How is constructor being called without any instance associated with it?

Question

Please see the below code:

class A {
    private int b;
    A(){
        b=5;
    }
}

class B extends A {

}

class C {
    public static void main(String args[]){
        B b=new B();
    }
}

When I create an instance of B,the default constructor of B invokes the constructor of A which assigns a value to instance variable b. My query is since instance variables are associated with instances of classes, and we have not created any instance of class A,what does this assignment(b=5) really mean? Also what does the call to A's constructor really mean when there is no instance of A?

But there _is_ an instance of `A`. It just happens to also be an instance of `B`. — Dawood ibn Kareem, Mar 24 '15 at 18:41
I'm voting to close this question as off-topic because answering this question displays a lack of basic knowledge of OO principles and should be taught by reading a good tutorial since it's too broad to answer on SO. https://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html — Jeroen Vannevel, Mar 24 '15 at 18:45

score 5 · Answer 1 · answered Mar 24 '15 at 18:41

5

B extends A means that an instance of B is also an instance of A, just like a dog is also an animal. It's both at the same time, so it's perfectly normal for b=5 to make sense, since the B is also an A and that is initializing the b field in A.

answered Mar 24 '15 at 18:41

Louis Wasserman

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But b is private and hence is not inherited by class B – paidedly Mar 24 '15 at 18:46
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It doesn't matter if b is private. There may be public or protected methods that perform operations that include b in those operations. Therefore, even if it is not always obvious, all of A gets initialized when B is created. – markbernard Mar 24 '15 at 18:48
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B still has a b, it just can't see it; only its inherited A methods can see it. – Louis Wasserman Mar 24 '15 at 18:51
But we never instantiated A,so where is b when there is no instance of A in memory? – paidedly Mar 24 '15 at 18:52
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@user3678484 Did you read my comment under the question? Why do you keep asserting there is no instance of A in memory? – Dawood ibn Kareem Mar 24 '15 at 19:02
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There _is_ an instance of `A` in memory. It's the instance of `B`, which is also an `A`. – Louis Wasserman Mar 24 '15 at 19:17

score 0 · Answer 2 · edited May 23 '17 at 12:12

0

Check this discussion about class inheritance vs object inheritance in Java. According to JLS you would say that class B doesn't inherit the private field b from class A.

Members of a class that are declared private are not inherited by subclasses of that class.

However an instance of B inherits all the data from A including the private field b.

edited May 23 '17 at 12:12

Community

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answered Mar 24 '15 at 18:51

S. Pauk

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so even without an instance of A in memory, the field b with value 5 is residing in memory? – paidedly Mar 24 '15 at 18:53
do you mean an instance of B? – S. Pauk Mar 24 '15 at 18:54
the instance variable b defined in class A.Is it present in memory even though there is no instance of A in memory? – paidedly Mar 24 '15 at 18:55
when you instantiate B private field b is also created in memory – S. Pauk Mar 24 '15 at 18:58
ok.so private variables of superclass are not inherited by subclass but are inherited by instances of subclass? Is this statement correct? – paidedly Mar 24 '15 at 19:07
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Have you checked the link? When you talk about class inheritance then `does not inherit` means `is not visible/cannot be accessed`. When you talk about object inheritance it means that parent's fields are also created in memory (inherited). – S. Pauk Mar 24 '15 at 19:11

How is constructor being called without any instance associated with it?

2 Answers2