Although Pete’s post is awesomely comprehensive, I’m using this method to draw a very simple line with a little triangle at its end.
// create an AffineTransform
// and a triangle centered on (0,0) and pointing downward
// somewhere outside Swing's paint loop
AffineTransform tx = new AffineTransform();
Line2D.Double line = new Line2D.Double(0,0,100,100);
Polygon arrowHead = new Polygon();
arrowHead.addPoint( 0,5);
arrowHead.addPoint( -5, -5);
arrowHead.addPoint( 5,-5);
// [...]
private void drawArrowHead(Graphics2D g2d) {
tx.setToIdentity();
double angle = Math.atan2(line.y2-line.y1, line.x2-line.x1);
tx.translate(line.x2, line.y2);
tx.rotate((angle-Math.PI/2d));
Graphics2D g = (Graphics2D) g2d.create();
g.setTransform(tx);
g.fill(arrowHead);
g.dispose();
}