code: public static String randomString(int length) {
          StringBuilder sb = new StringBuilder();
          for (int i = 0; i < length; i++) {
              int index = (int) random(0, ALL_CHAR.length() - 1);
              sb.append(ALL_CHAR.charAt(index));
          }
          return sb.toString();
      }
docs: Returns a random string.
  
  @param length The random string's length
  @return A random string.