code: public List<AnnounceEntry> trackers() {
          if (!th.is_valid()) {
              return Collections.emptyList();
          }
          return TorrentInfo.trackers(th.trackers());
      }
docs: Will return a sorted list with the trackers of this torrent.
  <p>
  The announce entry contains both a string {@code url} which specify the
  announce url for the tracker as well as an int {@code tier}, which
  specifies the order in which this tracker is tried.
  
  @return the list of trackers