If I remember correctly, wolfenstein use bsp map format.
Correct me if I m wrong but there is directly visibility data inside the bsp tree, so you just have to render all sprites inside the visible zone computed by vis.

Look at Nat-punchtrough ;-)
It's a way to make connection possible with server behind routers.
I've achieved that in C# but because sockets and IP protocol are the same in all languages I think you can do that in java.