Kotlin

Build robust Android and multiplatform apps with Kotlin idioms, coroutines, and null safety.

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