docs: public static boolean isInteger(CharSequence str, int off, int len) {
          if (len == 0) return false;
          int i = off;
          if (str.charAt(i) == '-') {
              if (len == 1) return false;
              ++i;
          }
          for (; i < len; ++i) {
              char c = str.charAt(i);
              if (c < '0' || c > '9') return false;
          }
          return true;
      }