Way I see it, Java and SGML are orthogonal. They can't even be categorized within the same descriptive framework. The only thing they have in common is that they can be transferred from one computer to another using HTTP.
No, strike that. It is in principle possible to write an SGML parser in Java. Indeed, I'd be surprised if nobody is yet writing such a brute.
SGML is about device-independent textual information transfer and storage; Java is about embedded distributable software. The SGML philosophy suggests that a web browser is a whacking great file viewer that should eventually be able to handle other SGML DTD's (beside the HTML one). The Java perspective is that a web browser is a glorified mainframe terminal for the internet, with a little local processing.
What have these visions got in common?
Sod all, that's what.
One web for the librarians, one web for the programmers, and never the twain shall meet.