class PointInPlane {
    public float x;
    public float y;
    static class CircleInPlane {
        public static float r;  
        public static float xcentr;
        public static float ycentr;
        static void solve(PointInPlane a, PointInPlane b, PointInPlane c) {
            float A = (b.y-a.y)/(b.x-a.x); //geting NullPointerExeption 
            float B = (c.y-b.y)/(c.x-b.x); //probably will get in all next steps
            xcentr = (A*B*(a.y-c.y)+B*(a.x+b.x)-A*(b.x+c.x))/(2*(B-A));
            ycentr = A*(xcentr-a.x)+a.y;
            r = sqrt((pow((a.x - xcentr), 2) + pow((a.y - ycentr), 2))); 
        }
    }
}
So   IDK how I can handle this problem. I'm getting NullPointerException when declare float A. I think the problem is that I'm using fields from one class in another or trying to use PointInPlane objects a, b and c with null fields. How this problem could be solved?