Java Program to Find Factorial of a Number

You can find the factorial of a number in Java by using a loop to multiply all integers from 1 to that number, like for (int i = 1; i <= n; i++) result *= i;.

Examples

Input0

OutputFactorial of 0 is 1

Input5

OutputFactorial of 5 is 120

Input10

OutputFactorial of 10 is 3628800

How to Think About It

To find the factorial of a number, think of multiplying all whole numbers from 1 up to that number. For example, factorial of 5 means 1 × 2 × 3 × 4 × 5. We start with 1 and keep multiplying by each number until we reach the input number.

Algorithm

1

Get input number n

2

Set result to 1

3

For each number i from 1 to n, multiply result by i

4

After the loop ends, return or print the result

Code

java

import java.util.Scanner;

public class Factorial {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int n = scanner.nextInt();
        long result = 1;
        for (int i = 1; i <= n; i++) {
            result *= i;
        }
        System.out.println("Factorial of " + n + " is " + result);
        scanner.close();
    }
}

Output

Enter a number: 5
Factorial of 5 is 120

Dry Run

Let's trace the input 5 through the code

1

Input number

User enters n = 5

2

Initialize result

result = 1

3

Loop iteration 1

i = 1, result = 1 * 1 = 1

4

Loop iteration 2

i = 2, result = 1 * 2 = 2

5

Loop iteration 3

i = 3, result = 2 * 3 = 6

6

Loop iteration 4

i = 4, result = 6 * 4 = 24

7

Loop iteration 5

i = 5, result = 24 * 5 = 120

8

End loop and print

Print 'Factorial of 5 is 120'

i	result
1	1
2	2
3	6
4	24
5	120

Why This Works

Step 1: Start with 1

We use result = 1 because multiplying by 1 does not change the value and it is the identity for multiplication.

Step 2: Multiply in loop

The for loop multiplies result by each number from 1 to n, building the factorial step by step.

Step 3: Print final result

After the loop, result holds the factorial, which we print to show the answer.

Alternative Approaches

Recursion

java

import java.util.Scanner;

public class Factorial {
    public static long factorial(int n) {
        if (n <= 1) return 1;
        else return n * factorial(n - 1);
    }

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

Uses function calling itself; elegant but can cause stack overflow for very large n.

Using Java Streams

java

import java.util.Scanner;
import java.util.stream.LongStream;

public class Factorial {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int n = scanner.nextInt();
        long result = LongStream.rangeClosed(1, n).reduce(1, (a, b) -> a * b);
        System.out.println("Factorial of " + n + " is " + result);
        scanner.close();
    }
}

Uses Java 8 streams for a functional style; concise but less intuitive for beginners.

Complexity: O(n) time, O(1) space

Time Complexity

The loop runs from 1 to n once, so the time grows linearly with n.

Space Complexity

Only a few variables are used, so space is constant regardless of n.

Which Approach is Fastest?

The iterative loop is fastest and safest; recursion adds overhead and risk of stack overflow; streams are elegant but slightly slower.

Approach	Time	Space	Best For
Iterative loop	O(n)	O(1)	Most efficient and simple
Recursion	O(n)	O(n)	Elegant but risks stack overflow
Java Streams	O(n)	O(1)	Functional style, less beginner-friendly