Questions on package

package com.hyunjaecho.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);
    }

}

package com.hyunjaecho.model;

import java.util.ArrayList;
import java.util.List;

public class SongBook {
  private List<Song> mSongs;

  public SongBook() {
    mSongs = new ArrayList<Song>();
  }

  public void addSong(Song song) {
    mSongs.add(song);
  }

  public int getSongCount() {
    return mSongs.size();
  }
}