Why is my array of indices from 0-16129 is not null but the rest are?

Question

I have an array of indices with a length of 97536, when it gets to 16,129 it gives me a null pointer exception. Index[] indices = new Index[6 * (VERTEX_COUNT - 1) * (VERTEX_COUNT * 1)];. I then have a double for loop and a pointer to loop through all of the objects in the array and initial them.

int pointer = 0;

    for(int gz = 0; gz < VERTEX_COUNT - 1; gz++)
    {
        for(int gx = 0; gx < VERTEX_COUNT - 1; gx++)
        {
            indices[pointer] = new Index((gz * VERTEX_COUNT) + gx);
            indices[pointer] = new Index(((gz + 1) * VERTEX_COUNT) + gx);
            indices[pointer] = new Index(((gz * VERTEX_COUNT) + gx) + 1);
            indices[pointer] = new Index(((gz * VERTEX_COUNT) + gx) + 1);
            indices[pointer] = new Index(((gz + 1) * VERTEX_COUNT) + gx);
            indices[pointer] = new Index((((gz + 1) * VERTEX_COUNT) + gx) + 1);
            pointer++;
        }
    }

This error did not come up when the array was a int[] indices. I am very confused and wondering why the null pointer exception happens at 16129. The first thought was that the for loops never reach the correct number, if so how would I calculate that? - Thanks in advance

You also seem to be overwriting the same position in your array 6 times - you may want to relook at how arrays work. — Evan Knowles, Jun 12 '15 at 16:01
why do you keep replacing all the objects you're instantiating? that'd just cause them to get GC'd since they'll have gone out-of-scope. and since you're running nested loops, you're going to be ending up with `pointer = VERTEX_COUNT * VERTEX_COUNT` iterations... — Marc B, Jun 12 '15 at 16:01

score 0 · Answer 1 · answered Jun 12 '15 at 19:50

You say your array is 97536 items. So I deduce VERTEX_COUNT=128. I arrive at this by noticing that 6*127*128 = 97536.

You are running through a nested for loop, where each loop runs from 0 to VERTEX_COUNT-1, or from 0 to 127. 127*127=16129, and as a commenter above pointed out you overwrite the same position in your array 6 times in each inner loop, so you are only instantiating from 0 to 16128. Thus when you try to access position 16129 (and above), you will retrieve null, which naturally throws your NPE if you dereference it without checking.

I'm assuming you don't want to write to the same index six times, but rather six consecutive indices, so my suggestion for fixing this would be

    { // inner loop body
        indices[pointer++] = new Index((gz * VERTEX_COUNT) + gx);
        indices[pointer++] = new Index(((gz + 1) * VERTEX_COUNT) + gx);
        indices[pointer++] = new Index(((gz * VERTEX_COUNT) + gx) + 1);
        indices[pointer++] = new Index(((gz * VERTEX_COUNT) + gx) + 1);
        indices[pointer++] = new Index(((gz + 1) * VERTEX_COUNT) + gx);
        indices[pointer++] = new Index((((gz + 1) * VERTEX_COUNT) + gx) + 1);
        // DO NOT DO pointer++ HERE
    }

I bet you had the same problem when it was an int[] but you never noticed it before because the default value of each item in an int[] is 0 (not null) which will not give you NPEs...

I got all the values to Initialize but it gave me the same error — danyguag, Jun 27 '15 at 19:48
Pretty much by definition, if you are getting a `NullPointerException`, you did not get all the values to initialize. But let's keep the details out of comments--you can update this question with further details as needed, or ask a new question. — dcsohl, Jun 29 '15 at 15:12

Why is my array of indices from 0-16129 is not null but the rest are?

1 Answers1