Does Java has any concept of Subpackage?

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Java: Subpackage visiblity? 

This is a continuation of question Java: Subpackage visiblity?. The reason for asking is to draw attention of broader community.
Question Is there any relation between two packages: odp.proj and odp.proj.subpackage?
Someone tried to answer it here but failed to describe it.

No, there's no relation between odp.proj and odp.proj.subpackage from the Java language perspective. 
For code, odp.proj.subpackage would reside in a sub directory of odp.proj, and some tools and IDE's might group them together in a UI, but regarding language access control, or other langage feature there's no relation.
Java does not have the concept of a subpackage, so odp.proj.subpackage and odp.proj are as different from each other as any other differently named packages.

Short answer
The exact feature you are looking for doesn't exist.
Long Answer
They considered doing it for Java 1.7, but the case is dormant.
As far as what's actually real, there is no relationship at all between odp.proj and odp.proj.subpackage other than a directory structure, the protected keyword and things like import odp.proj.*; - except that statement doesn't get any classes from odp.proj.subpackage
One way to approach what you want is to release an API specification for what you want to expose and leave the rest undocumented or privately documented. Possibly you could even obfuscate the private classes in each release using something like ProGuard.