code: protected String getRequestUrl(String key) throws Exception {
          String result = "";
          try {
              result = this.methods.get(key)[0];
          } catch (Exception ex) {
              throw new Exception("Unknown method key: " + key);
          }
          return result;
      }
docs: Gets the URL of REST Mango Pay API.
  @param key   The method key to get URL of.
  @return      The URL string of given method.