I've just pushed an update that should prevent roll-backs in progress in scenarios you've described. I'm really sorry for the inconvenience you've experienced. Hopefully, this won't be an issue anymore. Please let me know if you still experience the weirdness with the progress saving.

Hi!

Well, it suppose to save your progress. Do you, by any chance, open topics in new tabs? That might cause issues with saving the progress.

Hi Rafael,

Thanks for your feedback. Why do you think it's incorrect?

Thanks, Paulo!

Thanks!

Thanks for your feedback!

Hi Sergii,

Thanks for your feedback! I plan to publish the book in paper in future, but I'm still not sure when exactly that will happen. If you wish, I'll keep you posted.

Thanks for the idea, Marton. I'll think how those icons can be improved.

Hi, Sunil!

Thanks for asking! The examples are compatible with Java 8, but the course itself doesn't really focus on certain Java features, it's more language neutral.

I'm closing this for now, because it seems that current illustration does contain simple "isDefined" and "canHandle" checks: