public void acceptUrls(final String groupName, final List<String> urls) {
    for (String url : urls) {
      final Set<String> set;
      if (urls2groups.containsKey(url)) {
        set = urls2groups.get(url);
      }
      else {
        set = new HashSet<>();
        urls2groups.put(url, set);
      }
      set.add(groupName);
    }
  }
public void acceptUrls(final String groupName, final List<String> urls)