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.moveTo(0, 0);

  double l = drawArrowHead_length*size;

  arrowHead.lineTo(-size, -l);

  arrowHead.lineTo(size, -l);

  arrowHead.closePath();

  new AffineTransform tx;

  double angle = Math.atan2(y2-y1, x2-x1);

  tx.translate(x2, y2);

  tx.rotate(angle-Math.PI/2);

  AffineTransform old = g.getTransform();

  g.transform(tx);

  g.fill(arrowHead);

  g.setTransform(old);

}