static void drawArrowHead(Graphics2D g, int x1, int y1, int x2, int y2) {
  if (y2 == y1 && x2 == x1) ret;
  
  new Polygon arrowHead;
  arrowHead.addPoint(0, 5);
  arrowHead.addPoint(-5, -5);
  arrowHead.addPoint(5, -5);

  new AffineTransform tx;
  double angle = Math.atan2(y2-y1, x2-x1);
  tx.translate(x2, y2);
  tx.rotate(angle-Math.PI/2d);

  Transform old = g.getTransform();
  g.setTransform(tx);
  g.fill(arrowHead);
  g.setTransform(old);
}