public void actionPerformed(ActionEvent e) {

        if (aliens.size()==0) {
            ingame = false;
        }

        ArrayList ms = craft.getMissiles();

        for (int i = 0; i < ms.size(); i++) {
            Missile m = (Missile) ms.get(i);
            if (m.isVisible()) 
                m.move();
            else ms.remove(i);
        }

        for (int i = 0; i < aliens.size(); i++) {
            Alien a = (Alien) aliens.get(i);
            if (a.isVisible()) 
                a.move();
            else aliens.remove(i);
        }

        craft.move();
        checkCollisions();
        repaint();  
    }