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I have a problem understanding protected members inheritance and visibility.

I know it is visible in the same package and subclasses.

But in the following code it is not visible in a subclass.

A.java

package a; public class A { public static void main(String[] args) { } protected void run() { } }

B.java

package b; import a.A; public class B extends A { public static void main(String[] args) { B b = new B(); b.run(); // this works fine } }

C.java

package b; import a.A; public class C extends A{ // it will not work also if extends B public static void main(String[] args) { B b = new B(); b.run(); // this is the problem; not visible } }

Why b.run() in the last class is invisible?

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  • What is the access modifier of run() ? Commented Aug 18, 2015 at 15:20

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This is because class C can see A's protected methods from within its own inheritance tree. But it's not allowed to access A's protected methods for another class (B) from a different inheritance tree. C is not part of B's inheritance tree (by this, I mean that's it's not a parent of B), therefore the behavior is normal.

EDIT: Added documentation reference as requested

6.6.2.1. Access to a protected Member:

If the access is by a field access expression E.Id, or a method invocation expression E.Id(...), or a method reference expression E :: Id, where E is a Primary expression (§15.8), then the access is permitted if and only if the type of E is S or a subclass of S.

Applying the above to this case, because the variable b is not an instance of C or a subclass of C, access to the protected method b.run() is not allowed.

Also addressing Codebender's comment about the packages. Note that, if the C class would have been defined in the same package as the A class, where the protected run() method is defined, then the above rule wouldn't apply, and you would be able to access the method as shown in your code.

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3 Comments

Can you put an official doc link explaining this? Since B and C are in the same package, and protected fields can be accessed in the same package, this seems a bit weird.
What about C extends B. It is the same problem.
Added more specific details from the documentation. Should clarify things for you hopefully.

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