code: private static String hashFile(final File file, final int pieceSize)
            throws InterruptedException, IOException {
      return hashFiles(Collections.singletonList(file), pieceSize);
    }
docs: Return the concatenation of the SHA-1 hashes of a file's pieces.
  
  <p>
  Hashes the given file piece by piece using the default Torrent piece
  length (see {@link #DEFAULT_PIECE_LENGTH}) and returns the concatenation of
  these hashes, as a string.
  </p>
  
  <p>
  This is used for creating Torrent meta-info structures from a file.
  </p>
  
  @param file The file to hash.