I just came across an error when using NetBeans, and I wanted to post my findings (possible this is old news). Perhaps this will save some people the headache of debugging a similar problem that I just encountered. I don't think I'm abusing anything here... stack overflow's SEO is pretty good :)
I refactored a class in NetBeans (JavaFX project), and from that point on I got a "java.lang.NoClassDefFoundError" for that particular refactored class.
I fixed the error by simply doing a "Clean and Build Project." This apparently deleted whatever cache file or erroneous file/class reference. I don't think this is a Netbeans bug... hoping it was just one of those "flukey" moments.
Cheers!