@Override
  public Map<String, Integer> getCounts() {
    return new ImmutableMap.Builder<String, Integer>()
            .put(PLAYLIST_COUNT_DATA_NAME, playlists.size())
            .put(PLAYLIST_ITEM_COUNT_DATA_NAME, playlistItems.size())
            .put(TRACK_COUNT_DATA_NAME, tracks.size())
            .put(RELEASES_COUNT_DATA_NAME, releases.size())
            .build();
  }
A Wrapper for a group of music entities.
portability-types-common/src/main/java/org/datatransferproject/types/common/models/music/MusicContainerResource.java