code: static public boolean containsOnlyDigits(String string0) {
          // are they all digits?
          for (int i = 0; i < string0.length(); i++) {
              if (!Character.isDigit(string0.charAt(i))) {
                  return false;
              }
          }
          return true;
      }
docs: Checks if a string contains only digits.
  @return True if the string0 only contains digits, or false otherwise.