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Java Java Data Structures Efficiency! Menu UI

4,599 Points

java.lang.NullPointerException at com.teamtreehouse.KaraokeMachine.promptAction(KaraokeMachine.java:27)

When running (run time error)

Exception in thread "main" java.lang.NullPointerException
at com.teamtreehouse.KaraokeMachine.promptAction(KaraokeMachi ne.java:27)
at com.teamtreehouse.KaraokeMachine.run(KaraokeMachine.java:4 0)
at Karaoke.main(Karaoke.java:10)

Florian Tönjes
.a{fill-rule:evenodd;}techdegree seal-36
Florian Tönjes
Full Stack JavaScript Techdegree Graduate 50,856 Points

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5 Answers

Jason Wolfe
Jason Wolfe
Courses Plus Student 12,908 Points

Check your code for:

mReader = new BufferedReader(new InputStreamReader(System.in));

In the KaraokeMachine constructor. If mReader isn't initialized there, then it will throw a null.

It is hard to tell what the exact problem is without seeing any code, but from looking at your error it looks like your program is trying to pull info from a variable or method that doesn't have any data in it.

4,599 Points

This is the function for the Java course : The error is displayed for

choice = promptAction();
package com.teamtreehouse;

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

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

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

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

  public KaraokeMachine(SongBook mSongBook) {
    mSongBook = mSongBook;
    mReader = new BufferedReader(new InputStreamReader(System.in));
    mMenu = new HashMap<String, String>();
    mMenu.put("add", "Add a new song to the menu");
    mMenu.put("quit", "Give up. Exit the program");

  private String promptAction() throws IOException {
    System.out.printf("There are %d songs available. Your options are: %n", mSongBook.getSongCount());
    for (Map.Entry<String, String> option : mMenu.entrySet()){
      System.out.printf("%s - %s %n", 
                        option.getKey(), option.getValue());
    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 "quit":
          System.out.print("Thanks for playing");
          System.out.printf("Unknown choice: %s. Try again%n", choice);
      }catch(IOException ioe){
        System.out.println("Problem with input");
  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 int promptFroIndex(List<String> options) throws IOException {
    int counter = 1;
    for(String option : options){
      System.out.printf("%d.) %s %n", counter, option);
      counter ++;
    String optionAsString = mReader.readLine();
    int choice  = Integer.parseInt(optionAsString.trim());
    System.out.printf("Your choice %d", choice);
    return choice - 1;

Matt Y
Matt Y
1,595 Points

Check mSongBook = songBook;

Phil Spelman
Phil Spelman
6,664 Points

I had this same error. My code is essentially identical to Oussama DJEDIDI's code. Here's my setup and what I figured out using JShell (available as of Java 9).

This was on Windows 10 I was using IntelliJ to follow along with Craig's lesson and write the code along with him. In IntelliJ I was using a JShell environment through the IntelliJ Terminal (I replaced the default IntelliJ Terminal with the windows MINGW32 terminal that I was using to learn Git stuff)

For the commands that Craig writes in the java-repl, I wrote those in a file called "junks.java," which I saved in the same folder as my Karaoke.java and KaraokeMachine.java files (the root of my project)

I changed the terminal directory to the folder in which my Karaoke.java file was stored and THEN I opened JShell by typing "jshell" I have a file called "junks.java" and in that file I have written all my jshell commands for opening the .java files I made from following Craig's lesson:

JShell opens and I type: /open junks.java

///////////////junks.java FILE STARTS HERE/////////////////////

/open model/Song.java
/open model/Songbook.java
/open KaraokeMachine.java
System.out.printf("Print this!%n");

Songbook songbook = new Songbook("BOOK ONE!");

Song song = new Song(
    "Michael Jackson",
    "Beat It",
System.out.printf("Adding %s %n",song);
System.out.printf("There are %d songs in the book.%n", songbook.getSongCount());

KaraokeMachine machine = new KaraokeMachine(songbook);

///////////////FILE ENDS HERE/////////////////////

In the JShell terminal I saw the following:

jshell> /open junks.java Print this! Adding Song: Beat It by Michael Jackson There are 1 songs in the book.


jshell> Songbook songbook = new Songbook("Book three"); songbook ==> Book: Book three

jshell> KaraokeMachine machine = new KaraokeMachine(songbook) machine ==> KaraokeMachine@18a70f16

jshell> machine.run() There are 0 songs available. Your options are: add - Add a new song to the songbook quit - Give up. Exit program What do you want to do (type your answer): | java.lang.NullPointerException thrown: | at KaraokeMachine.promptAction (#792:33) | at KaraokeMachine.run (#792:42) | at (#816:1)

I double and triple checked my code, which had worked previously, and there were no mistakes that I could see.

I figured there might have been some hangup with objects in memory that I had created through messing around with JShell, so I QUIT and began a new instance:

jshell> /exit | Goodbye

phil_@DESKTOP-88PHKT2 MINGW32 ..javabasics/src/javaintermediate/karaoke (treehouse_rewrite) $ jshell | Welcome to JShell -- Version 9 | For an introduction type: /help intro


jshell> /open junks.java Print this! Adding Song: Beat It by Michael Jackson There are 1 songs in the book.

jshell> machine.run() There are 1 songs available. Your options are: add - Add a new song to the songbook quit - Give up. Exit program What do you want to do (type your answer):


I have no idea if that will help anyone, but that's how I figured out that my code was typed correctly!