Why does my Biginteger.multiply() shows NullPointerException?

Question

I tried to initialize factorials from 1 to 1000 to an biginteger array and calculating the sum of the digits. Why this code showing java.lang.NullPointerException? I think everything was initialized correctly.

class Main {

public static void main(String[] args) {

    BigInteger[] b = new BigInteger[1010];
    int[] ara = new int[1010];
    BigInteger c;

    b[0] = BigInteger.ONE;
    b[1] = BigInteger.ONE;

    ara[0] = ara[1] = 1;
    String s;
    int l, sum;

    for (int i = 2; i <= 1001; i++) {
        c = b[i - 1];
        b[i] = b[i].multiply(c);
        s = b[i].toString();
        l = s.length();
        sum = 0;

        for (int j = 0; j < l; j++) {
            sum += Character.getNumericValue(s.charAt(j));
        }
        ara[i] = sum;
    }

Answer 1

Problem :

    b[i] = b[i].multiply(c);

And look at your b array which you initialised

 b[0] = BigInteger.ONE;
 b[1] = BigInteger.ONE;

And now look at the for loop

 for (int i = 2; i <= 1001; i++) {
        c = b[i - 1];
        b[i] = b[i].multiply(c);

You have only 0,1 indexes. It will throw NPE for index 2.

You are trying loop on 1001 elements and there are only 2 elements inside your array. Fill the b array with zeros first.

Solution :

Change your for loop as below and keep everything same. It works.

for (int i = 2; i <= 1001; i++) {
         b[i] = BigInteger.ONE;
        c = b[i - 1];
        b[i] = b[i].multiply(c);
        s = b[i].toString();

Answer 2

The algorithm for factorial is to take some value n and multiply that n by n - 1 until the value 1 is arrived at. Your algorithm doesn't appear to do that (it generates ones). I think you wanted something like

int len = 1010;
BigInteger[] b = new BigInteger[len];
int[] ara = new int[len];
for (int i = 0; i < len; i++) {
    // calculate factorial.
    b[i] = BigInteger.valueOf(i + 1);
    for (int j = i; j > 1; j--) {
        b[i] = b[i].multiply(BigInteger.valueOf(j));
    }
    // now sum digits.
    for (char ch : b[i].toString().toCharArray()) {
        ara[i] += Character.getNumericValue(ch);
    }
}