kotlin

Kotlin

Null-safety and closed types win !!. Language first; Android UI: jetpack-compose / android-development. Real async: kotlin-coroutines.

Tooling

• Kotlin 2.x / K2 in new code; respect kotlin in Gradle/version catalog.
• kotlinc warnings handled in scope. kotlinOptions.jvmTarget aligned with the project's minSdk/JDK.
• Style: ktlint / detekt if the repo already uses them.

Organization

src/main/kotlin/.../
  domain/
  data/            # se houver I/O

• Packages by domain. internal for module API.
• Prefer files by public type; avoid Utils.kt giant.
• Top-level functions only for pure and stable helpers.

Language

• val default; var only with a clear lifecycle.
• data class for values; class for identity. value class for IDs.
• sealed class / sealed interface for states (Loading / Ready / Err).
• Exhaustive when without else blind on sealed.
• Extensions for APIs that the type should not know about; not to hide side effects.
• inline / reified only with real gain (not cosmetic).
• Collections: listOf, buildList; mutable only when the contract requires it.

@JvmInline
value class UserId(val value: String)

sealed interface LoadState<out T> {
    data object Loading : LoadState<Nothing>
    data class Ready<T>(val value: T) : LoadState<T>
    data class Err(val reason: String) : LoadState<Nothing>
}

Null-safety

• T? explicit. No !! except local invariant (Java platform type).
• ?. / ?: / requireNotNull with message.
• Java annotations (@Nullable) at JNI/SDK boundaries.

Functions and APIs

• Default args > explosive overloads.
• Named args in booleans (notify = false).
• Result<T> or sealed error — one policy per boundary. Do not mix runCatching silent with domain exceptions.
• typealias only when it clarifies; not to hide Pair<String, String>.

JVM / Android Interop

• @JvmName / @JvmStatic / @JvmOverloads only in the API called from Java.
• Do not expose Unit as a “magic void” in Java callbacks without documentation.
• Coroutines on Android: viewModelScope — details in kotlin-coroutines.

Anti-patterns

• Any / !! cascading
• object singleton with global mutable state
• Assume Compose/@Composable in this skill
• apply/also/let unreadable nested
• lateinit without init guarantee

Integration with other skills

• Coroutines/Flow: kotlin-coroutines
• Android app: android-development
• Compose: jetpack-compose
• Persistence: android-room
• JVM tests: android-unit-testing

Completion criteria

• Honest null-safety; sealed for states
• APIs with clear defaults and named args
• No !! unjustified
• Formatted code; core tests if touched