Why does this problem print 10?

Question

public class A{

int x = 10;

}

public class B extends A{

int x = 20;

}

public class Main{
public static void main(String[] args){

A a = new B();
System.out.println(a.x);

}
}

Answer 1

~ swim ~

But what is the point of creating an object of type B when I am actually creating an instance of A although at no time do I create an object of type A?

This is polymorphism?

Answer 2

~ swim ~

Yes, my fault. I deleted it.

A a = new B(); //upcasting
B b = (B) a; //downcasting

Answer 3

Juan Debenedetti

Inside an inheritance hierarchy it is possible to cast an object.

A a = new B() is called upcasting. Because class B is a child class of A. And every child class is an instance of the parent class (when you create an object B -> constructor of A and B is called)

Printing a.x shows you 10 instead of 20 because your object is from type A not B.

The parent class of all classes is the Object class.
Object o = new A();
-> this would be an object from type Object