Questions on Karaoke byArtist method. Would appreciate feedback.

package com.teamtreehouse.model;

import java.util.List;
import java.util.ArrayList;
import java.util.Set;
import java.util.Map;
import java.util.HashMap;

public class SongBook {
  private List<Song> mSongs;//?? // dont have initialize Song bc in same package

  public SongBook() { //constructor takes nothing
    mSongs = new ArrayList<Song>();//initializing list of songs, 
    // dont have initialize Song bc in same package
      }

  public void addSong(Song song) {//pass in a song
    mSongs.add(song);
   }

  public int getSongCount() {
    return mSongs.size();
  }
  //FIXME: This should be cached!
  private Map<String, List<Song>> byArtist() {// method creating Map of artists to songs
    Map<String, List<Song>> byArtist = new HashMap<>();// How, where does the Song object with 3 string variables, artist,          title, video url get stored into a list data structure? 
    for (Song song : mSongs) { // for each song currently in mSongs list
      List<Song> artistSongs = byArtist.get(song.getArtist());// How can we get the list from the key value out of                  byArtist map when we haven't fully declared artistSongs as a new ArrayList, which we then do 2 lines down? 
      if (artistSongs == null) {// see if yet to find an artist, if not make new one 
        artistSongs = new ArrayList<>();// 
        byArtist.put(song.getArtist(), artistSongs);//put into map the artist name and new list we made but we have                 artistSongs ArrayList evaluating to null at this point??
      }
      artistSongs.add(song);// how does List add method put an objects' string variables into a comma separated list                sequence?
    }
    return byArtist;
  }

  public Set<String> getArtists() {
    return byArtist().keySet();
  }

  public List<Song> getSongsForArtist(String artistName) {
    return byArtist().get(artistName);
  }



}

package com.teamtreehouse.model;

public class Song {
  private String mArtist;
  private String mTitle;
  private String mVideoUrl;

  public Song(String artist, String title, String videoUrl){

    mArtist = artist;
    mTitle = title;
    mVideoUrl = videoUrl;
  }

  public String getTitle() {
    return mTitle;
  }

  public String getArtist() {
    return mArtist;
  }

  public String getVideoUrl() {
    return mVideoUrl;
  }

  @Override
  public String toString() {
    return String.format("Song: %s by %s", mTitle, mArtist);
  }
}