Java + Kotlin = Javalin :D

Except Oracle lawyers wouldn't like Java being literally used as part of the name, though I hate to say this.

That's why javaslang was renamed to Vavr: https://github.com/vavr-io/vavr

I always thought Kava would be a good name for a Kotlin/JVM framework.

Java was named from the coffee from Java, Indonesia. Kava is a beverage consumed throughout the Pacific Ocean cultures. Similar enough :)

Yeah, I don't see this as an attempt to be another Codepen or jsfiddle. This is just a way to easily test out features of a language, without going through the (sometimes very tedious) installation and setup processes required to code in certain languages locally.

What do they do for languages other than JavaScript?

Ok, then Codeanywhere would be a better choice. Don't you think? ;)

Saying that Codeanywhere is a better choice than TIO is like saying that GMail is a better choice than Netflix.

In both cases, both services were built for different purposes, and neither one can or intends to replace the other.