Kotlin

## Quick Reference | Topic | File | |-------|------| | Null safety operators and patterns | `nullsafety.md` | | Coroutines, flows, structured concurrency | `coroutines.md` | | Collections, sequences, data classes | `collections.md` | | Scope functions, extensions, sealed classes | `idioms.md` | | Java interop and common Kotlin mistakes | `interop.md` | | Android lifecycle, Compose state | `android.md` | | Delegation, inline, reified, multiplatform | `advanced.md` | ## Critical Rules ### Null Safety - `!!` asserts non-null — crashes on null, use only when you've already checked - Platform types from Java are risky — add null checks or use `@Nullable`/`@NonNull` annotations - Elvis with `return`/`throw` for early exit — `val name = user?.name ?: return` ### Coroutines - `viewModelScope` auto-cancels on ViewModel clear — don't use `GlobalScope` in Android - Structured concurrency: child coroutine failure cancels parent — use `supervisorScope` to isolate - `StateFlow` needs initial value and never completes — `SharedFlow` for one-shot events - Inject dispatchers for testability — don't hardcode `Dispatchers.IO` ### Collections & Data Classes - `first()` throws on empty — use `firstOrNull()` for safe access - Only constructor properties in `equals`/`hashCode` — body properties ignored - `mutableStateListOf` for Compose — wrapping `mutableListOf` in state won't track changes ### Scope Functions & Extensions - Don't nest scope functions — readability drops fast, extract to named functions - Extensions are resolved statically — not polymorphic, receiver type matters at compile time ### Android/Compose - `repeatOnLifecycle(STARTED)` for flow collection — `launchWhenStarted` is deprecated - `remember` survives recomposition only — use `rememberSaveable` for config changes - `collectAsStateWithLifecycle` is the gold standard — lifecycle-aware + Compose state ### Java Interop - `==` is structural equality in Kotlin — `===` for reference, opposite of Java - SAM conversion only for Java interfaces — Kotlin interfaces need explicit `fun interface` - `@JvmStatic`, `@JvmOverloads`, `@JvmField` for Java-friendly APIs