private void openHashFile() throws IOException {
        if (mapFileReader != null) {
          mapFileReader.close();
        }
        Path dataDir = new Path(TableHash.this.hashDir, HASH_DATA_DIR);
        Path dataFile = new Path(dataDir, getDataFileName(hashFileIndex));
        mapFileReader = new MapFile.Reader(dataFile, conf);
      }
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