Opening file using JFileChooser with File filtering

Question

ok guys I am trying set a file filter object to my file chooser it compile but i have a run time error, the error says:
Exception in thread "AWT-EventQueue-0" java.land.IllegalArgurmentException: Extension must be non-null and not empty
at javax.swing.filechooser.FileNameExtensionFilter..........
I have declared
JFileChooser jfc = new JFileChooser();
as a global variable
and in to my ActionListener, ActionPerform the following:
ActionListener listener = new ActionListener()
          String selection, file, copy;
     public void actionPerformed(ActionEvent e)
int tipe =0, size = 16;
     String Stile = " ";
     if (e.getSource() == openItem)
          FileNameExtensionFilter filter = new FileNameExtensionFilter("txt");
jfc.setFileFilter(filter);
          jfc.showOpenDialog(null);
          try
          File f = jfc.getSelectedFile();
          String file = f.getAbsolutePath();
          File f2 = new File(file);
          BufferedReader in = new BufferedReader(new FileReader(f2));
          String text = "";
String textl = text;
while( text != null)
               text = in.readLine() ;
               textl = textl + text + "\n";
               System.out.println(text);
Can some one please tell me whats wrong? and if possible how to fix it? Thank you guys

Answer 1

A couple of things might help:
It's a good idea to post the entire stack trace (even though it may appear a bit long). Specifically it helps to know which line in your code triggered the runtime error. You read the stack trace down from the top and the first line you find belonging to your code is a good place to begin looking hard at.
Another debugging technique is the SSCCE. The idea with this is that you come up with a really brief example illustrating your problem. (ie it compiles, runs and spits out the runtime error). It can be made brief by getting rid of anything extraneous to the problem. A bare button in a frame which when clicked sets up and displays the file chooser should be enough. I describe this as a debugging technique because in the course of constructing such things I often find I isolate - and therefore can see and fix - an error.
Anyway, sorry this is so general. Maybe someone will come along with a "missing semicolon in line 3"-type solution but, if not, it may be useful.