Java Program for factorial of a number

Factorial of a non-negative integer, is multiplication of all integers smaller than or equal to n. In this article, we will learn how to write a program for the factorial of a number in Java.

Formulae for Factorial

n! = n * (n-1) * (n-2) * (n-3) * ........ * 1

Example of Factorial in Java

6! == 6*5*4*3*2*1 = 720. 
5! == 5*4*3*2*1 = 120
4! == 4*3*2*1 = 24

Methods to Find Factorial of a Number

1. Iterative Solution for Factorial in Java

Below is the implementation of the above method:

Java

// Java program to find factorial
// of given number

// Driver Class
class Test {
    // Method to find factorial
    // of given number
    static int factorial(int n)
    {
        int res = 1, i;
        for (i = 2; i <= n; i++)
            res *= i;
        return res;
    }

    // main method
    public static void main(String[] args)
    {
        int num = 5;
        System.out.println("Factorial of " + num + " is "
                           + factorial(5));
    }
}

Output

Factorial of 5 is 120

The complexity of the above method:

Time Complexity: O(n)
Auxiliary Space: O(1)

2. Factorial in Java Using Recursive Method

Below is the implementation of the above method:

Java

// Java program to find factorial
// of given number

// Driver Class
class Test {
    // method to find factorial
    // of given number
    static int factorial(int n)
    {
        if (n == 0)
            return 1;

        return n * factorial(n - 1);
    }

    // main method
    public static void main(String[] args)
    {
        int num = 5;
        System.out.println("Factorial of " + num + " is "
                           + factorial(5));
    }
}

Output

Factorial of 5 is 120

The complexity of the above method:

Time Complexity: O(n)
Auxiliary Space: O(n)

3. One-line Solution (Using the Ternary operator)

Below is the implementation of the above method:

Java

// Java program to find factorial
// of given number

// Driver Class
class Factorial {
    // function to find factorial
    int factorial(int n)
    {

        // single line to find factorial
        return (n == 1 || n == 0) ? 1
                                  : n * factorial(n - 1);
    }

    // main function
    public static void main(String args[])
    {
        Factorial obj = new Factorial();
        int num = 5;
        System.out.println("Factorial of " + num + " is "
                           + obj.factorial(num));
    }
}

Output

Factorial of 5 is 120

The complexity of the above method:

Time Complexity: O(n)
Auxiliary Space: O(n)

The above solutions cause overflow for small numbers. Please refer factorial of large numbers for a solution that works for large numbers. Please refer complete article on the Program for factorial of a number for more details!