Code from p552 implied the existence of a Jukebox4 since 552 has Jukebox 5

import java.util.ArrayList;

import java.util.Collections;

public class Jukebox4 {

    ArrayList<SongV3> mySongs;

    MockSongV2File mockFile = new MockSongV2File();

    public static void main(String[] args) {

        new Jukebox4().go();

    }

    public void go() {

        mySongs = mockFile.getSongsV3();

        System.out.println(mySongs);

        Collections.sort(mySongs);

        System.out.println(mySongs);

    }

}

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