That's fair - what I meant by it being a different career isn't that skills aren't transferable at all but that you solve different kinds of problems entirely, as distinct from a difference in stack. And if you hired me then as a mobile developer, I'd have quit, so you don't want to hire non mobile engineers into mobile roles. Language/stack is a red herring here in the sense that Java backend development is a lot closer to Ruby backend development than it is to Android development.