code: public static StringBuilder join(final StringBuilder sb, final String sep, final String... tokens) {
          boolean first = true;
          for (Object t: tokens) {
              if (!first) sb.append(sep);
              else first = false;
              sb.append(t);
          }
          return sb;
      }
docs: Appends {@code tokens} to {@code target}, separated by {@code sep}.
  @param tokens
  @param sb