public void finishBatch() {
      if (!batchStarted) {
        throw new RuntimeException("Cannot finish batch that has not started.");
      }
      batchStarted = false;
      batchHash = new ImmutableBytesWritable(digest.digest());
    }
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