Hey, thanks for the answer, yes I am using Eclipse and stateless4j-2.4.0.jar is in the classpath. I have now realised that this also appears for each stateless4j-2.4.0.jar class:
Dx bad class file magic (cafebabe) or version (0033.0000)
…while parsing com/github/oxo42/stateless4j…
Maybe this library is only supported by java 7 or above?