Why, when trying to populate an array, am I getting a NullPointerException?

Question

I'm trying to Populate an array of BitSets with information about an int array

    //ar is a SIZE*SIZE 1-D int array, SIZE is a constant

    //decare BitSet array
    BitSet bs[] = new BitSet[SIZE];

    //initialize BitSet array
    for (BitSet x:bs)
        x = new BitSet();

    //populate BitSet array
    for (int i = 0 ; i < ar.length ; i++)
        if (ar[i] > 0)
            bs[(i/SIZE)].set(SIZE-(i%SIZE));

The last line is what seems to be the problem but I can't see anything wrong with the logic. Any ideas?

To make this a bit more clear, if I had an array like 0,3,4,5,0,1,0,0,1,2,7,0,2,3,0,3,2,0,0,0,2,1,5,8,0,0,0,0,0,1,0,0,0,6,0,0

I want to get a bitset array that could be represented as 011101,001110,110110,001111,000001,000100

Your initialization technique is wrong. Check [this question][1] for details. [1]: http://stackoverflow.com/questions/2556419/initializing-an-array-in-java-using-the-advanced-for-each-loop — gkuzmin, Nov 16 '12 at 09:21

score 7 · Accepted Answer · answered Nov 16 '12 at 09:24

You're problem is a miss understanding of the enhanced for loop. Basically:

for (BitSet x:bs)
    x = new BitSet();

Is just a piece of syntatic sugar. The compiler expands it to the following:

for (int i = 0; i < bs.length; i++) {
    BitSet x = bs[i];
    x = new BitSet();
}

In this expansion it's quite clear that you're simply assigning the new bit set to x and throwing it away. You're not actually modifying the array at all. You need to initialize the array as follows:

for (int i = 0; i < bs.length; i++) {
    bs[i] = new BitSet();
}

Sweet tooth comes back to bite me. This is quite the facepalm moment. Thank you. — 1279343, Nov 16 '12 at 09:28

score 3 · Answer 2 · answered Nov 16 '12 at 09:24

In java your 'foreach' construct works like this:

for (BitSet x:bs)
  x = new BitSet();

// is the same as:

for (int i = 0; i < bs.length; i++) {
  BitSet x = bs[i];
  x = new BitSet();
}

Your problem is you don't assign back to array, and there is no way to do it with 'foreach' construct. You have to use plain 'for' loop:

for (int i = 0; i < bs.length; i++) {
  bs[i] = new BitSet();
}

score 1 · Answer 3 · answered Nov 16 '12 at 09:24

1

You are assigning the newly created object to a temporary variable x

Try something like below..

        int i=0;
        for (BitSet x:bs)
            {
                x = new BitSet();
                bs[i++] = x; //This will initialize the array elements
            }

answered Nov 16 '12 at 09:24

Shashank Kadne

7,993
6
41
54

shouldnt it be bs[i] rather than bs[i++]?? – PermGenError Nov 16 '12 at 09:38
2

@GanGnaMStYleOverFlowErroR he has to increment i somehow, since he's not incrementing it in the for, it either needs to be i++ in the array assignement or as another line in the block. Since i++ is post increment this code does work. – EdC Nov 16 '12 at 09:42

Why, when trying to populate an array, am I getting a NullPointerException?

3 Answers3