I’m into Kotlin, because it’s a new (quite new) programming language, that solves all the Java problems, especially on Android. You can simply use Lambda expressions and much more on any API version. I also used Kotlin in production in my open sourced newsreader app NewsCatchr. Here I’ll share some of the sources, that helped me getting some knowledge in Kotlin:
The documentation is a really helpful source. I learned Kotlin myself through practice and every time I didn’t understand something I went looking in the docs and they already answered most of my questions, for example how to use any of the many helper functions in the standard library.
There are a couple of well explaining videos about Kotlin and how to use Kotlin, for example this one by Jack Wharton (for people, who want to code for Android):
Antonio Leiva, developer at Plex, is an experienced Android developer and also Kotlin programmer. On his website he posts from time to time some tutorials about Kotlin (most of them are in relation to Android). When you want to check his site out, get started with this one:
If you are still unsure about Kotlin, this post on Medium.com might be interesting:
Learning by doing
That’s possibly the best way to become a power Kotlin developer. Maybe just try to convert your existing Java program / app to Kotlin. You may get started with the automatic converting tool in the IntelliJ IDEA (or Android Studio). I did it and with doing so, I became very familiar with Kotlin.