language-kotlinlisted

# Kotlin ## Null safety - `?` makes nullability explicit in the type — `String?` vs `String`. - Safe-call `?.` returns null instead of throwing. Elvis `?:` provides a default. - **Never use `!!` in production code** — it is a promise you will never break that can't be verified. - Jackson does not null-check collection *elements* even with `jackson-module-kotlin` — a declared `List<Content>` can hold nulls from `{"content":[null]}`. Use `filterNotNull()` (not `filter { it != null }`, which leaves the type as `List<Content?>`), or `@JsonSetter(contentNulls = Nulls.SKIP)` at the boundary. ```kotlin val length = name?.trim()?.length ?: 0 user?.email?.let { send(it) } // null-guard + scoping val ids = payload.content.filterNotNull().map { it.id } ``` ## Data classes and sealed interfaces - `data class` for value containers: auto-generates `equals`, `hashCode`, `toString`, `copy`, and destructuring. - `sealed interface` closes a hierarchy and enables exhaustive `when` without an `else` branch. ```kotlin sealed interface Result<out T> data class Success<T>(val value: T) : Result<T> data class Failure(val error: Throwable) : Result<Nothing> fun handle(r: Result<User>) = when (r) { is Success -> show(r.value) is Failure -> log(r.error) } ``` ## Coroutines — structured concurrency - `suspend` functions must be called from a coroutine or another `suspend` function. - Use `coroutineScope { }` for fan-out — child coroutines are cancelled if one fails. - `withContext(