NullPointerException thrown in JOptionPane

[John Jaspers]

I don't understand why this happens in this code.
The only difference I see is that the first two numbers are doubles and the third is an int, but the exception is caught for the first two and not the third. The third throws the NumberFormatException.
But if I hit Cancel BEFORE parseInt() is called, why am I getting a NumberFormatException?
I couldn't find anything in the docs that might explain this.
Thanks in advance.
import javax.swing.*;
import java.text.*;
public class FutureValue
{
public static void main(String[] args)
{
String choice = "";
try
{
while(!(choice.equalsIgnoreCase("x")))
{
//Works as expected here.
String paymentString = JOptionPane.showInputDialog(null,
"Enter monthly payment: ", "Future Value", JOptionPane.PLAIN_MESSAGE);
double monthlyPayment = Double.parseDouble(paymentString);

//Works as expected here.
String rateString = JOptionPane.showInputDialog(null,
"Enter yearly interest rate: ", "Future Value", JOptionPane.PLAIN_MESSAGE);
double interestRate = Double.parseDouble(rateString);
double monthlyInterestRate = interestRate/12/100;

//NumberFormatException thrown here.
String yearsString = JOptionPane.showInputDialog(null,
"Enter number of years: ", "Future Value", JOptionPane.PLAIN_MESSAGE);
int years = Integer.parseInt(yearsString);
int months = years * 12;
double futureValue = calculateFutureValue(monthlyPayment,
months, monthlyInterestRate);
NumberFormat currency = NumberFormat.getCurrencyInstance();
NumberFormat percent = NumberFormat.getPercentInstance();
percent.setMinimumFractionDigits(2);
String message =
"Monthly payment: " + currency.format(monthlyPayment) + "\n"
+ "Yearly interest rate: " + percent.format(interestRate/100) + "\n"
+ "Number of years: " + years + "\n"
+ "Future value: " + currency.format(futureValue) + "\n\n"
+ "To continue, press Enter.\n"
+ "To exit, enter 'x': ";
choice = JOptionPane.showInputDialog(null,
message, "Future Value", JOptionPane.PLAIN_MESSAGE);
}//end while
}//end try
catch(NullPointerException e)
{
System.exit(0);
}//end catch

System.exit(0);
}//end main
private static double calculateFutureValue(double monthlyPayment,
int months, double interestRate)
{
int i = 1;
double futureValue = 0;
while (i <= months)
{
futureValue = (futureValue + monthlyPayment) *
(1 + interestRate);
i++;
}//end while

return futureValue;

}//end calculateFutureValue
}//end class

[John Jaspers]

I'm sorry. I should have made my question clearer.
The following code is supposed to catch a NullPointerException in any of the JOptionPanes if one presses cancel. This works just fine in the first two, but the third seems to always try to assign to the int. The NullPointerException should occur before the assignment, but it seems to always occur at the assignment and throw a NumberFormatException.
Could anyone explain to me why this happens here and not in the two previous boxes?
Thanks again.
[ March 28, 2003: Message edited by: John Jaspers ]

[Barry Gaunt]

It has nothing to do with JOptionPane as you can see from this:

class A1
{
    public static void main( String[] args )
    {
        try
        {
            int x = Integer.parseInt(null);  // gives NumberFormatException
        }
        catch ( NumberFormatException e )
        {
            System.out.println( "Got NFE" );
            e.printStackTrace();
        }
        
        try
        {
            double y = Double.parseDouble(null); // gives NullPointerException
        }
        catch ( NullPointerException e )
        {
            System.out.println( "Got NPE" );
            e.printStackTrace();
        }
    }
}

Any Java source code geeks around?
[ March 28, 2003: Message edited by: Barry Gaunt ]