JTextField textField =  new  JTextField();

AbstractDocument abstractDocument = (AbstractDocument) textField.getDocument();
abstractDocument.setDocumentFilter( new  DocumentFilter() {

     public   void  insertString(FilterBypass fb,
                              int  offset,
                             String string,
                             AttributeSet attr)
             throws  BadLocationException {
        
        replace(fb, offset,  0 , string, attr);
    }

     public   void  replace(FilterBypass fb,
                         int  offset,
                         int  length,
                        String text,
                        AttributeSet attrs)
             throws  BadLocationException {

         for  ( char  ch : text.toCharArray()) {
             if  (!Character.isDigit(ch)) {
                 throw   new  BadLocationException( null , offset);
            }
        }

        fb.replace(offset, length, text, attrs);
    }

});