/**
 * This program computes and prints the factorial of 
   a number pass as an argument on the command line.
   Usage: java Factorial <n>
 */

public class Factorial {                   	// Define a class

 public static void main(String[] args) { 	// The program starts here
   if (!(args.length==1)) {                     // Right number of args?
    System.out.println("Usage: java Factorial <integer>");
     }
   else {
    int input = Integer.parseInt(args[0]); 	// Get the user's input
    System.out.println(factorial(input)); 	// Call facorial method, print result
    }
 }                                      	// The main() method ends here

 public static double factorial(int x) {	// This method computes x!
   if (x<0) return 0.0;                 	// if input's bad, return 0  
   double fact = 1.0;                     	// Begin with an initial value
   while(x>1) {                         	// Loop until x equals 1
     fact = fact*x;                      	//   multiply by x each time
     x = x-1;                         	        //   and then decrement x
     }                                  	// Jump back to the star of loop
   return fact;                           	// Return the result
   }                                      	// factorial() ends here
}                                          	// The class ends here