GNU Classpath does not accept any code that has a non-free license, or that was automatically generated from code with a non-free license. The standard Java API contains numerous classes from the omg.org domain that are normally generated from the IDL files, released by the Object Management Group. The "use, but no modify" license of these files counts as non-free. For this reason, the mentioned classes in the GNU Classpath project were written from scratch, using only the official printed OMG specifications. Hence this part of GNU Classpath is as free as any other code in the project.