**Why does this java code throw a `StackOverflowError` exception?**

```
public class factorial2 {

     public BigInteger fact( BigInteger n)
     {
         BigInteger one = new BigInteger("1");
         if(n.equals("0"))
              return one;
         else
             return n.multiply(fact(n.subtract(one)));      
     }

     public static void main(String[] args) {       
        @SuppressWarnings("resource")
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        factorial2 f = new factorial2();
        for(int i=0;i<n;i++)
        {
           BigInteger b = sc.nextBigInteger();
           System.out.println(f.fact(b));
        }
        sc.close();
    }
}

```

I have attempted to generate a factorial using `BigInteger`. But, why does my code give the reference exception on input?