I've dug myself part way through the Evolution Plugin documentation, and while they're a bit coy about it, I'm given to believe that their interface mechanisms are rather C-centric.
This means you could write plugins in Lua, Python, Perl, probably Ruby and many other languages that support C-style interfaces. There's a chance that you could do it from Java using the JNI (Java Native Interface) mechanism, though chances are you'd do a lot of type translating and data copying along the way.
Let's say it might be possible to do in Java if you really, really wanted to; but it would be painful. I wouldn't recommend it.
You might consider building a plugin in a more C compatible language and then let it do TCP/IP I/O over sockets with a Java app that presents the pretty GUI and does the heavy lifting.