how to show the negative number factorial in this code

Question

im trying to make a calculator and im was unable to continue because of some confusion in my codes. i was trying to make a factorial of a number, if its a positive number there is no error but every time i input a negative number it results to 1, here is my code .

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

public class Factorial2 {

   public static void main(String[] args) {
       Scanner s = new Scanner(System.in);
       System.out.print("Enter a number: ");
       int n = s.nextInt();
       String fact = factorial(n);
       System.out.println("Factorial is " + fact);
   }

   public static String factorial(int n) {
       BigInteger fact = new BigInteger("1");
       for (int i = 1; i <= n; i++) {
           fact = fact.multiply(new BigInteger(i + ""));
       }
       return fact.toString();
   }
}

i already tried making if statements but still it results to 1.i also want to make the negative factorial into a display text not the value of the negative factorial

Answer 1

You need to validate the input before the calculation, example:

public static String factorial(int n) {
    if(n < 1) return "0";
    BigInteger fact = new BigInteger("1");
    for (int i = 1; i <= n; i++) {
        fact = fact.multiply(new BigInteger(i + ""));
    }
    return fact.toString();
}

Of course you can define any default return value or throw an error:

if(n < 1) throw new RuntimeException("Input must be > 0");