public void actionPerformed( ActionEvent e) 
    {
        if (this.aliens.size() == 0) 
        {
            if (this.level1) 
            {
                this.level1 = false;
                this.level2 = true;
                this.level  = "2";
                this.initAliens(30);
            }
            else if (this.level2) 
            {
                this.level2 = false;
                this.level3 = true;
                this.level  = "3";
                this.initAliens(40);
            }
            else if (this.level3) 
            {
                this.level3 = false;
                this.level4 = true;
                this.level  = "4";
                this.initAliens(1);
            }
        }
        ArrayList ms = craft.getMissiles();
        for (int i = 0; i < ms.size(); i++) 
        {
            final Missile m = (Missile)ms.get(i);
            if (m.isVisible()) 
            {
                m.move();
            }
            else 
            {
                ms.remove(i);
            }
        }
        for (int i = 0; i < aliens.size(); i++) 
        {
            final Alien a = (Alien)this.aliens.get(i);
            if (a.isVisible())
            {
                a.move();
            }
            else 
            {
                this.aliens.remove(i);
            }
        }
        craft.move();
        this.checkCollisions();
        this.repaint();
    }