There’s the Debian Java Policy, but I’m not sure how up-to-date it is.
Then, it really depends on the buildsystem your package is using. Java programmers use a variety of build systems — ant, maven, no buildsystem at all — and packaging heavily depends on this.
In the simplest case, if your package is using Ant, you might come with a simple debian/rules containing just dh $@; if you have ant installed, then it’ll be automatically picked as the buildsystem.
If your package uses maven, you might take a look at maven-debian-helper and maven-repo-helper. Also read the related mh_* manpages, you’ll need them :)
If your package doesn’t use any buildsystem, then it’s your duty to build it. There’s the javahelper package taking care of this: it provides jh_build (and other commands), with which you can build a .jar. You can find the documentation at /usr/share/doc/javahelper/tutorial.html.
Obviously, there might be other buildsystems I'm not aware of, and, if it's not supported by Debian tools, javahelper could be a last resort. :)