Difficulty with BigInteger

Question

I am trying to do Factorial with Recursion and BigIntegers but eclipse is complaining about the BigInteger. I know the program is supposed to be simple but it is giving me headache. Here is the code.

import java.util.Scanner;
import java.math.BigInteger;

public class Factorial
{
    public static void main(String[] args) 
    {
        Scanner input = new Scanner(System.in);
        System.out.println("Enter integer");
        BigInteger n = input.nextBigInteger();
        System.out.println("Factorial of " + n + " is "  + fact(n));

    }

    public static  int fact(BigInteger n)
    {
        if(n ==0)
        {
            return 1;
        }
        else
        {
            return n * fact(n-1);
        }
    }
}

Answer 1

BigInteger does not support comparison using == and multiplication using *. Instead, you have to call the appropriate methods of the BigInteger class (equals() and multipy()).

Also note that there exist BigInteger.ZERO and BigInteger.ONE.

Finally, the return type of your fact method should be BigInteger and not int. Whether you want the argument to be of type BigInteger or int is up to you.

Answer 2

In addition to what @aix mentioned regarding invoking the arithmetics on BigInteger - I can also see another issue with this code.

Your method signature is

public static  int fact(BigInteger n)

This is problemantic - factorial grows fast, so you are very likely to overflow the result.
I think what you realy wanted is:

public static  BigInteger fact(int n)

which makes much more sense, since the return value should probably be the BigInteger (since it grows fast) and not the parameter, or possibly - both of them.