I created the system of classes describes a shape and its variations. I’ve found out public-access modifier of variable “volume” can be replaced on private so result will not change. It isn’t lead to errors. Part of my code is here:

`
class Shape {
private double volume;

    public Shape(double volume) {
        this.volume = volume;
    }
    public double getVolume() {
        return volume;
    }
}
class SolidOfRevolution extends Shape {
    private double radius;

    public SolidOfRevolution(double volume, double radius) {
        super(volume);
        this.radius = radius;
    }

    public double getRadius() {
        return radius;
    }
}

class Ball extends SolidOfRevolution {
    public Ball(double radius) {
        super(Math.PI * Math.pow(radius, 3) * 4 / 3, radius);
    }
}`

I considered private access variables aren’t inherited in Java. Why do constructors in sub-classes initialize a private-access variable “volume” as absolutely legitimate part of sub-class? Please tell me what I missed.
I’ve read some similar questions but I still can’t get it: what differ inheritance of class to inheritance of instance?