code: public static void signParams(Map<String, String> params) {
  
      String requestString =
          params.entrySet().stream()
              .sorted(Comparator.comparing(Map.Entry::getKey))
              .map(e -> e.getKey() + "=" + e.getValue())
              .map(String::toUpperCase)
              .collect(Collectors.joining("&"));
  
      String sign = md5(requestString).toLowerCase();
  
      params.put("sign", sign);
      params.remove("secret");
    }
docs: Sign Coinbene private API request parameters.
  
  <p>Docs: <a href="https://github.com/Coinbene/API-Documents/wiki/0.1.0-Sign-Verification">
  
  @param params parameters
  @return the sign