In addition to the ever fantastic Kotlin Playground, I regularly use to answer quick questions, as of last summer Matt Godbolt’s Compiler Explorer has gained the ability to analyse various Android formats, including Java, Dex and Kotlin.

Godbolt is suited to answer questions about rather smaller, self-contained snippets of code. Larger binaries and whole apps are best dissected using Apktool or jd-gui, which thankfully are both open source1, 2.

Additionally, JetBrains’ Fernflower comes in handy when you want to dissect some code. RIP Stiver.