Null Pointer Exception when trying to set a value

Question

So pretty much I'm making an array of obvjects called Spots which symbolise the different faces of a dice.

It takes user input (manually set to three for this example), and then creates that make Spots and rolls a random number from 1 to 6.

However when I go to use the rollAgain() method on the aleady created array of Spots I get a null pointer even though I am using the same variable length in both for loops (the one that creates and one that rolls the spots).

My code

Global Variables

private Spots[] spots;

private int x = 3;

Contructor

  public Director(JFrame window, String args[]) {
    JMenuBar menus = new JMenuBar();
    window.setJMenuBar(menus);
    menus.add(makeFileMenu());

    window.getContentPane().add(makePanel(), BorderLayout.WEST);
    window.getContentPane().add(makeSpots(x), BorderLayout.CENTER);

    rollAgain();
}

rollAgain() method

public void rollAgain() {
    int v = 1 + (int) (Math.random() * 6);
    for (int i = 0; i < x; i++) {
        spots[i].setValue(v);
    }
}

makeSpots() method

private JComponent makeSpots(int x) {
    JPanel p = new JPanel();
    p.setBorder(BorderFactory.createTitledBorder("Dice"));

    Spots[] spots = new Spots[x];

    for (int i = 0; i < x; i++) {

        spots[i] = new Spots(200, 200);
        spots[i].setBorder(BorderFactory.createEtchedBorder());
        p.add(spots[i]);
    }
    return p;
}

possible duplicate of [What is a Null Pointer Exception, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-how-do-i-fix-it) — Jeroen Vannevel, Jun 04 '14 at 16:25
Is it too much to ask if I can see the exception stack trace? — Matias Cicero, Jun 04 '14 at 16:25
@MatiCicero yep, the error should be obvious from the code given — Ordous, Jun 04 '14 at 16:26
@Ordous What if the exception is thrown inside setValue(v), because he is doing some inner logic with v?. I know this isn't the case, but we can't know for certain — Matias Cicero, Jun 04 '14 at 16:27
@MatiCicero fair enough, with slightly more code and more complex logic it would be a necessity — Ordous, Jun 04 '14 at 16:30

score 4 · Accepted Answer · answered Jun 04 '14 at 16:25

4

You are setting a local variable

Spots[] spots = new Spots[x];

This doesn't change the field (which happens to have the same name)

private Spots[] spots;

The simplest solution is to not have a local variable

this.spots = new Spots[x];

answered Jun 04 '14 at 16:25

Peter Lawrey

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score 0 · Answer 2 · answered Jun 04 '14 at 16:25

0

You need to instantiate a new Spots array in your constructor.

this.spots = new Spots[x];

answered Jun 04 '14 at 16:25

Josef E.

2,179
4
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score 0 · Answer 3 · answered Jun 04 '14 at 16:26

spots[i].setValue(v);

Judging from this line, my guess is that the Spot object in the array in null.

The error is in your makeSpots() method. You don't update the value of the field x, but use a local variable. Add this.x = x at the beginning of the method.

score 0 · Answer 4 · answered Jun 04 '14 at 16:26

0

In your makeSpots() method, you are creating a new Spots object called spots:

Spots[] spots = new Spots[x];

This effectively hides your private member variable spots from the method. Instead, do this in your makeSpots() method:

spots = new Spots[x];

answered Jun 04 '14 at 16:26

Edwin Torres

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score 0 · Answer 5 · answered Jun 04 '14 at 16:29

0

You are declaring a global 'spots' array and then in the makeSpots() you create the spots with a local variable also named 'spots'. Just substitute

Spots[] spots = new Spots[x];

by

spots = new Spots[x];

so the global variable gets a value.

answered Jun 04 '14 at 16:29

yombo

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Null Pointer Exception when trying to set a value

5 Answers5