public void startBatch(ImmutableBytesWritable row) {
      if (batchStarted) {
        throw new RuntimeException("Cannot start new batch without finishing existing one.");
      }
      batchStarted = true;
      batchSize = 0;
      batchStartKey = row;
      batchHash = null;
    }
Get the current hash
@return the current hash or null if there is no current hash
hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapreduce/HashTable.java