Kotlin

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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