code: public List<String> getPaymentRequestUrls() {
          ArrayList<String> urls = new ArrayList<>();
          while (true) {
              int i = urls.size();
              String paramName = FIELD_PAYMENT_REQUEST_URL + (i > 0 ? Integer.toString(i) : "");
              String url = (String) parameterMap.get(paramName);
              if (url == null)
                  break;
              urls.add(url);
          }
          Collections.reverse(urls);
          return urls;
      }
docs: Returns the URLs where a payment request (as specified in BIP 70) may be fetched. The first URL is the main URL,
  all subsequent URLs are fallbacks.