Logic behind behavior of the Java class protected members

Question

Can please someone explain me the logic (not the behavior itself but the logic) behind the peculiar fact that access to protected class members, both methods and fields I believe, is allowed through inheritance only i.e. by the reference of the child type – and not by the reference of the parent type where protected member was declared – in case child class is located in a different package ?

And what if both parent and child classes are in the same package, does call by inheritance is only allowed option as well?

Also why it's not allowed to access A's protected methods for another class (B) from a different inheritance tree (C)?

@j.seashell I disagree it's a duplicate. This question is mainly about "why", the logic behind `protected`. — lexicore, Apr 18 '18 at 20:10

score 1 · Answer 1 · answered Apr 18 '18 at 19:58

access to protected class members <...> is allowed through inheritance only i.e. by the reference of the child type – and not by the reference of the parent type where protected member was declared – in case child class is located in a different package ?

This is because protected means access from children classes OR same package. From different classes you can access to members only from the same package. That's why you need to refer to members using child class reference.

what if both parent and child classes are in the same package, does call by inheritance is only allowed option as well?

If the caller in the same package -- you can use whatever reference (either parent or child)

why it's not allowed to access A's protected methods for another class (B) from a different inheritance tree (C)?

Code would be nice to see, but from my understanding of the question, that's exactly what protected modifier need to do.

score 1 · Answer 2 · answered Apr 18 '18 at 20:05

When implementing something you will normally want to reduce the "attack surface" and not expose implementation details. This is why different access levels are needed. The reason for protected is inheritance. We need something which is more strict than public (to hide implementation details), but less strict than private - so that subclasses would have access.

I think this logic is best explained by JLS §6.6.2. Details on protected Access:

A protected member or constructor of an object may be accessed from outside the package in which it is declared only by code that is responsible for the implementation of that object.

I think this should explain the logic of "class, package, subclass in same or different package":

class should be clear, the class itself should of course have access to its members.
subclass (same or different package) is also understandable.
package is not so logical, to be honest. Possible explanation: it is likely that other classes in the same package will be used in the implementation of this object.

I agree that protected is somewhat illogical. It combines restrictions on horizontal access (this package/other package) with restrictions on vertical access (subclass/not subclass).

And what if both parent and child classes are in the same package, does call by inheritance is only allowed option as well?

No. Other classes from the same package also have access to protected members of this class.

Also why it's not allowed to access A's protected methods for another class (B) from a different inheritance tree (C)?

This is specifically the intention of protected. This is done to hide implementation details of A from the "outside world".

Logic behind behavior of the Java class protected members

2 Answers2