I found a software package (R library on CRAN) distributed under GPL license, which makes use of Java classes. The classes are distributed, but for the sources, a README states: "for access to the java source of .class files, contact XXX". The same author apparently developed both, so that it's not just a modified version of some GPL Java library.
I would like to know if it's possible to distribute part of a software package with a non-free license, while the package is itself under GPL. I have not yet contacted the author, and the Java sources might be under GPL too, but it looks smelly: if they are, why not disrtibute them with the package? I would like to clarify what is permitted by the license first.