Question

I’m getting a null pointer exception despite initialising all of my objects correctly (to my knowledge). Baiscially, I need someone to point out where I’ve been an idiot.

The runtime NPE occurs here:

public class CDAlbum implements Album{
    private String title;
    private String genre;
    private String artist;
    private ArrayList<String> tracks;
    private int i=0;

    public CDAlbum() {    //constuctor
        title   = "Unknown CD";
        genre   = "Unknown genre";
        artist = "Unknown artist";
        ArrayList<String> tracks = new ArrayList<String>(); //NPE occurs on this line
        tracks.add("-- no tracks --");
    }
    //snip

And the code calling the constructor:

CDAlbum tempCD;
tempCD = new CDAlbum();

I think I’m initialising everything properly, so I can’t see where this null pointer exception is coming from. A little help?

Answer 1

As it turns out, your problem was because you had forgotten to recompile your code! After downloading the code from your dropbox, I decompiled CDAlbum to reveal an outdated constructor:

public CDAlbum() {
  i = 0;
  title = "Unknown CD";
  genre = "Unknown genre";
  artist = "Unknown artist";
  tracks.add("-- no tracks --");
}

After recompiling (after following my suggestions), the code appeared to work fine.

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As I stated in my comments, your code here is probably modified and not the actual code producing the exception; you’re more than likely trying to add to the tracks field, however you never initialized it — you only initialized the local variable tracks.

Try doing something like the following…

tracks = new ArrayList<>(Collections.singleton("-- no tracks --"));

You should post an SSCCE or at least the unmodified code to receive accurate help. It’s going to be tough to solve the issue if you treat it as a black box.

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Note you can move all of your initialization to the field declarations, e.g.

private String title = "Unknown CD";