code: public static String joinByComma( List<String> items ) {
          StringBuilder sb = new StringBuilder();
          for( String item : items ) {
              sb.append(",").append(item);
          }
          if (sb.length() == 0) {
              return "";
          }
          return sb.substring(1);
      }
docs: Join a list of strings by comma.
  
  @param items the list of strings.
  @return the resulting string.