JPanelに矢印付きの直線を描画する

class Arrow {
  private final Point start = new Point();
  private final Point end = new Point();
  private final Path2D arrowHead;

  protected Arrow(Point start, Point end) {
    this.start.setLocation(start);
    this.end.setLocation(end);
    arrowHead = makeArrowHead(new Dimension(8, 8));
  }

  protected Path2D makeArrowHead(Dimension size) {
    Path2D path = new Path2D.Double();
    double t = size.height;
    double w = size.width * .5;
    path.moveTo(0d, -w);
    path.lineTo(t, 0d);
    path.lineTo(0d, w);
    path.closePath();
    return path;
  }

  public void draw(Graphics2D g2) {
    g2.drawLine(start.x, start.y, end.x, end.y);
    arrowHead.transform(AffineTransform.getRotateInstance(
        end.getX() - start.getX(), end.getY() - start.getY()));
    arrowHead.transform(AffineTransform.getTranslateInstance(
        end.getX(), end.getY()));
    g2.fill(arrowHead);
    g2.draw(arrowHead);
  }
}