NullPointerException Error

Question

Hello, I hope you can help me out with a Java excercise I am working on. I am using "Learn to Program with Java by John Smiley.
It involves error checking to see if the user clicks cancel without sending a required string or sending an empty string on a JOptionPane
prompt.
If you look at the code you will see what I mean. On my machine
it compiles fine. But when I run it, instead of returning a string with
message "You clicked on CANCEL button" or you sent an empty screen the program bombs
Specifically when click Cancel I get
NullPointerException error.
When I click OK on an empty string the program bombs black screen without any error messages.
Here is the code:
import javax.swing.JOptionPane;
class Excercise6_1
          static double adjustment=0;
          static double balance =0;
          static double newBalance=0;
     public static void main(String args[])
          String response;
          String moreBankingBusiness;
          moreBankingBusiness=JOptionPane.showInputDialog
          ("Do you want to do some banking?");
          moreBankingBusiness=moreBankingBusiness.toUpperCase();
          while(moreBankingBusiness.equals("YES"))
               response=JOptionPane.showInputDialog
               ("***Select Banking Options***\n\n"+
               "1 for deposit" + "\n"+"2 for withdrawal"+
               "\n" +"3 for balance query");
          response=JOptionPane.showInputDialog
          ("Enter subject number,(English:1,Maths:2,Science:3");
          if(response==null)
               JOptionPane.showMessageDialog
               (null,"You clicked CANCEL");
               System.exit(0);
          else if(response.equals(""))
               JOptionPane.showMessageDialog
               (null,"You sent and empty string");
               System.exit(0);
          }// end of while
     } // end of main
}// end of class
The curious thing is that the program actually worked initially which lead me to think it had to do with SDK getting corrupt.
I have reinstalled JDK and the IDE JCreator several times and still get
the same problem.
Would you be kind enough to provide some insight into the problem.?
I thank you in advance for your support and co-operation.
Regards
SS

Answer 1

The solution is already given but why it doesn't happen when pressed ok is because at that condition it returns an empty string i.e("") whereas if you press cancel it returns a null value.Try replacing the code below and you can understand everything.
try
moreBankingBusiness=JOptionPane.showInputDialog
("Do you want to do some banking?");
if(moreBankingBusiness.equals(""))
System.out.println("empty String");
System.exit(0);
moreBankingBusiness=moreBankingBusiness.toUpperCase();
catch(NullPointerException e)
System.out.println("we caught it");
System.exit(0);
}