Subclass PlainDocument, then set the document of the JTextfield to the subclass as shown below:
JTextField myJTextfield;myJTextfield.setDocument(new DigitsDocument());class DigitsDocument extends PlainDocument { public void insertString(int offs, String str, AttributeSet a)throws BadLocationException { if (str == null) { return; } char[] addedFigures = str.toCharArray(); char c; for (int i = addedFigures.length; i > 0; i--) { c = addedFigures[i-1]; if(Character.isDigit(c)){ //System.out.println(