import java.awt.geom.*;

static float drawArrowHead_length = 2f;

static void drawArrowHead(Graphics2D g, double x1, double y1, double x2, double y2, double size) {
  if (y2 == y1 && x2 == x1) ret;
  
  Path2D.Double arrowHead = new Path2D.Double;
  arrowHead.addPoint(0, 0);
  double l = drawArrowHead_length*size;
  arrowHead.addPoint(-size, -l);
  arrowHead.addPoint(size, -l);

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

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