code: public static long readNumberFromFile(final Path filePath)
        throws IOException, NumberFormatException {
      final List<String> allLines = Files.readAllLines(filePath);
      if (!allLines.isEmpty()) {
        return Long.parseLong(allLines.get(0));
      } else {
        throw new NumberFormatException("unable to parse empty file " + filePath.toString());
      }
    }
docs: Reads a number from a file.
  
  @param filePath the target file