How to use collections for a music app

Question

I am writing a basic music app for my school. What our teacher asked from us is that wee need to create three classes; Song, PlayList and MusicCollection. A play list object contains songs and music collection contains play lists.I write my Song class and also a big part of my PlayList class. But there are two methods that I don't know very well. One is sortByName and the other is sortByPlayingTime. I need to use collections and I write something until now but it is not working. Can you have a look?

public class PlayList {

private String playListName;
private ArrayList<Song> songsPlayList = new ArrayList<>();

public PlayList(String playListName) {
    this.playListName = playListName;
}

public void addSong(Song songObject){
    songsPlayList.add(songObject);
}

public void removeSong(Song songObject) {
    songsPlayList.remove(songObject);
}

public void countOfPlaylist(){
    System.out.println("You have " + songsPlayList.size() + " songs on your list!");
}

public void displayPlayList(){
    for (int i =0;i<songsPlayList.size();i++){
        System.out.println((i+1) + ". " + songsPlayList.get(i));
    }
}

public void sortByName(){
    System.out.println("You can see your list as sorted by name.\n");
    Collections.sort(song); //this code give an error!
    for (Song temp : songsPlayList){
        System.out.println(temp);
    }
}

public void sortByPlayingTime(){
    System.out.println("Your playlist sorted by playing time.");
    //I stock here.
}

Also with my MusicCollection class I need to do the same thing. It was easy to add a song to a play list but now I need to choose (or say) which play list I need to add a song. I write my methods but not define yet. I need your help :)

Here is my MusicCollection class:

public class MusicCollection {

private String musicCollectionName;
private ArrayList<PlayList> collection = new ArrayList<>();

public MusicCollection(String musicCollectionName) {
    this.musicCollectionName = musicCollectionName;
}

public void addSongToPlayList(String playListName,Song songObject){


}

public void removeSongFormPlaylist (String playListName, Song songObject){

}

public void addPlayList (PlayList  playlistObject){

}

public void removePlayList (PlayList  playlistObject){

}

public int countOfPlaylistsInCollection(){

    return 0;
}

public int countsOfSongsInPlayList (String playlistName){

    return 0;
}

public void displayPlayListInCollection(){

}

public void displaySongsFromPlaylist (String playlistName){

}

}

Collections.sort(song); Where is your song variable? You have to sort songsPlayList I suppose. — Sergei Podlipaev, Jun 19 '18 at 09:36
..and what is the error? Either your `Song` class needs to implement `Comparable` or you can use `Collections.sort(list, Comparator)` — Jack Flamp, Jun 19 '18 at 09:39
@SergeiPodlipaev I didn't put a song variable. I missed that variable and because of it it is not working good.Right? — Eren Arican, Jun 19 '18 at 09:53
@JackFlamp Error was: reason: no instance(s) of type variable(s) T exist so that Song conforms to Comparable super T> But when I received that error, this was like Collections.sort(songsPlayList); — Eren Arican, Jun 19 '18 at 09:53
@ErenArican no, this wasnt the full root cause. Please refer to my answer. — Sergei Podlipaev, Jun 19 '18 at 09:55
@ErenArican `Collections.sort(songsPlayList)` means that the list is sorted according to its natural sorting order. An object has no natural order so you have to implement that yourself. See Maurice's answer or use `Comparable` — Jack Flamp, Jun 19 '18 at 09:55

score 0 · Accepted Answer · answered Jun 19 '18 at 09:38

0

Collections.sort(song);

Where is your song variable? You have to sort songsPlayList I suppose. And you have to write comparators for your Song class. For example implement Comparable interface. Please, refer to this article or this question for more information about comparators.

answered Jun 19 '18 at 09:38

Sergei Podlipaev

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Thank you for your help. I guess i see that where i made a mistake. – Eren Arican Jun 19 '18 at 10:05
Please mark this answer as resolution after you fix and test it – Sergei Podlipaev Jun 19 '18 at 10:07

score 0 · Answer 2 · answered Jun 19 '18 at 09:44

You need a Comparator<Song> that compares two songs' playing time:

public static final Comparator<Song> BY_PLAYINGTIME = new Comparator<Song>() {
    @Override
    public int compare(Song a, Song b) {
        return a.getPlayingTime().compareTo(b.getPlayingTime());
    }
};

You can then sort your list with:

Collections.sort(songsPlayList, BY_PLAYINGTIME);

How to use collections for a music app

2 Answers2