Do subclasses inherit private fields?

Solution 1:

Most of the confusion in the question/answers here surrounds the definition of Inheritance.

Obviously, as @DigitalRoss explains an OBJECT of a subclass must contain its superclass's private fields. As he states, having no access to a private member doesn't mean its not there.

However. This is different than the notion of inheritance for a class. As is the case in the java world, where there is a question of semantics the arbiter is the Java Language Specification (currently 3rd edition).

As the JLS states (https://docs.oracle.com/javase/specs/jls/se8/html/jls-8.html#jls-8.2):

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.

This addresses the exact question posed by the interviewer: "do subCLASSES inherit private fields". (emphasis added by me)

The answer is No. They do not. OBJECTS of subclasses contain private fields of their superclasses. The subclass itself has NO NOTION of private fields of its superclass.

Is it semantics of a pedantic nature? Yes. Is it a useful interview question? Probably not. But the JLS establishes the definition for the Java world, and it does so (in this case) unambiguously.

EDITED (removed a parallel quote from Bjarne Stroustrup which due to the differences between java and c++ probably only add to the confusion. I'll let my answer rest on the JLS :)

Solution 2:

Yes

It's important to realize that while there are two classes, there is only one object.

So, yes, of course it inherited the private fields. They are, presumably, essential for proper object functionality, and while an object of the parent class is not an object of the derived class, an instance of the derived class is mostly definitely an instance of the parent class. It could't very well be that without all of the fields.

No, you can't directly access them. Yes, they are inherited. They have to be.

It's a good question!

Update:

Err, "No"

Well, I guess we all learned something. Since the JLS originated the exact "not inherited" wording, it is correct to answer "no". Since the subclass can't access or modify the private fields, then, in other words, they are not inherited. But there really is just one object, it really does contain the private fields, and so if someone takes the JLS and tutorial wording the wrong way, it will be quite difficult to understand OOP, Java objects, and what is really happening.

Update to update:

The controversy here involves a fundamental ambiguity: what exactly is being discussed? The object? Or are we talking in some sense about the class itself? A lot of latitude is allowed when describing the class as opposed to the object. So the subclass does not inherit private fields, but an object that is an instance of the subclass certainly does contain the private fields.

Solution 3:

No. Private fields are not inherited... and that's why Protected was invented. It is by design. I guess this justified the existence of protected modifier.

Now coming to the contexts. What you mean by inherited -- if it is there in the object created from derived class? yes, it is.

If you mean can it be useful to derived class. Well, no.

Now, when you come to functional programming the private field of super class is not inherited in a meaningful way for the subclass. For the subclass, a private field of super class is same as a private field of any other class.

Functionally, it's not inherited. But ideally, it is.

OK, just looked into Java tutorial they quote this:

Private Members in a Superclass

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.

refer: http://download.oracle.com/javase/tutorial/java/IandI/subclasses.html

I agree, that the field is there. But, subclass does not get any privilege on that private field. To a subclass, the private field is same as any private field of any other class.

I believe it's purely matter of point-of-view. You may mould the argument either side. It's better justify both way.