drawArrow in Graphics2D (http://stackoverflow.com/questions/4112701/drawing-a-line-with-arrow-in-java)

drawArrow.java

void drawArrow(Graphics g1, int x1, int y1, int x2, int y2) {
    Graphics2D g = (Graphics2D) g1.create();

    double dx = x2 - x1, dy = y2 - y1;
    double angle = Math.atan2(dy, dx);
    int len = (int) Math.sqrt(dx*dx + dy*dy);
    AffineTransform at = AffineTransform.getTranslateInstance(x1, y1);
    at.concatenate(AffineTransform.getRotateInstance(angle));
    g.transform(at);

    // Draw horizontal arrow starting in (0, 0)
    g.drawLine(0, 0, len, 0);
    g.fillPolygon(new int[] {len, len-ARR_SIZE, len-ARR_SIZE, len},
                  new int[] {0, -ARR_SIZE, ARR_SIZE, 0}, 4);
}