修复NullPointerException？

Question

我正在制作一个简单的程序，用户必须避开一些障碍（来自抽象的障碍类），如岩石，水母和游泳者（所有扩展障碍类）。我在我创建的构造函数Background中创建了其中的三个对象，但是，我在这些对象上遇到了NullPointerException。任何人都可以告诉我，为了得到这个例外，我做错了吗？

Background对象是在Runner类中创建的，但我只是在构造函数中得到了异常。

public class Background extends JPanel implements Runnable
{
    public static Dimension d;
    public Thread game;
    public int hlth;
    private ArrayList<Obstacle> os;
    private User u;
    private JLabel pic;
    private ImageIcon picture;
    private Tools t;

    public Background(Dimension dim)
    {
        //set up the JPanel dimension
        d = dim;
        this.setPreferredSize(d);

        //unrelated - just set up the background in a jlabel

        //unrelated - just set up a health jlabel at top of screen

        //set up the arraylist of obstacles
        os = new ArrayList<Obstacle>();
        os.add(new JellyFish(t.rX(), t.rY())); //the program is crashing here
        os.add(new Rock(t.rX(), t.rY()));
        os.add(new Swimmer(t.rX(), t.rY()));

        u = new User();

        game = new Thread(this);
    }
    //not related
}

public class JellyFish extends Obstacle
{
    public JellyFish()
    {
        super();
    }

    public JellyFish(int useX, int useY)
    {
        super(useX, useY);
    }

    //unrelated - draw the JellyFish using graphics
}

提前致谢！