Fibonacci sequence has a negative number

Question

I have this code for fibonacci sequence :

public class Fibonacci {
    private List<Integer> fibonacci;

    public void fillFibonacci(){
        fibonacci = new ArrayList<>();
        int n1 = 1 , n2 = 1 , n3;
        fibonacci.add(n1);
        fibonacci.add(n2);
        for(int i = 2 ; i < 4000 ; i ++){
            n3 = n1 + n2;
            fibonacci.add(n3);
            n1=n2;
            n2=n3;
        }
    }

    public void printFibonacci(){
        for(int i = 0 ; i < fibonacci.size() ; i ++){
            System.out.print(fibonacci.get(i) + " ");
            System.out.println(i);
        }
    }
}

this code show negative number what's wrong with it?

Ints have a limited range. When they get too high, they overflow. — khelwood, Oct 30 '20 at 09:55
See [Often big numbers become negative](https://stackoverflow.com/q/17218964/3890632) — khelwood, Oct 30 '20 at 09:57

score 1 · Accepted Answer · answered Oct 30 '20 at 09:56

You cannot go up to index= 4000 in this loop, you have integer overflowed. Maximum you can go to is 49. Change the loop as such.

for(int i = 2 ; i < 49 ; i ++){
            n3 = n1 + n2;
            fibonacci.add(n3);
            n1=n2;
            n2=n3;
        }

score 1 · Answer 2 · answered Oct 30 '20 at 09:58

The 4000-th Fibonacci number has 836 digits. The java int type has 32-bits and can fit numbers up to 10 digits. After that it overflows and may go negative. If you want big numbers, change your code to use BigInteger.

Fibonacci sequence has a negative number

2 Answers2