Why is there no subclass visibility modifier in Java?

Question

More than once, I have found that I want a variable visibility that is not possible in Java. I wanted some members to be visible within their class and within any subclasses, but not for the rest of the package or for the rest of the world. In other words, I wanted this:

Modifier Class Package Subclass World sub-class YNYN 

However, the Java developers gave me this :

 Modifier Class Package Subclass World public YYYY protected YYYN no modifier YYNN private YNNN 

A typical case is when I want something like this when creating an abstract class. Sometimes I find that an abstract parent needs access to certain members, but specific children too. I can grant them this access by creating protected members, but this makes the rest of the package available when I don't want it.

To be completely understandable, I know that such a modifier is not possible in Java. My question is: why is such a modifier not included in Java? It seems to me that for me a more natural level of visibility than protected or the default. Is it the reason it is not so important to be included, or is it more related to possible side effects that I have not considered?

Answer 1

I believe that they want to avoid extra complexity by having a hierarchy of non-linear access.

You must have control over your package, so just do not call these protected methods there.

(By the way, protected not exactly the same as sub-class and package , because non-static protected methods (if not in one package) cannot be called on arbitrary objects of the declaration class, but only on the objects of the subclass is the code. (You can see this on Object.clone() , which can only be called by a class whose object is being cloned.))

Answer 2

Apparently, they are considering being-a-subtype - not as a "close relationship", but somehow as a package. And for the modifier that you propose, it will not differ much from protected , unless the subclass is in another package, and I would argue that being in one package should be considered as a closer relationship than being a subclass in a completely different package.

Answer 3

You must put your class in your own package and mark it (instance of the variable or method) as protected. Thus, no classes other than subclasses can access this member, which you sell as protected. You will get one class in one package if you desperately want only subclasses to have access to this protected member.