Why does the do-while loop not run as expected in this Java program?

Question

I've been writing a program that will take an array of the names of music files and play them. I succeeded in doing that, but, I wanted to touch up on some things and make it a little nicer. I am trying to make the music play in a random order but not repeat any songs before the entire list has been played. I was almost able to do it but I think there is something wrong with my do-while loop. The program runs as intended for about eight songs but then it stops playing the music and the JVM continues to run. I am using BlueJ as I am still an AP Comp Sci student so I realize that I might not be able to accomplish this task but any help would be greatly appreciated. I have a driver, "MusicDriver" that is a "has-a" relationship with two other classes: "MP3" and "Music".

my MP3 class:

import java.io.BufferedInputStream;
import java.io.FileInputStream;
import javazoom.jl.player.Player;

public class MP3 {
    String filename;
    Player player; 

    public void stopMP3() { if (player != null) player.close(); }

    // play the MP3 file to the sound card
    public void playMP3(String filename) {
    try {
        FileInputStream fis = new FileInputStream(filename);
        BufferedInputStream bis = new BufferedInputStream(fis);
        player = new Player(bis);
    }
    catch (Exception e) {
        System.out.println("Problem playing file " + filename);
        System.out.println(e);
    }

    // run in new thread to play in background
    new Thread() {
        public void run() {
            try { player.play(); }
            catch (Exception e) { System.out.println(e); }
        }
    }.start();
}  
}

my Music class:

import java.util.*;

public class Music{
private ArrayList<String> music;

public Music(){music = new ArrayList<String>();}

public int size(){return music.size();}

public void addSong(String song){music.add(song);}

public String getSong(){return music.get(music.size());}

public String getSong(int num){return music.get(num);}

public void removeSong(String song){
    for(int i = 0; i < music.size(); i++){
        if(music.get(i).equals(song)) {music.remove(i); return;}
    }
}

public String toString(){
    String s = "";
    for(int i = 0; i < music.size(); i++){
        s += music.get(i);
    }
    return s;
}
}

my MusicDriver class:

import java.util.*;
import java.io.*;
import javazoom.jl.player.Player;
import java.util.Random;
import java.util.Scanner;
import java.io.FileNotFoundException;

public class MusicDriver{
public static void main(String[] args) throws FileNotFoundException{
    Random r = new Random();
    Scanner s = new Scanner(System.in);
    String line = "";
    int number;

    Music song = new Music();
    song.addSong("1-01-overture.mp3");
    song.addSong("1-03-fortune-teller-2.mp3");
    song.addSong("1-07-prayer.mp3");
    song.addSong("1-08-island-atlas.mp3");
    song.addSong("1-12-warren-report.mp3");
    song.addSong("1-13-avilla-hanya.mp3");
    song.addSong("1-20-war-situation.mp3");
    song.addSong("2-10-fog-of-phantom.mp3");
    song.addSong("2-12-religious-precepts.mp3");
    song.addSong("2-14-box-of-sentiment.mp3");
    song.addSong("3-02-light-everlasting.mp3");
    song.addSong("3-09-viking-spirits.mp3");
    song.addSong("3-12-unsealed.mp3");
    song.addSong("3-16-notice-of-death-reprise-.mp3");
    //14 songs

    ArrayList<Integer> songNums = new ArrayList<Integer>();
    MP3 mp3 = new MP3();
    do{
        if(songNums.size() == song.size()) songNums.clear();

        number = r.nextInt(song.size());
        boolean done = false;
        int counter = 0;
        while(!done){
            for(int i = 0; i < songNums.size(); i++){
                if(number == songNums.get(i).intValue()) {number = r.nextInt(song.size()); counter++;}
            }
            if(counter == 0) done = true;
            else done = false;
        }

        songNums.add(number);
        mp3.playMP3(song.getSong(number));
        System.out.println("Now Playing " + song.getSong(number));
        System.out.println("Enter \"Stop\" to stop playing the song");
        System.out.println("Enter \"n\" to play the next song");
        line = s.nextLine();
        mp3.stopMP3();
    }while(line.equals("n"));
    mp3.stopMP3();
}
}

I have done a lot of research on why my program just stops playing my songs but I haven't been able to find anything. I did, find that BlueJ programs don't open the terminal window (the thing that comes up when you do a "System.out.print()") if you ask for input before you have any output but I don't think that comes into account for this program. I have also made sure that I have input a String "n" when I want to play the next song and for the first couple songs, it works, but after the eighth song, it just stops. I am thoroughly confused.

Answer 1

I think the only issue lies in the logic that you are using for shuffling the list.

number = r.nextInt(song.size());
boolean done = false;
int counter = 0;
while(!done){
    for(int i = 0; i < songNums.size(); i++){
        if(number == songNums.get(i).intValue()) {number = r.nextInt(song.size()); counter++;}
    }
    if(counter == 0) done = true;
    else done = false;
}

When the random number generated is already existing in the songNums list, you are generating a new random number. This new random number is not checked with all numbers of songNums list. The following change should solve your problem.

    boolean done = false;
    while(!done){
        number = r.nextInt(song.size());
        if(!songNum.contains(number)) done = true;
    }

Alternatively, you can use Sasha 's suggestion in the comments for shuffling the list(Collections.shuffle()).

