Working with objects in java?

Question

Java

I'm trying to figure out how to create objects. There is a certain class A to create an object, we write A a1 = new A (). It seems to be simple. Then class B appears which inherits from class A. What does this object creation mean:
A a2 = new B();
Why are the class names at the beginning and end different and what does this mean?
There is also this code:

class A{
    int q = 33;
    int sum(int a,int b){
        return a+b;
    }
}

class B extends A{
    int q = 32;

    int sum(int a, int b) {
        return a-b;
    }
}

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

        A a = new B();

        System.out.println(a.sum(1,2));
        System.out.println(a.q);

    }
}

Why is calling a.sum(1,2) a method of class B being called, while calling aq is calling a class variable A?

Answer 1

1. A a2 = new B();means that you put the child class object into the parent class variable . This is possible and often necessary when you do not know exactly which of the heirs will be used or you want to abstract from a specific implementation.
2. a.sum(1,2)calls the method overridden in the child class. Because it doesn't have a private modifier, it works.
3. When aq is called, the class variable A is called because the fields are not redefined.
I advise you to read about the OOP base: inheritance. polymorphism and encapsulation.