Java Factorial Program with Loop, Recursion and BigInteger

The factorial of n is n x (n-1) x ... x 1, for example 5! = 120. It grows very fast: 21! is already too big for a long.

Loop and recursion

public class Main {
    static long factorialLoop(int n) {
        long result = 1;
        for (int i = 2; i <= n; i++) {
            result *= i;
        }
        return result;
    }

    static long factorialRec(int n) {
        return n <= 1 ? 1 : n * factorialRec(n - 1);
    }

    public static void main(String[] args) {
        System.out.println(factorialLoop(5));
        System.out.println(factorialRec(10));
        System.out.println(factorialLoop(20));     // the biggest one for a long
        System.out.println(factorialLoop(21));     // overflow: wrong value, no error
    }
}

BigInteger for big results

import java.math.BigInteger;

public class Big {
    static BigInteger factorial(int n) {
        BigInteger result = BigInteger.ONE;
        for (int i = 2; i <= n; i++) {
            result = result.multiply(BigInteger.valueOf(i));
        }
        return result;
    }

    public static void main(String[] args) {
        System.out.println(factorial(21));
        System.out.println(factorial(30));
        BigInteger f100 = factorial(100);
        System.out.println(f100.toString().length() + " digits");
        System.out.println(f100.toString().substring(0, 20) + "...");
    }
}

BigInteger has no size limit. It is slower than long, so use it only when you need it.

Check the negative input

public class Neg {
    static long factorial(int n) {
        if (n < 0) {
            throw new IllegalArgumentException("Factorial is not defined for negative numbers: " + n);
        }
        return n <= 1 ? 1 : n * factorial(n - 1);
    }

    public static void main(String[] args) {
        System.out.println(factorial(-3));
    }
}

Related: Java Recursion and Java Exceptions.

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