The factorial function using the iterative technique

factorial_iterative.java

    private static long factorialForLoop(long n) {
        long result = 1;
        
        for (int i = 1; i <= n; i++) {
            result *= i;
        }
        
        return result;
    }

    private static long factorialWhileLoop(long n) {
        if (n <= 1) 
            return 1;

        long result = n;
        while (n-- > 1) {
            result *= n;
        }

        return result;
    }

    private static long factorialReduceOptional(long n) {
        if (n <= 1)
            return 1;

        List<Integer> factorsList = IntStream.range(1, (int) n+1)
            .boxed()
            .collect(Collectors.toList());

        return factorsList.stream()
            .reduce((a, b) -> a * b)
            .orElseThrow(() -> new RuntimeException("Failed to perform reduce operation"));
    }