapper<K1,V1,K2,V2>>) 
           job.getConfiguration().getClass("mapred.map.multithreadedrunner.class",
                                           Mapper.class);
    }
docs: Get the application's mapper class.
  @param <K1> the map's input key type
  @param <V1> the map's input value type
  @param <K2> the map's output key type
  @param <V2> the map's output value type
  @param job the job
  @return the mapper class to run