You don't calculate the MBID. The MBID is attached on every entity you retrieve from MusicBrainz.
When getting releases by DiscID you get a list. Each entry is a release and has an MBID, accessible with getId()
:
for (ReleaseWs2 rel : disc.getReleases()){
log.info("MBID: " + rel.getId() + ", String: " + rel.toString());
}
You then probably want to try the CoverArtArchive (CAA) for every release and take the first cover art you get.
Unfortunately I don't know of any API documentation for musicbrainzws2 on the web. I recommend running javadoc
on all source files.