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David Pollard
David Pollard
4,324 Points

Struggling with understanding of this Java code

I'm trying to get my head around this code. What I understand is added as remarks. The question comes towards the end :-

// Create a method called byArtist() that returns a hashmap of Artist name
// and for each artist a list of songs by them.
 private Map<String, List<Song>> byArtist() {

// Create the object we're eventually going to return (byArtist) that is a new hashmap of
// artist and their list of songs
    Map<String, List<Song>> byArtist = new HashMap<>();

// For each song in the songlist
    for (Song song : mSongs) {

// Check if there is a array list of songs for the artist in question
      List<Song> artistSongs = byArtist.get(song.getArtist());

// if there's not
      if (artistSongs == null) {

// then create one.
        artistSongs = new ArrayList<>();

// and initialize it with the artist name, and the new blank arraylist
        byArtist.put(song.getArtist(), artistSongs);

// this is where i struggle. we seem to only add the song to the list,
// we don't ever seem to update byArtist with the list of songs ! :-(
// apart from when there is no arraylist (i.e. it's null) ?!?!?!


    return byArtist;

Thanks ! :-)


1 Answer

Eitan Vinsonneau
Eitan Vinsonneau
2,498 Points

artistSongs is actually a reference (a pointer to place in memory if you will). So when you do artistSongs.add(song) it modify the ArtistSong that is in byArtist.

I suggest you lookup "java object reference" or "C pointer" for more info and there's also a treehouse workshop in the java course (concerning Strings if I remember correctly) where this is explained in great details.

Also why do you hate other programmers so much as to give the same name to a function and a variable ?

David Pollard
David Pollard
4,324 Points

Eitan - thanks so much - that is definitely what I was missing. Makes sense now - much appreciate your time.

As for this :-

Also why do you hate other programmers so much as to give the same name to a function and a variable ?

I also didn't feel comfortable with that, but that's the code used in the tutorial, not my code ! :-)