In a standard scripting language the error will show up on the screen. In a java app, it will show in the java console.
I have done some java web app testing and I know as far as compatibility testing goes, you will need to not only test your supported browsers and operating systems but also the various JVMs. (So make sure you have a "clean" box might be more in order)
Otherwise I believe I used our general testing practices. (I might be missing something because it has been a little while since I tested a java application)