// File: FactTest.java

import javax.swing.JOptionPane;

/**
 *   A class to compute factorials
 */
class FactFinder
{
   // instance var
   private int number ; 	// the number to compute factorial of
   
  /**
   * Constructs a FactFinder object 
   * @param number the number to compute factorial of
   */
   public FactFinder(int number)
   {
      this.number = number ;
   }

  /**
   * Computes the factorial of the number.
   * @return the factorial
   */
   public int getFact()
   {
      // trivial cases
      if (number < 0) return 0 ;		// fact fn not defined for negatives
      if (number == 0) return 1 ;		// 0! defined as 1

	  // recursive call with reduced case
      FactFinder previous = new FactFinder(number - 1) ;
      int previousFact = previous.getFact() ;
      return previousFact * number ;
   }
}

public class FactTest
{
   public static void main(String[] args)
   {
      String input = JOptionPane.showInputDialog
                                 ("Enter number to find factorial of") ;
      int number = Integer.parseInt(input) ;
      FactFinder f = new FactFinder(number) ;
      int fact = f.getFact() ;
      System.out.print(number + " factorial is " + fact) ;
      
      // print messages (nothing to do with recursion)
      
      if ( number < 0 )      					// input was negative
        System.out.println(" (factorial not defined for negative numbers)") ;
	  else if ( number >= 0 && fact <= 0 )      // checking for overflow
        System.out.println(" (overflow occurred)") ;
      else
        System.out.println() ;
    }
}

/*  sample o/p from several runs:

5 factorial is 120

1 factorial is 1

0 factorial is 1

10 factorial is 3628800

-5 factorial is 0  (factorial not defined for negative numbers)

37 factorial is 0  (overflow occurred)

*/