Why adding objects to a static Collection causing a NullPointerException?

Question

I am attempting to add 2 sub-class objects to a static Collection of super-class objects. However, when I try to add the objects to the Collection, I'm receiving a null pointer exception and I'm trying to figure out why.

public class MoverLogic{
    static Collection<Super> superCollection;

    public static void main(String[] args) {
    SubAlpha sub1 = new SubAlpha();
    SubBeta sub2 = new SubBeta();
    superCollection.add(sub1); //I'm getting the null pointer exception here
    superCollection.add(sub2);
}

How is it that questions like this come up every day? Either teachers are so bad, or it is for some reasons hard to understand that references are initialized with `null`. Anyway, it shows that it is counter-intuitive to have stated "superCollection is a Collection" and then to find that it is not. Remedy would be if java would demand explicit initialization of all variables. — Ingo, Oct 21 '13 at 17:03
So, @TomFang, do you think it would have helped you if the compiler would have said something like: "error on line 2: please iniialize variable superCollection!" — Ingo, Oct 21 '13 at 17:15
@TomFang So pick one of the answers as answering your question and move on. Now you know. A reference has nothing in it unless you put something in it. — Radiodef, Oct 21 '13 at 17:18

score 3 · Answer 1 · answered Oct 21 '13 at 17:00

Members of a class are initialized to default values when a class is constructed (or loaded statically as in your case)--like ints to 0, booleans to false, etc. Similarly, objects are initialized to null.

So in your case, superCollection is initialized to null, and when you use it, you get a NullPointerException. It is common to think collections are initialized to empty collection objects, but that isn't the case. Besides, which Collection do you mean? ArrayList? LinkedList?

So something like

static Collection<Super> superCollection = new ArrayList<>(); //Java 7

would work. Or with any other Collection you prefer.

score 1 · Answer 2 · edited Oct 21 '13 at 16:51

1

You will have to initialize superCollection before using it.

edited Oct 21 '13 at 16:51

Luiggi Mendoza

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answered Oct 21 '13 at 16:49

JoeC

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Darkhogg · Answer 3 · 2013-10-21T16:55:35.133

You have not initialized superCollection. Before anything you have in your main, write:

superCollection = new ArrayList<Super>();

If you aren't going to change the actual collection reference, it might be beneficial of creating it directly in the declaration and adding the final modifier:

static final Collection<Super> superCollection = new ArrayList<Super>();

You will still be allowed to add or remove elements, but you can't re-assign it to anything else.

NOTE: You might change ArrayList to a different concrete impelementation of Collection if it doesn't suit your needs.

score 0 · Answer 4 · answered Oct 21 '13 at 16:50

0

You need to create your collection object eg.

 static List<Super> superCollection=new LinkedList<Super>();

answered Oct 21 '13 at 16:50

Antoniossss

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Why adding objects to a static Collection causing a NullPointerException?

4 Answers4