null pointer exception prob in my coding

Question

I am always getting nullPointerException on line validateCarPlate(nStr) in the main method and on line if(y.matches(rex)). How should i edit to remove the nullPointerException?

import javax.swing.*;
import java.lang.Exception;

public class Q2{
public static void main(String[]args){
boolean loop = true;
while(loop){
   String nStr = JOptionPane.showInputDialog("Enter car plate number: ");
   try{
       validateCarPlate(nStr);
}
catch(InvalidCarPlateException e){
}
}
}
public static void validateCarPlate(String y)throws InvalidCarPlateException{
String rex = "[a-zA-Z]{3}[0-9]{1,4}";
if(y.matches(rex)){
    computeCheckDigit(y);
}else{
    throw new InvalidCarPlateException();
}
}

public static void computeCheckDigit(String x){
char [] arr = new char[x.length()];
for(int i=0; i<x.length();i++){
    arr[i] = x.charAt(i);
}

Use a consistent and logical indent for code blocks. The indentation of the code is intended to help people understand the program flow. — Andrew Thompson
– Andrew Thompson, Commented Apr 15, 2013 at 6:07

Bhavik Shah · Accepted Answer · 2013-04-15 06:08:18Z

2

Looks like

String nStr = JOptionPane.showInputDialog("Enter car plate number: ");

this returns null

change the code like this

public static void main(String[]args){
  boolean loop = true;
  while(loop){
   String nStr = JOptionPane.showInputDialog("Enter car plate number: ");
   if(nStr != null)
   {
     try{
       validateCarPlate(nStr);
        }
     catch(InvalidCarPlateException e){
     }
   }
  }
}

Since, javadocs for JOptionPane#showInputDialog() says Shows a question-message dialog requesting input from the user I think you forgot to give a input.

edited Apr 15, 2013 at 6:08

answered Apr 15, 2013 at 5:59

Bhavik Shah

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Comments

Mayank Tiwari · Accepted Answer · 2013-04-15 05:59:10Z

1

change you method like that

public static void validateCarPlate(String y)throws InvalidCarPlateException{
String rex = "[a-zA-Z]{3}[0-9]{1,4}";
if(y == null){
     // put some message to handle that exception such as
     // JOptionPane.showMessageDialog(null,"Some Message");
}else if(y.matches(rex))
    computeCheckDigit(y);
}else{
    throw new InvalidCarPlateException();
}
}

Also put the code of computeCheckDigit(y); method.

answered Apr 15, 2013 at 5:59

Mayank Tiwari

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Comments

Summer_More_More_Tea · Accepted Answer · 2013-04-15 06:04:55Z

1

If you click the Cancel button, JOptionPane.showInputDialog will return a null. You can check the return value before passing nStr to validateCarPlate. If a null returned, just drop this nStr and continue the loop (or break it according to your requirement).

answered Apr 15, 2013 at 6:04

Summer_More_More_Tea

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