Java recursion and bounded factorial

Use a base case and validate the representable range.

Java

class Main {
    static long factorial(int n) {
        if (n < 0 || n > 20) throw new IllegalArgumentException("use 0 through 20");
        if (n == 0) return 1;
        return n * factorial(n - 1);
    }
    public static void main(String[] args) {
        System.out.println(factorial(4));
        if (factorial(0) != 1) throw new AssertionError("base");
    }
}

Output

Explanation

Each recursive call gets its own parameter and waits for the smaller call's result. factorial(0) returns one and stops descent.

This version accepts 0 through 20 because 20! fits in long while 21! does not. Rejecting values outside the contract avoids silently overflowing the result.

Explore the dry run

Guided walkthroughReady

1class Main {

2 static long factorial(int n) {

3 if (n < 0 || n > 20) throw new IllegalArgumentException("use 0 through 20");

4 if (n == 0) return 1;

5 return n * factorial(n - 1);

6 }

7 public static void main(String[] args) {

8 System.out.println(factorial(4));

9 if (factorial(0) != 1) throw new AssertionError("base");

10 }

11}

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Start at 4.	`calls: 4`	—
Descend through 3, 2, 1, and 0.	`calls: 4 -> 3 -> 2 -> 1 -> 0`	—
Return the base result.	`factorial(0) = 1`	—
Unwind with results 1, 2, 6, 24.	`factorial(4) = 24`	—
Print the final result.	`factorial(4) = 24`	24

Cost and common mistakes

O(n) time and O(n) stack frames for accepted n.

A long still has a fixed maximum value.
Recursive algorithms may exhaust stack space even when their numeric result fits; this bounded example has at most 21 calls.

Try it yourself

What happens for factorial(21)?

Reveal answer and reasoning

It throws IllegalArgumentException before attempting the multiplication.

Java recursion and bounded factorial

Explanation

Follow the example

Cost and common mistakes

Try it yourself

Further reading