Issue with adding instance of class object to ArrayList, what am I doing wrong? [JAVA]

Question

I am trying to add an object of class Track to my ArrayList of Track, playlist.

import java.util.ArrayList;

public class PlayList {
    private String playList;
    private ArrayList <Track> myTracks;

    //constructors
    public PlayList(String name) {
        this.playList = name;
    }

    public PlayList(String name, ArrayList<Track> tracks) {
        this.playList = name;
        this.myTracks = tracks;
    }

    public addTrack(Track track){
        myTracks.add(0,track)
    }
}

But both my constructor and addTrack method fails, showing java.lang.NullPointerException

Where am I going wrong?

you hven't initialize your arraylist hence your method fails, try only constructing an object your constructors must work — SSH
– SSH, Commented Oct 29, 2015 at 6:01
Your code doesn't compile. addTrack() should have a return type — TheLostMind
– TheLostMind, Commented Oct 29, 2015 at 6:08

Avinash A · Accepted Answer · 2015-10-29 06:04:04Z

2

You never get an exception in your constructor. There is no chance at all.

And the reason for add method is you are using myTracks.add(0,track) and you never initialize your list

private List<Track> myTracks = new ArrayList<Track>();

answered Oct 29, 2015 at 6:04

Avinash A

915 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

TheLostMind · Accepted Answer · 2015-10-29 06:02:43Z

0

You need to initialize the ArrayList and code to the interface. Use : private List<Track> myTracks = new ArrayList<>(); (for java versions below java -7, use new ArrayList<Track>();)

edited Oct 29, 2015 at 6:02

answered Oct 29, 2015 at 6:01

TheLostMind

36.3k12 gold badges72 silver badges109 bronze badges

3 Comments

Suresh Atta Over a year ago

But why exception is constructor ?

TheLostMind Over a year ago

@sᴜʀᴇsʜᴀᴛᴛᴀ - I guess the OP's code is incomplete. He must be trying to add items to myTracks . His current code doesn't compile

Suresh Atta Over a year ago

Yah. Probably he failed to understand the stacktrace.

karim mohsen · Accepted Answer · 2015-10-29 06:14:52Z

0

I think your constructor fails because you try to initialize object from playList like this :-

playList myList = new playList();

This will give you error because there is no defualt constructor and it is not created by default (The default constructor is automatically generated unless you define another constructor)

If you didn't create your playList object like this there is no way you got an exception in the constructor

And the reason for addmethod error is you initialized your list

You have to do this :-

private List<Track> myTracks = new ArrayList<Track>();

answered Oct 29, 2015 at 6:14

karim mohsen

2,2741 gold badge17 silver badges20 bronze badges

Comments

Aritro Sen · Accepted Answer · 2015-10-29 06:31:29Z

0

You are getting "NullPointerException" in the Arraylist because you have not initialized it first. You can add items in list only after initializing it first. So write like this instead and try -

private List<Track> myTracks = new ArrayList<Track>();

As far as the constructor is concerned i think there you didnt give the required parameters. When you are calling the constructor of PlayList class you need to pass the correct values as parameters which you defined(2 Constructors in this case). if you don't then it will throw an error because there is no default(no argument) constructor. So try like this -

PlayList myList=new PlayList("//give name","//ArrayList name which contains the tracks")

answered Oct 29, 2015 at 6:31

Aritro Sen

3571 gold badge7 silver badges15 bronze badges

Collectives™ on Stack Overflow

Issue with adding instance of class object to ArrayList, what am I doing wrong? [JAVA]

4 Answers 4

Comments

3 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

3 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related