Timerを使用してJPanelにアナログ時計の針を描画する

class AnalogClock extends JPanel {
  protected LocalTime time = LocalTime.now(ZoneId.systemDefault());
  protected final Timer timer = new Timer(200, e -> {
    time = LocalTime.now(ZoneId.systemDefault());
    repaint();
  });
  private transient HierarchyListener listener;

  @Override public void updateUI() {
    removeHierarchyListener(listener);
    super.updateUI();
    listener = e -> {
      if ((e.getChangeFlags() & HierarchyEvent.SHOWING_CHANGED) != 0) {
        if (e.getComponent().isShowing()) {
          timer.start();
        } else {
          timer.stop();
        }
      }
    };
    addHierarchyListener(listener);
  }

  @Override protected void paintComponent(Graphics g) {
    Graphics2D g2 = (Graphics2D) g.create();
    g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
    Rectangle rect = SwingUtilities.calculateInnerArea(this, null);
    g2.setColor(Color.BLACK);
    g2.fill(rect);
    double radius = Math.min(rect.width, rect.height) / 2d - 10d;
    g2.translate(rect.getCenterX(), rect.getCenterY());

    // Drawing the hour markers
    double hourMarkerLen = radius / 6d - 10d;
    Shape hourMarker = new Line2D.Double(0d, hourMarkerLen - radius, 0d, -radius);
    Shape minuteMarker = new Line2D.Double(0d, hourMarkerLen / 2d - radius, 0d, -radius);
    AffineTransform at = AffineTransform.getRotateInstance(0d);
    g2.setStroke(new BasicStroke(2f));
    g2.setColor(Color.WHITE);
    for (int i = 0; i < 60; i++) {
      if (i % 5 == 0) {
        g2.draw(at.createTransformedShape(hourMarker));
      } else {
        g2.draw(at.createTransformedShape(minuteMarker));
      }
      at.rotate(Math.PI / 30d);
    }

    // Calculate the angle of rotation
    double secondRot = time.getSecond() * Math.PI / 30d;
    double minuteRot = time.getMinute() * Math.PI / 30d + secondRot / 60d;
    double hourRot = time.getHour() * Math.PI / 6d + minuteRot / 12d;

    // Drawing the hour hand
    double hourHandLen = radius / 2d;
    Shape hourHand = new Line2D.Double(0d, 0d, 0d, -hourHandLen);
    g2.setStroke(new BasicStroke(8f));
    g2.setPaint(Color.LIGHT_GRAY);
    g2.draw(AffineTransform.getRotateInstance(hourRot).createTransformedShape(hourHand));

    // Drawing the minute hand
    double minuteHandLen = 5d * radius / 6d;
    Shape minuteHand = new Line2D.Double(0d, 0d, 0d, -minuteHandLen);
    g2.setStroke(new BasicStroke(4f));
    g2.setPaint(Color.WHITE);
    g2.draw(AffineTransform.getRotateInstance(minuteRot).createTransformedShape(minuteHand));

    // Drawing the second hand
    double r = radius / 6d;
    double secondHandLen = radius - r;
    Shape secondHand = new Line2D.Double(0d, r, 0d, -secondHandLen);
    g2.setPaint(Color.RED);
    g2.setStroke(new BasicStroke(1f));
    g2.draw(AffineTransform.getRotateInstance(secondRot).createTransformedShape(secondHand));
    g2.fill(new Ellipse2D.Double(-r / 4d, -r / 4d, r / 2d, r / 2d));

    g2.dispose();
  }
}