Kotlin

Quick Reference

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