Adding multiple boolean arrays together

Question

I have this method that adds two 2d boolean arrays together. So for a given cell, if any array is true for that cell, the cell in the resulting array is set to true.

private boolean[][] addBooleanArrays(List<boolean[][]> arrays) {
    boolean[][] result = new boolean[8][8];
    for (int i = 0; i < 8; i++) {
        for (int j = 0; j < 8; j++) {
            for (boolean[][] b : arrays) {
                if (b[i][j] == true) {
                    result[i][j] = true;
                }
            }
        }
    }
    return result;
}

I'm feeding this method 16 boolean arrays that I know contain true and false values but I am getting a null pointer exception at the if statement. I can't see why though, maybe I'm missing something. Any help appreciated.

musical_coder · Accepted Answer · 2013-11-19 01:33:30Z

2

Add a null check to ensure that a particular b isn't null:

if (b != null && b[i][j] == true) {

A for-each loop like you're using will still return each value in arrays, even if the value is null.

answered Nov 19, 2013 at 1:33

musical_coder

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4 Comments

Ogen Over a year ago

Yeah but my arrays only contain true and false I know that for a fact

musical_coder Over a year ago

Most definitely, since boolean is a primitive type and can't be set to null. But I'm talking about making sure that b, which is a 2-d boolean array, isn't null (see the difference?)

Ogen Over a year ago

Ok I added that check and a b is null. But I can see how it can be null. My arrays List only has boolean[][]?

musical_coder Over a year ago

That's because arrays (whatever their dimensions; in this case 2-d) are objects, so they can be set to null.

user207421 · Accepted Answer · 2013-11-19 02:29:25Z

1

A row in the input array is null instead of being a boolean[8].

edited Nov 19, 2013 at 2:29

answered Nov 19, 2013 at 1:36

user207421

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2 Comments

Ogen Over a year ago

I thought booleans initialise to false

ajb Over a year ago

@Clay You have to realize that there are 9 objects involved. Each array is an object, so a boolean[][] is an object that could be null. Since this is a 2-D array, each element of the array is itself an object, another array, so any of those array elements could also be null. (And if you had said new boolean[8][], the array elements would be initialized to null.) Those last eight arrays are the arrays of boolean, though, and I think those are indeed initialized to false.

Ted Yu · Accepted Answer · 2013-11-19 01:39:21Z

0

Maybe some b[i] is null that causes the exception, I suggest you checking the code generating elements of arrays.

answered Nov 19, 2013 at 1:39

Ted Yu

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