public void initAliens(int amount) 
    {
        this.aliens = new ArrayList();
        for (int i = 0; i < amount; i++) 
        {
            if (this.level4) 
            {
                this.aliens.add(new Alien(this.getRandomXPos(), 200, this.level, this.mode));
            }
            else 
            {
                this.aliens.add(new Alien(this.getRandomXPos(), this.getRandomYPos(), this.level, this.mode));
            }
        }
    }