How to use user defined exception class

Question

Hi all
I just need som help with creating a user defined exception class.
Im writing a small/simple text editor.
My exception class looks like this:
public class myExcp extends Throwable
     private String message;
     public myExcep(String message)
          this.message = message;
     public void display()
          System.out.println(message);
I would like to use it when a user tries to open a exe-file instead of a txt file.
Here is some code from the editor:
if (e.getSource() == open)
saveOld();
if (fc.showOpenDialog(null)== JFileChooser.APPROVE_OPTION)
readFile(fc.getSelectedFile().getAbsolutePath());           
saveas.setEnabled(true);                
So, should I use exception here or at the readFile method?
readfile:
private void readFile(String fileName)
try
String tmp = fileName.substring(fileName.length() -4, fileName.length());
if (!tmp.equals(".exe"))
FileReader r = new FileReader(fileName);
textarea.read(r, null);
r.close();
currentFile = fileName;
label.setText(currentFile);
changed = false;
catch (IOException e)
JOptionPane.showMessageDialog (this, "Cannot find the file " + fileName);
Where and how do I use my exception class.
Do I need to create an instance? Where?
Should the exception class extend Exception instead?
Thank you in advance /

Answer 1

Extend Exception, not Throwable. It's a checked exception that way.
Follow the Sun coding standards and make that exception class name start with a capital letter.
When you extend Exception, override all four ctors.
What's that display method you added? Isn't getMessage() good enough?
You need to create a new instance just before you throw the exception, of course.
Sounds like a terrible design, by the way. Exceptions shouldn't be used like "go to" for app logic. They should signal unrecoverable conditions. You can easily recover from the situation you've described simply by displaying a pop-up that tells the user to open only text-readable file types. I think that's a better solution.
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