@Override
  public RecordReader<NullWritable, Cell> createRecordReader(InputSplit split,
    TaskAttemptContext context) throws IOException, InterruptedException {
    return new HFileRecordReader();
  }
A private cache of the key value so it doesn't need to be loaded twice from the scanner.
hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapreduce/HFileInputFormat.java