Calculate Factorial in Java: Solution, Explanation & Practice

Calculate factorial of a number

Problem summary

Write a program that calculates n! (factorial). Your code should work for any non-negative integer.

Starter code

public class Main {
    public static void main(String[] args) {
        int n = 5; // Test case 1
        
        // Calculate n! = n * (n-1) * ... * 1
        // Print: Factorial: <result>
    }
}

Expected output and test cases

5! = 120
```
Factorial: 120
```
4! = 24
```
Factorial: 24
```
0! = 1
```
Factorial: 1
```

Hints

Start from 1 and multiply up to n
Or start from n and multiply down to 1
0! is defined as 1

Validated solution

Reveal Java solution

public class Main {
    static long factorial(int n) {
        if (n < 0) throw new IllegalArgumentException("n must be non-negative");
        long result = 1;
        for (int value = 2; value <= n; value++) result *= value;
        return result;
    }

    public static void main(String[] args) {
        System.out.println("Factorial: " + factorial(5));
    }
}

How to approach the problem

Factorial is a product, so start at the multiplicative identity 1 and multiply each integer from 2 through n. The guard prevents a loop from quietly assigning a meaning to negative factorials.

Approach

Reject negative input.
Initialize result to 1 so 0! stays 1.
Multiply each remaining factor exactly once.

Time and space complexity

Time: O(n). Space: O(1).

Edge cases to test

0! is 1.
long overflows after 20!, so large exact values need BigInteger.

Common mistakes

Starting at zero, which makes every result zero.
Using int and assuming a positive result proves correctness.

Follow-up challenge

Implement factorial with BigInteger and compare iterative and recursive stack usage.

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