JTextArea - need event rised on text changed.

Question

I am making an application which contains a JTextArea. when user writes some data on in, some event must be invoked changing the text of the same JtextArea.
Problem is, when i change the text, Java automatically adds to JtextArea typed character, what i don't want.
program works in this way:
user presses "d" on keyboard -> on JTextArea "e" appears.
thanks for help.

Answer 1

This is an example of a text area that forces upper case.
You can modify it to your needs.
     private static class UpperCaseTextArea extends JTextArea {                
          protected Document createDefaultModel() {
                 return new UpperCaseDocument();
          static class UpperCaseDocument extends PlainDocument {
              public void insertString(int offs, String str, AttributeSet a)
                     throws BadLocationException {
                     if (str == null) {
                          return;
                     if (str.length() == 1) {
                          char c = str.charAt(0);
                          char cUpper = Character.toUpperCase(c);
                          super.insertString(offs, Character.toString(cUpper), a);
                     else
                         super.insertString(offs, str, a);
      }