Better than initially assessed: Chapter 2 introduces collections, good variance coverage (with a Pet hierarchy), covers extensions and their role in the stdlib, two lambda/HOF chapters. Unusual ordering: generics before lambdas. map/filter/fold deferred to lambda chapters. Coroutines appendix exists but dated.
Head First Kotlin uses the usual Head First spiral approach - introduce a concept lightly, let it sit, circle back with depth later - which meant it deferred anything not strictly load-bearing to the appendices. Labeled breaks and enum classes, both things I’d consider core language features, got pushed all the way to Appendix C rather than covered in the main chapters. That appendix is even honest about it in the title: “The Top Ten Things (We Didn’t Cover)” - ten separate topics, including packages, visibility modifiers, sealed classes, and object declarations, that never made it into the twelve main chapters at all.
Compare that to Head First Go, which felt reasonably complete without needing to punt a top-ten list to the back of the book - but that’s not really a fair fight. Go is a deliberately small language by design (that’s Rob Pike’s whole philosophy. Minimal surface area, few ways to do anything), so a Head First treatment of it has a real shot at covering the whole language in the main chapters. Kotlin’s surface area is just bigger: null safety, data classes, sealed classes, extension functions, coroutines, object expressions/declarations, operator overloading. So a book aimed at teaching it from scratch in a similarly-sized page count is going to hit a wall and have to defer something. It’s less “Head First Kotlin did a worse job” and more “the two books picked structurally different-sized languages to cover with the same format.”
Full book notes.