Android: nullpointerexception attempt to read from null array

Question

I have a suspicion that the makeLine method created a null array somehow, but I'm not quite sure.

Any help would be greatly appreciated! A snippet of my code is below:

public void all() {

        int[][] line = new int[3][];

        for (int i = 0; i < 3; i++) {
            if (i > 0 && line[i - 1][0] == 2) { // Error occurs at this line
                int start = line[i - 1][2];
                int pos = line[i - 1][1];
                int xy = line[i - 1][3]++;

                if (line[i - 1][3] == 1) {
                    int end = y()[1];
                    line[i] = new int[]{1, start, end, pos, xy};
                } else {
                    int end = x()[1];
                    line[i] = new int[]{1, start, end, pos, xy};
                }
            } else {
                line[i] = makeLine();
            }
        }
}

private int[] makeLine() {
                Random r = new Random();

                int startX = x()[0];
                int endX = x()[1];
                int startY = y()[0];
                int endY = y()[1];

                int xy = r.nextInt(3 - 1) + 1;

                if (xy == 1) {
                    return new int[]{1, startX, endX, startY, xy};
                }

                return new int[]{1, startY, endY, startX, xy};
            }

private int[] x() {
        DisplayMetrics displaymetrics = context.getResources().getDisplayMetrics();
        int xLeft = (int) (13 * displaymetrics.density);
        int xRight = (int) (displaymetrics.widthPixels - (13 * displaymetrics.density));

        Random r = new Random();
        int startX = r.nextInt(xRight - xLeft) + xLeft;
        int endX = r.nextInt(xRight - xLeft) + xLeft;

        return new int[]{startX, endX};
    }

private int[] y() {
        DisplayMetrics displaymetrics = context.getResources().getDisplayMetrics();
        int yTop = (int) (60 * displaymetrics.density);
        int yBottom = (int) (displaymetrics.heightPixels - (51 * displaymetrics.density));

        Random r = new Random();
        int startY = r.nextInt(yBottom - yTop) + yTop;
        int endY = r.nextInt(yBottom - yTop) + yTop;

        return new int[]{startY, endY};
    }

I don't think there's enough information here to help you. You need to provide us with a complete example we can run. At the moment this is impossible because some methods (e.g. x() and y()) are missing. — Paul Boddington
– Paul Boddington, Commented Jan 2, 2017 at 0:54

yakobom · Accepted Answer · 2017-01-02 05:23:36Z

1

The exception is because you tried accessing line[0][0] before it was initialized, there is nothing there.

By doing:

int[][] line = new int[3][];

You are only creating 1 dimension for this multidimensional array. The way to do this is either:

int[][] line = new int[3][SOME_NUMBER];

or to explicitly initialize each row:

int[][] line = new int[3][];
for (int i = 0; i < 3; i++) {
  line[i] = new int[SOME_NUMBER];
}

If you need to better understand it, have a look here, for example: [Multi-Dimensional Arrays in Java

]1

answered Jan 2, 2017 at 5:23

yakobom

2,7111 gold badge27 silver badges35 bronze badges

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

Comments

John S. · Accepted Answer · 2017-01-02 01:55:29Z

0

I think you are getting a null pointer because you made a reference to an element outside the bounds of the array. The line where you got the error on should look like this:

if (i > 0 && line[i][0] == 2) {

Instead of:

if (i > 0 && line[i - 1][0] == 2)

There are mistakes similar to this all over this program. Remember that arrays start indexing at 0, not 1.

answered Jan 2, 2017 at 1:55

John S.

6461 gold badge5 silver badges17 bronze badges

1 Comment

yakobom Over a year ago

There is nothing wrong with accessing line[i - 1][0], or accessing line[i][0], if the memory has been allocated.

Collectives™ on Stack Overflow

Android: nullpointerexception attempt to read from null array

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related