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This simple example written in Java, shows how to calculate the factorial of an integer.
To calculate the factorial of a number n, start the program with java-cp. Factorial n; for example:

java-cp. Factorial 9

calculates the factorial of 9

public class Factorial {
	public static void main(String [] args) {
		int n = 0;
		if(args.length>0) {	
			try {
				n = Integer.parseInt(args[0]);
			}
			catch(NumberFormatException nfe) {
				System.out.println("Invalid input value!");
				System.exit(1);
			}
		}
		if (n < 0) {
		System.out.println("Invalid input value!");
		} 
		else {
		long fact = 1;
		for (int i = 2; i <= Math.abs(n); i++){
		fact = fact * i;
		}
		System.out.println("Factorial of " + Math.abs(n) + " = " + fact);
		}
	}
}

 
For example:

java -cp . Factorial
Factorial of 0 = 1
 
java -cp . Factorial 13
Factorial of 13 = 6227020800
 
java - cp . Factorial four
Invalid input value!