堆中创建的抽象类的私有实例变量在哪里？

Question

abstract class A {
    private int a;
    public A(int x) {
        a = x;
    }
    public int getA() {
        return a;
    }
}

class B extends A {
    public B(int x) {
        super(x);
    }
}

class TestB {
    public static void main(String args[]) {
        B b = new B(5);
        System.out.println(b.getA());
    }
}

In this situation when i say B b=new B(5); the super class constructor is called and the private instance variable is initialized to 5. So when i say getA() on class B 's object reffered by b it returns 5 . As the instance variable a of class A is private it will not be inherited by the class B . So where does the instance variable a is created(on heap) . Had it been a public it would have been a part of class B instance on Heap. Also class A is an abstarct class so it can not be instantiated.

Answer 1

there is no difference where the instance variables are allocated no matter if they are private, public, from a super class, from a abstract super class

typically the sequence will be something like

1. reference to the B.class object
2. block of Object instance variables (including helper fields for GC, the monitor for synchronisation,...)
3. block of A instance variables (only a in this case)
4. block of B instance variables (none in this case)

however each implementation of a JVM is free to choose how it allocates each of them

and access control is enforced by both the compiler and the JVM

Answer 2

Instance of variable 'a' would be created on heap inside object of class 'B'. and, it would still be created inside object of class 'B' where instance 'a' being public.

OBJECT of a subclass must contain its superclass's private fields. Having no access to a private member doesn't mean its not there.

As the JLS states.

Members of a class that are declared private are not inherited by subclasses of that class. Only members of a class that are declared protected or public are inherited by subclasses declared in a package other than the one in which the class is declared.

The sub-class doesn't private fields of super class. OBJECTS of subclasses contain private fields of their superclasses. The subclass itself has NO NOTION of private fields of its superclass.

Answer 3

All abstract does is say "you can't create this class on its own". Everything defined within the class works exactly the same as for a non-abstract class.

So in other words remove the "abstract" keyword above and the resulting compiled bytecode will look almost identical.

It changes how you can use the class, not the behavior of what the class contains.

Answer 4

Basic Rule of Inheritance!!!

From oracle documentation

A subclass does not inherit the private members of its parent class. However, if the superclass has public or protected methods for accessing its private fields, these can also be used by the subclass.