// Calculator.java program designed to use Java Swing OptionPane   R.Metcalfe July 2013
import javax.swing.JOptionPane;
import java.io.*;

// Begin class Calculator
public class Calculator {

// Define main method
public static void main( String args[] ) {
 // Declaring string variables 
 String  numberOne, numberTwo, x, choices = "+-*/^!";
 int pos = 0;
 // Declaring variables as type double
 double fnumber1, fnumber2, fresult;

 // Using a do while statement as loop
 do {

  // Read Calculator Action from user as a string
  x = JOptionPane.showInputDialog( "Enter Calculator Action out of +-*/^! or " + 
                  "type anything else to exit : " );
  if (x != null) {
  pos = -1;
  if (x.length() > 0) pos = choices.indexOf(x.substring(0,1));
 
  switch (pos) {
   case 5:
   case 0:
   case 1:
   case 2:
   case 3:
   case 4:
     // Read in first number from user as string
     numberOne = JOptionPane.showInputDialog( "Enter number : ");
     // Convert number from type string to type double
     fnumber1 = Double.parseDouble( numberOne );
     if (x.substring(0,1).equals("!") && fnumber1 >= 1.0) {
      fresult = fnumber2 = 1.0;
      while (fnumber2 < fnumber1) {      
         fnumber2 += 1.0;
         if (fnumber2 > fnumber1) fnumber2 = fnumber1;
         fresult *= fnumber2;
      }
      JOptionPane.showMessageDialog( null, numberOne + "! = " + fresult, 
                    "Calculator Result", JOptionPane.PLAIN_MESSAGE );
     } else {
      // Read in first number from user as string
      numberTwo = JOptionPane.showInputDialog( "Enter second number : " );
      // Convert number from type string to type double
      fnumber2 = Double.parseDouble( numberTwo );
      if (x.substring(0,1).equals("+")) {
       JOptionPane.showMessageDialog( null, numberOne + " + " + numberTwo + " = " + 
         (fnumber1 + fnumber2), "Calculator Result", JOptionPane.PLAIN_MESSAGE );
      } else if (x.substring(0,1).equals("-")) {
       JOptionPane.showMessageDialog( null, numberOne + " - " + numberTwo + " = " + 
         (fnumber1 - fnumber2), "Calculator Result", JOptionPane.PLAIN_MESSAGE );
      } else if (x.substring(0,1).equals("*")) {
       JOptionPane.showMessageDialog( null, numberOne + " * " + numberTwo + " = " + 
         (fnumber1 * fnumber2), "Calculator Result", JOptionPane.PLAIN_MESSAGE );
      } else if (x.substring(0,1).equals("/")) {
       JOptionPane.showMessageDialog( null, numberOne + " / " + numberTwo + " = " + 
         (fnumber1 / fnumber2), "Calculator Result", JOptionPane.PLAIN_MESSAGE );
      } else if (x.substring(0,1).equals("^")) {
       JOptionPane.showMessageDialog( null, numberOne + " ^ " + numberTwo + " = " + 
         Math.pow(fnumber1, fnumber2), "Calculator Result", JOptionPane.PLAIN_MESSAGE );
      }
     }
     break;
    
   default:
     x = "stop";
     break;
  }
 } else {
  x = "stop";
 }

 // ... until forced exit
 } while( ! x.equals( "stop" ));

// Ends program
System.exit(0);
}
}