public void step()
    {
        int w = getWidth();
        int h = getHeight();

        if (pos_x < 0) {
            delta[0] = 1;
        } else if (pos_x > w - radius) {
            delta[0] = -1;
        }

        if (pos_y < 0) {
            delta[1] = 1;
        } else if (pos_y > h - radius) {
            delta[1] = -1;
        }

        pos_x += delta[0];
        pos_y += delta[1];
    }