Kotlin as the official docs teach it — the language of modern Android and much JVM backend work. Null safety end-to-end (nullable types, safe calls, the Elvis operator, platform types), val/var and immutability, data classes, sealed classes and exhaustive when, extension functions and lambdas with receivers, the scope functions (let, run, with, apply, also — and which returns what), collections and sequences (eager vs lazy), coroutines (suspend functions, structured concurrency, Dispatchers), delegation (by lazy, class delegation), generics with declaration-site variance (in/out), and Java interop. Predict-the-output and spot-the-bug items test real understanding; every explanation names the concept, the classic confusion, and a portable rule. Authored from kotlinlang.org at the current stable release.