?????

private synchronized void buildListsByPlaylist(){
        ConcurrentMap<String, List<MediaMetadata>> newMusicListByPlaylist = new ConcurrentHashMap<>();
        for(MutableMediaMetadata m : mMusicListById.values()){
            String playlist = m.metadata.getString(CUSTOM_METADATA_PLAYLIST_NAME);
            List<MediaMetadata> list = newMusicListByPlaylist.get(playlist);
            Log.e("buildListsByPlaylist", playlist);
            if(list == null){
                list = new ArrayList<>();
                newMusicListByPlaylist.put(playlist, list);
            }
            list.add(m.metadata);
        }
        mMusicListByPlaylist = newMusicListByPlaylist;
    }