Java classes don't see the scala package

If you want to freely mix Java and Scala sources, meaning some Scala files depend on some Java files and some Java files depend on some Scala files, afair you should put your Java files into src/main/scala as the Scala plugin supports joint compilation.

If you only have one-sided dependencies, meaning you only depend from Java classes to Scala classes but not the other way around, you could also configure the compilation and dependency order to be like that.