It's not so much detect and erase, but just prevent from inserting. 
If you look at 
http://java.sun.com/j2se/1.4.2/docs/api/javax/swing/JTextField.html
they have an example of modifying what is being inserted.

Probably I misunderstand the first message "Prevent illegal characters from 
ever being typed in any URL forms". The way to prevent the illegal characters 
ever being enter was to detect and erase them. A popup dialog may be a 
distraction. Displaying information at the bottom of the window is better.

So do we want to detect illegal characters and erase them with a message being 
displayed at the bottom of the window while the user is typing? Or just do a 
check after the user types enter or clicks apply button? I think the latter 
has been done by the current SWOOP.

Oops, that log message doesn't make sense.
It should read:
I wouldn't worry at this point about whether to make sure the entire field is
legal at all points in time. Just make sure it consists of legal characters.

Modified:
   swoop/trunk/src/org/mindswap/swoop/popup/PopupNew.java
Log:
Issue212 chatting
Ok, I've merged the changes out for now, since people were having problems.
Since we have many areas that require URI or ID entries, you might want
to consider subclassing JTextField as both URIField and IDField.  I wouldn't
worry at this point about whether to make sure the entire field consists
of legal characters.  Refer to the XML Spec for what's allowed.

The Java documentation on JTextField has a good example of this.

Oh, note that you don't need to add that dialog box, I think it's already there.

Ack!
Don't display a dialog box that takes away focus!  Lets try little step at
first, just prevent obviously bad characters from going in.  Next step would be
to have a little notice at the bottom of the window with an explanation when you
type something wrong.

The dialog box should only come up if there is an error at the end.

Modified:
   swoop/trunk/src/org/mindswap/swoop/popup/PopupNew.java
Log:
Issue212 resolved
When illegal characters are typed for URI during class, instance or property creation, an error message is displayed and the illegal characters will be removed automatically

Prevent illegal characters from ever being typed in any URL forms (Class
creation, etc)