Answer 2

The actual problem with your existing algorithm is you are not resetting counter when you discover a song that has already been played. So as soon as you hit a repeat, you get stuck in an infinite loop - done will never be true.

(Actually, it won't be infinite - once counter reaches Integer.MAX_VALUE it will wrap around to Integer.MIN_VALUE and eventually reach 0 again, so if you left it long enough it would eventually play another song)

There are some useful suggestions here already about improvements to the code and I won't repeat them here, but the minimum change that will fix what you have is to move the initialization of counter to 0 inside the loop:

boolean done = false;

while(!done){
    int counter = 0; // reset counter every time

    for(int i = 0; i < songNums.size(); i++){
        if(number == songNums.get(i).intValue()) {number = r.nextInt(song.size()); counter++;}
    }

    if(counter == 0) done = true;
    else done = false;
}

Answer 3

Sasha said it in the comments: use Collections.shuffle(). In practice that would look a lil' something like this:

In the Music class have a method to get all the songs:

public List<String> getSongs() {return music;}

The loop in MusicDriver would be along the lines of:

List<String> songs = song.getSongs();
do{
    Collections.shuffle(songs);
    for (String songToPly: songs) {
        mp3.playMP3(song.getSong(number));
        System.out.println("Now Playing " + song.getSong(number));
        System.out.println("Enter \"Stop\" to stop playing the song");
        System.out.println("Enter \"n\" to play the next song");
        mp3.stopMP3();
        line = s.nextLine();
        if (!line.equals("n")) break;
    }
}while(line.equals("n"));

On a variable-naming note, naming the instance of your Music class as "song" (singular) is kind of confusing. Maybe call it "music" or at least "songs".

Answer 4

What I would do is:

public class MainClass() {

    public static void main(String[] args) {
        PlayerWrapper player = new PlayerWrapper();
    }
}

public class PlayerWrapper() {
    private List<MP3> playlist;
    private Scanner userInputReader;
    private String currentUserInput;

    public PlayerWrapper() {
        userInputReader = new Scanner(System.in());
        System.out.println("Filepath to playlist?");
        String playlistFileName = userInputReader.nextLine();
        playlist = PlayListExtractor.extractPlaylist(playlistFileName);
        start();
    }

    public void start() {
        playlistCopy = new ArrayList<MP3>(playlist);
        shufflePlayList(playlistCopy);
        Iterator<MP3> songIterator = playlistCopy.iterator();
        while (songIterator.hasNext()) {
            MP3 song = songIterator.next();
            songIterator.remove();
            player = new Player(song.toStream());
            player.play();
            displayCurrentSongAndCommands(song);
            currentUserInput = userInputReader.nextLine();
            if ("Stop".equals(currentUserInput )) {
                player.close();
                break;
            } else if ("n".equals(currentUserInput )) {
                player.close();
                continue;
            }
        }

        if("Stop".equals(currentUserInput)) {
            System.out.println("Playlist stopped. Press q to quit or c to continue");
            currentUserInput = userInputReader.nextLine();
            if ("q".equals(currentUserInput)) {
                System.exit(0);
            } else if ("c".equals(currentUserInput)) {
                start();
            }
        }
        start();
    }

    private void shufflePlayList(final List<MP3> playlistToBeShuffled) {
        long seed = System.nanoTime();
        Collections.shuffle(playlistToBeShuffled, new Random(seed));               
    }

    private void displayCurrentSongAndCommands(final MP3 currentSong) {
        System.out.println("Now Playing " + currentSong.toString());
        System.out.println("Enter \"Stop\" to stop playing the song");
        System.out.println("Enter \"n\" to play the next song");
    }
}

public static class PlayListExtractor() {
    private PlayListExtractor();

    public static List<MP3> extractPlayList(final String playListFileName) {
        List<MP3> result = new ArrayList<>();
        try (BufferedReader br = new BufferedReader(new FileReader(file))) {
            String line;
            while ((line = br.readLine()) != null) {
                result.add(new MP3(line));
            }
            return result;
        } catch (IOException e) {
            System.out.println("Problem parsing playlist");
        }
    }
}

public class MP3 {
    private String filename;

    public MP3(final String filename) {
        this.filename = filename;
    }

    public BufferedInputStream toStream() {
        try {
            FileInputStream fis = new FileInputStream(filename);
            return new BufferedInputStream(fis);                  
        }
        catch (Exception e) {
            System.out.println("Problem playing file " + filename);
            System.out.println(e);
        } 
    }

    public String toString() {
        return filename;
    }
}