Código de Java - Calcular el factorial de un numero

import java.math.BigInteger;

public class x

{

    public static void main( String[] args ) {

        System.out.print(Factorial(25));

    }

    public static String Factorial(int n) {

        if(n < 0){

            return null;

        }

        if(n == 0 || n == 1){

            return "1";

        }

        BigInteger product = BigInteger.ONE;

        for(int i = 2; i <= n; i++){

            product = product.multiply(new BigInteger(String.valueOf(i)));

        }

        return product.toString();

    }

}