code: private List<Integer> extractDigits(final String value) throws NumberFormatException {
      final List<Integer> digits = new ArrayList<>(value.length());
      final char[] chars = value.toCharArray();
      for (final char c : chars) {
        digits.add(this.extractDigit(c));
      }
      return digits;
    }
docs: Parses the {@link String} value as a {@link List} of {@link Integer} objects.
  
  @param value the input string to be parsed
  
  @return List of {@code Integer} objects.
  
  @throws NumberFormatException in case any of the characters is not a digit