There's a dearth of materials on how to package Java applications for Nix that aren't pre-built.
So here's how to do it from first principles; no fancy Nix machinery at all, just bare bones Java source files all the way to a runnable script that can be directly invoked.
To test this simple example out, assuming you have Nix already installed, the following suffices:
nix-build simple-java-example.nix
result/bin/Hello
This should result in Hello World! being printed out to the screen.