用一个实例对列表进行排序

Question

Hello I am trying to pass to my jsp page two sorted Lists which I get from the database like that

   @RequestMapping(value = "/sortDate", method= RequestMethod.GET)
  public void sortedLists(Model model, HttpSession session){


    List<Song> songsByDate;
    try {
    songsByDate = SongDAO.getInstance().getAllSongs();
    Collections.sort(songsByDate, new UploadTimeComparator());
    session.setAttribute("songs", songsByDate);
     /////////////////////////////////
    List<Song> songsByLikes;
    try {
        songsByLikes = SongDAO.getInstance().getAllSongs();
    Collections.sort(songsByLikes, new LikesComparator());
    session.setAttribute("songs2", songsByLikes);
   }}}

However the problem is since I use getInstance() , songs and songs2 are the same Lists sorted by Likes,how can I overcome this problem?

Answer 1

You need to copy each "all songs" list to a new array list, which you can then proceed to sort.

So:

List<Song> songsByDate = new ArrayList<>( SongDAO.getInstance().getAllSongs() );
Collections.sort( songsByDate, new UploadTimeComparator() );

List<Song> songsByLikes = new ArrayList<>( SongDAO.getInstance().getAllSongs() );
Collections.sort( songsByLikes, new LikesComparator() );