Struggling with understanding of this Java code

// 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) ?!?!?!

  artistSongs.add(song);
    }

    return byArtist;
  }