code: public static String randomString(int length, @NonNull CharMatcher validChar) {
        return randomString(length, 0, Integer.MAX_VALUE, validChar);
     }
docs: Generates a random string of a given length
  
  @param length    The length of the string
  @param validChar CharPredicate that must match for a character to be returned in the string
  @return A string of random characters