code: public static String join(String[] strings, String separator) {
        StringBuilder sb = new StringBuilder();
        boolean first = true;
        for (String s : strings) {
           if (first) {
              first = false;
           } else {
              sb.append(separator);
           }
           sb.append(s);
        }
        return sb.toString();
     }
docs: Join an array of strings with the given separator.
  
  @param strings
  The strings to join
  @param separator
  The separator to use
  @return The concatenation of the collection of strings with the given
  separator.