Java Self Reference with Inhertiance

I understand why this code below doesn't work. That's because convolusion will call Base, not Derived. This code is soooo simple, and have self-referencing. I extended 'self-referencing class' and I stucked with that problem.

class Base
{
    public    int  important_data;
    protected Base child;

    public int sum()
    {
        if(child != null)
        {
            return important_data + child.sum();
        }
        else
        {
            return important_data;
        }
    }
}

class Derived extends Base
{
    public int more_important;

    public int convolusion()
    {
        if(child != null)
        {
            return more_important*important_data + child.convolusion();
        }
        else
        {
            return more_important*important_data;
        }
    }
}

Then, is there any available method to do that?