In any case, being the good developer I am, after designing the core interfaces, I wrote some unit tests using JUnit 4.  Right click on the file in Package Explorer, hit "Run as JUnit Test" and it fires up in a fast view to show you how you did.  All very handy.  After a few tests, i figured I should build some test suites, but I really didn't want to.  I don't know about anyone else, but I always end up forgetting to register some test or another, and then I create a regression that would get caught by that test, but since it's not registered it doesn't run, and it's just bad.  So I didn't, and continued to run the tests individually.

After half an hour or so so, I had the idea to try right clicking on a package and see if I could run all the test in that package.  I was quite pleased to see that you can.  Same thing works at the source folder and project levels.  Little efficiency improvements like that make Barney a happy boy.  Of course, still have to write the suites if I want to run them from CI or the like, but this project at least will very likely never get there.

If you use Ant, definitely useful.  If not, reenforcement that it's worth a bit of digging to learn about your tools, so you can leverage them most effectively.