SetDocument doesn't seem to work correctly

Question

I'm obviously using the setDocument method for my JTextField incorrectly, but how? The DocumentSizeFilter works correctly, allowing only the number of characters I specify, but the DigitDocument.insertString(...) doesn't seem to work at all (it should allow for only digits to be entered into the JTextField).
          AbstractDocument doc;
          pafTextField = new JTextField();
          pafTextField.setDocument(new DigitDocument());
          pafTextField.setPreferredSize(new Dimension(30, height));
          doc = (AbstractDocument)pafTextField.getDocument();
          doc.setDocumentFilter(new DocumentSizeFilter(2));The DigitDocument class:
public class DigitDocument extends PlainDocument{
     public DigitDocument(){
          super();
     public void insertString(int offs, String str, AttributeSet a) throws BadLocationException, NumberFormatException{
          try{
               System.out.println("String str = " + str);
               int i = Integer.parseInt(str);
               super.insertString(offs, str, a);
          catch(NumberFormatException e){
               // do nothing because a user input was not a digit
               Toolkit.getDefaultToolkit().beep();
}the DocumentSizeFilter code:
http://java.sun.com/docs/books/tutorial/uiswing/components/example-1dot4/DocumentSizeFilter.java

Answer 1

didn't try the abstract doc, but why not combine the two
something like this
import java.awt.*;
import javax.swing.*;
import javax.swing.text.*;
class Testing extends JFrame
  public Testing()
    setLocation(400,300);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    JTextField tf = new JTextField(new DigitDocument(2),"",10);
    JPanel p = new JPanel();
    p.add(tf);
    getContentPane().add(p);
    pack();
  public static void main(String[] args){new Testing().setVisible(true);}
class DigitDocument extends PlainDocument{
  int maxChars;
  public DigitDocument(int chars){
    super();
    maxChars = chars;
  public void insertString(int offs, String str, AttributeSet a) throws BadLocationException, NumberFormatException{
    try{
      System.out.println("String str = " + str);
      if(this.getLength()+str.length() > maxChars) throw new NumberFormatException();//<----
      int i = Integer.parseInt(str);
      super.insertString(offs, str, a);
    catch(NumberFormatException e){
      // do nothing because a user input was not a digit
      Toolkit.getDefaultToolkit().beep();
}