code: static public Class<? extends WritableComparable> getSequenceFileOutputKeyClass(JobConf conf) { 
      return conf.getClass("mapred.seqbinary.output.key.class", 
                           conf.getOutputKeyClass().asSubclass(WritableComparable.class),
                           WritableComparable.class);
    }
docs: Get the key class for the {@link SequenceFile}
  
  @return the key class of the {@link SequenceFile}