Functional Java: A Beginner's Guide

It's been a while since I've done Java and Android, but I just remember like I did everything for iOS first because my client is mostly a Mac and iOS based kind of company. And then part of their team always had Android, so then I had to duplicate the app and Android. It was all Objective C stuff. So it was a really long time ago. But this is before we had functional Java, which I think came out a long time ago,. This is before way before Kotlin, obviously, and this is before even Swift.