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, therefore the behavior is normal.

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.

This is because class C can see A's protected methods for its own class. But it's not allowed to access A's protected methods for another class (B).

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, therefore the behavior is normal.