Recursion is when a function calls itself repeatedly. The classical example of recursion the factorial function.

5! = 5 x 4 x 3 x 2 x 1
n! = n * (n-1) * (n-2) * (n-3) * (n-4) * (n-5)
n! = n * (n-1)!
5! = 5 * 4!
5! = 5 * 4 * 3!
...

To create a recursive definition of this function, we need to define a recursive case and a base case.

recursive case: n! = n * (n - 1)! => fact(n) = n x fact(n -1) when n >= 1
base case: if n = 0, fact(n) = 1

The program

public class Factorial {
	public static void main(String[] args) {
		int n = 12;
		System.out.println(n + "! = " + fact(n));
	}
	public static int fact(int n) {
		if (n == 0) { 
			return 1;
		} else {
			return n * fact(n-1);
		}
	}
}

output
12! = 479001600