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oh just found this: https://www.meetup.com/de-DE/Kotlin-User-Group-Munich/events/253612919/#
a meetup in Munich, where they talk about Kotlin for the backend - I will go there and tell, what I learned

Cool, I will wait your feedback.

yeah, tutorials... I mean, you can mix java and kotlin
write in whatever you want ... I think, as soon as you have enough experience, you can always use one for the other.
Just copy/paste from stackoverflow won't work that smooth :)