code: static String join(String sep, String... elements) {
      StringBuilder sb = new StringBuilder();
      for (String e : elements) {
        if (e != null) {
          if (sb.length() > 0) {
            sb.append(sep);
          }
          sb.append(e);
        }
      }
      return sb.toString();
    }
docs: Joins a list of strings using the given separator.