int score = 0;

    var bi = bullets.iterator();
    while (bi.hasNext()) {
        Rectangle b = bi.next();
        var ai = aliens.iterator();
        while (ai.hasNext()) {
            Rectangle a = ai.next();
            if (b.intersects(a)) {
                ai.remove();      // alien destroyed
                bi.remove();      // bullet used up
                score += 20;
                break;            // this bullet is gone
            }
        }
    }