code: public static String join(List<String> list, String separator) {
          if (list == null) {
              return null;
          }
  
          return join(list.toArray(), separator, 0, list.size());
      }
docs: Join a list of Strings
  
  @param list      strings to join
  @param separator the separator to insert between the strings
  @return a string made of the strings in list separated by separator