textfield1.addKeyListener(new KeyAdapter() {
                public void keyTyped(KeyEvent e) {
                  char c = e.getKeyChar();
                  if (!((c >= '0') && (c <= '9') ||
                     (c == KeyEvent.VK_BACK_SPACE) ||
                     (c == KeyEvent.VK_DELETE))) {
                    getToolkit().beep();
                    e.consume();
                  }
                }
              });