code: public static <K1, V1, K2, V2> void setMapperClass(Configuration conf,
          Class<? extends BaseMapper<?, ?, ?, ?>> internalMapperClass) {
          if (MultithreadedMapper.class.isAssignableFrom(internalMapperClass)) {
              throw new IllegalArgumentException("Can't have recursive "
                  + "MultithreadedMapper instances.");
          }
          conf.setClass(ConfigConstants.CONF_MULTITHREADEDMAPPER_CLASS, internalMapperClass,
              Mapper.class);
      }
docs: Set the application's mapper class.
  
  @param <K1>
  the map input key type
  @param <V1>
  the map input value type
  @param <K2>
  the map output key type
  @param <V2>
  the map output value type
  @param job
  the job to modify
  @param internalMapperClass
  the class to use as the mapper