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Java Java Data Structures Efficiency! Queueing


Unfortunately every time when I compile the program I keep getting a null pointer exception and I cannot find where I made a mistake...

The error is:

Exception in thread "main" java.lang.NullPointerException
at com.teamtreehouse.KaraokeMachine.promptAction(KaraokeMachine.java:35)
at com.teamtreehouse.KaraokeMachine.run(KaraokeMachine.java:50)
at Karaoke.main(Karaoke.java:10)

My code for KaraokeMachine.java:

package com.teamtreehouse;

import com.teamtreehouse.model.Song;
import com.teamtreehouse.model.SongBook;

import java.io.InputStreamReader;
import java.io.BufferedReader;
import java.io.IOException;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.List;
import java.util.HashMap;
import java.util.Map;
import java.util.Queue;

public class KaraokeMachine {
  private SongBook mSongBook;
  private BufferedReader mReader;
  private Queue<Song> mSongQueue;
  private Map<String, String> mMenu;

  public KaraokeMachine(SongBook songBook) {
    mSongBook = songBook;
    mReader = new BufferedReader(new InputStreamReader(System.in));
    mMenu = new HashMap<String, String>();
    mMenu.put("add", "Add a new song to the song book");
    mMenu.put("play", "Play next song in the queue");
    mMenu.put("choose", "choose a song to sing!");
    mMenu.put("quit", "give up. Exit the program");

  private String promptAction() throws IOException {
    System.out.printf("There are %d songs available and %d in the queue. Your options are: %n", 
                      mSongBook.getSongCount(), mSongQueue.size());
    for (Map.Entry<String, String> option : mMenu.entrySet()) {
      System.out.printf("%s - %s %n",
    System.out.print("What do you want to do: ");
    String choice = mReader.readLine();
    return choice.trim().toLowerCase();

  public void run() {
    String choice = "";
    do {
      try {
        choice = promptAction();
        switch(choice) {
          case "add":
            Song song = promptNewSong();
            System.out.printf("%s added! %n%n", song);
          case "choose":
            String artist = promptArtist();
            Song artistSong = promptSongForArtist(artist);
            // TODO: Add to a a song queue
            System.out.printf("You chose: %s %n", artistSong);
          case "play":
          case "quit":
           System.out.println("Thanks for playing!");
            System.out.printf("Unknown choice: '%s'. Try again. %n%n%n", choice);
      } catch(IOException ioe) {
        System.out.println("Problem with input");
    } while(!choice.equals("quit"));

  private Song promptNewSong() throws IOException {
    System.out.print("Enter the artist's name:  ");
    String artist = mReader.readLine();
    System.out.print("Enter the title:  ");
    String title = mReader.readLine();
    System.out.print("Enter the video URL:  ");
    String videoUrl = mReader.readLine();
    return new Song(artist, title, videoUrl);

  private String promptArtist() throws IOException {
    System.out.println("Availabe artists: ");
    List<String> artists = new ArrayList<>(mSongBook.getArtists());
    int index = promptForIndex(artists);
    return artists.get(index);

  private Song promptSongForArtist(String artist) throws IOException {
    List<Song> songs = mSongBook.getSongsForArtist(artist);
    List<String> songTitles = new ArrayList<>();
    for (Song song: songs) {
    System.out.printf("Available songs for %s: %n", artist);
    int index = promptForIndex(songTitles);
    return songs.get(index);

  private int promptForIndex(List<String> options) throws IOException {
    int counter = 1;
    for (String option : options) {
      System.out.printf("%d.) %s %n", counter, option);
    System.out.print("Your choice:   ");
    String optionAsString = mReader.readLine();
    int choice = Integer.parseInt(optionAsString.trim());
    return (choice - 1);

  public void playNext() {
    Song song = mSongQueue.poll();
    if (song == null) {
      System.out.println("Sorry there are no songs in the queue. Use choose from the menu to add some!");
    else {
        System.out.printf("%n%n%n Open %s to hear %s by %s %n%n%n", song.getVideoUrl(),


1 Answer

John Lack-Wilson
John Lack-Wilson
8,181 Points

It appears to me that your Queue<Song> mSongQueue variable has not been assigned a value, which would explain the NullPointerException, i.e. it is null but you are attempting to access it with the following: