Please Help!!!

[Stussy_3]

I have just started studying java and am working on our first class assignment. It's just a simple program that takes two integers and tell which one is larger or if they're equal. What I want to know is what I need to put in the program to tell if they just pressed enter without entering any numbers. Because if they did do this the program just crashes. I think it's some sort of null thing, but i can't figure it out. Here's the code so far:
import javax.swing.JOptionPane;

public class Java12 {
public static void main( String args[] )
{
// Create the variables

String firstNumber,
secondNumber;
int number1 = 0,
number2 = 0,
largest = 0;

firstNumber =
JOptionPane.showInputDialog(" Enter the first integer");

secondNumber =
JOptionPane.showInputDialog(" Enter the second integer");


number1 = Integer.parseInt( firstNumber );
number2 = Integer.parseInt( secondNumber );


// check to see which number is larger, smaller or equal

if (number1 > number2)
JOptionPane.showMessageDialog( null," " + number1 + " is larger " , "Results", JOptionPane.PLAIN_MESSAGE );
else

if (number2 > number1)
JOptionPane.showMessageDialog( null," " + number2 + " is larger " , "Results", JOptionPane.PLAIN_MESSAGE );
else

if (number2 == number1)
JOptionPane.showMessageDialog( null, "These numbers are equal ", "Results", JOptionPane.PLAIN_MESSAGE );



System.exit( 0 );

}
}