Java: JButton and ArrayList: adding to JPanel from ArrayList

Question

So I'm getting a NullPointerException at Container.add so I'm obviously missing something I just don't see it. ArrayList is of Cell which extends JButton. The method init() is in an extended JPanel class. This is my frist question so be harsh on question format if I'm doing it wrong, Thank you.

    public void init() {
    int k = cells.length / 2;
    for (int i = 0; i < (cells.length / 2); i++) {
        int ID = rand.nextInt(25);
        cells[i] = new Cell(this, ID);
        cells[i + k] = new Cell(this, ID);
        k--;
        cellList.add(cells[i]);
        cellList.add(cells[i + k]);
    }

    Collections.shuffle(cellList);

    for (Cell cell : cellList) {
        add(cell);
    }
}

Also, something I noticed is that people don't seem to use debuggers anymore nowadays do they? I mean this was an obvious error, which easily can be overseen after long hours in front of the screen but debugging the code would have revealed the mistake in an instant. So please learn about debuggers and how to use them. — stryba
– stryba, Commented Feb 23, 2012 at 19:59
You're right. I'm new to programming and overlook the debugger frequently. I'm getting more familiar with NetBeans and it's debugger now. — martin8768
– martin8768, Commented Feb 28, 2012 at 3:55

stryba · Accepted Answer · 2012-02-23 19:51:26Z

3

Plain and simple don't k-- before cellList.add(cells[i+k])

Even though I don't see what your are doing with the array anyways.

answered Feb 23, 2012 at 19:51

stryba

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kajacx · Accepted Answer · 2012-02-23 19:52:33Z

1

What if cells.length is odd (not divisionable by 2, like 3, 5, 7, 15)

Consider that cells.length=5, then k=2 and cells[4] will remain null.

answered Feb 23, 2012 at 19:52

kajacx

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1 Comment

stryba Over a year ago

valid point, but shouldn't matter since he wouldn't add cells[4] to cellList

Eric Hydrick · Accepted Answer · 2012-02-24 04:20:22Z

0

You're deprecating k before you add cells[i+k] to the cellList. As a result, cells[i+k] hasn't been initialized yet. Try changing it to:

    cellList.add(cells[i]);
    cellList.add(cells[i + k]);
    k--;

edited Feb 24, 2012 at 4:20

answered Feb 23, 2012 at 20:02

Eric Hydrick

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1 Comment

Eric Hydrick Over a year ago

Whoops, was trying to bang out a quick answer before I left work and forgot to modify the original code after I copied and pasted. Sorry about that, answer edited.

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