Inheritance for beginners

Question

I just started to learn java, so now i read about such possibility as inheritance, so try to create class that must create object - box. And using inheritance implement new properties to created object. I try to put each class in separate file, so after creating class, try to use it in

public static void main(String[] args)

So class Inheritance:

 public class Inheritance {
double width;
double height;
double depth;
Inheritance (Inheritance object){
    width = object.width;
    height = object.height;
    depth = object.depth;
}
Inheritance ( double w, double h, double d){
    width = w;
    height = h;
    depth = d;
}
Inheritance (){
    width = -1;
    height = -1;
    depth = -1;
}
Inheritance (double len){
    width=height=depth=len;
}
double volumeBox (){
    return width*height*depth;
}
class BoxWeight extends Inheritance {
    double weight;
    BoxWeight (double w, double h, double d, double m){
        super(w,h,d);
        weight = m;
    }
}

But, when i try to use BoxWeight in main class, during using i got an error

public class MainModule {
    public static void main(String[] args) {
    Inheritance.BoxWeight mybox1 = new Inheritance.BoxWeight(9, 9, 9, 9);
....

Error - No enclosing instance of type Inheritance is accessible. Where i'm wrong?

Answer 1

As it stands, BoxWeight requires an instance of Inheritance to work (just like accessing a non-static variable or function requires an instance, so accessing a non-static inner class also does). If you change it to static it would work, but this isn't required.

BoxWeight doesn't need to be inside the Inheritance class.

Instead, remove BoxWeight out of the Inheritance class.

And change Inheritance.BoxWeight to BoxWeight.

EDIT: Just for completeness, you could also make it:

Inheritance.BoxWeight mybox1 = new Inheritance().new BoxWeight(...);

Inheritance.BoxWeight is just the type, so the above does not apply. But to create an instance of BoxWeight, you need an Inheritance object, which you create with new Inheritance().