IncompatibleClassChangeError, very much like its cousin NoSuchMethodError, indicates that the class you depend on has changed since you’ve last compiled.
In my case, the jvm tells me that the method I’ve been calling has changed from static to non-static. That’s a decent enough explanation to start with. You could recompile, fix the compilation errors and should be good to go.
I compiled my changes against the latest build of the library, libA, added libA to the classpath. No luck, the jvm says class org.foo.Test.getInstance method has changed from static to non-static. Er, it was static when I compiled?
Turns out someone had copied over the class org.foo.Test into libB, and changed the method to non-static. A transitive dependncy I had pulled in libB to my class path and here I have two incompatible definitions of the same class in my classpath. One of them gets pulled when I compile and the other when I run. (Someone has been a lazy bum!)
Tip: to find the jar containing a class,
find . -iname *.jar | while read JAR; do jar tvf $JAR | grep 'org.foo.Test' && echo $JARF ; done