code: public static String joinBySeparator( List<String> items, String separator ) {
          int size = items.size();
          if (size == 0) {
              return "";
          }
          StringBuilder sb = new StringBuilder(items.get(0));
          for( int i = 1; i < size; i++ ) {
              sb.append(separator).append(items.get(i));
          }
          return sb.toString();
      }
docs: Join a list of strings by string.
  
  @param items the list of strings.
  @param separator the separator to use.
  @return the resulting string.