Showing all files of a given directory

Question

I'm trying to write a class that shows me all files of a given directory. When a run the class without parameters, everything seems to work fine, however, when I use a parameter that contains the directory, I get the following error:
java.lang.NullPointerException
     at be.hogelimb.ti.fundamentals.io.IOProgr4.<init>(IOProgr4.java:25)
     at be.hogelimb.ti.fundamentals.io.IOProgr4.main(IOProgr4.java:43)
Exception in thread "main"
this is my class without parameters:
public class IOProgr4 {
public IOProgr4(){
File dirName = new File("C:\\Documents and Settings\\FirstName LastName\\My documents\\test");
     if (dirName.exists()){
     if (dirName.isDirectory()){
     File[] dirFiles = dirName.listFiles();
     for (int i = 0; i<dirFiles.length; i++){
     System.out.println(dirFiles);
     else{
     System.out.println("dirName is not a directory");
     else{
     System.out.println("Can'f find directory");
     public static void main(String args[]){
          new IOProgr4();
You see, it's a very simple class.
This is my class with parameters :
public class IOProgr4 {
String dirString;
public IOProgr4(String dirString){
this.dirString = dirString;
File dirName = new File(dirString);
if (dirName.exists()){
if (dirName.isDirectory()){
     File[] dirFiles = dirName.listFiles();
     for (int i = 0; i<dirFiles.length; i++){
     System.out.println(dirFiles[i]);          
     else{
     System.out.println("dirName is not a directory");
     else{
     System.out.println("Can't find directory");
public static void main(String args[]){
String dirString = "";
for (int i = 0; i < args.length; i++){
dirString += args[i] + " ";
new IOProgr4(dirString);
I'm working with Eclipse and my parameter string is the same as the String I've written in the first class, and it's also recognized as a directory. Can anyone tell me why this doesn't work?
Thanks in advance!!!

Answer 1

I'm working with Eclipse and my parameter string is
the same as the String I've written in the first
class...In the end, it has nothing to do with whether you've hard-coded the directory name versus taken it as a parameter. It must not have been the same value, or it would have worked the same. So your problem lies elsewhere - however, you were given a nice stack trace pointing out the line in your code where it happened. Are you not familiar with a little concept known as debugging?