Kotlin idioms

Kotlin idioms

Kotlin's value is making illegal states unrepresentable with less ceremony: non-null types by default, values as data classes, asynchrony as suspending functions. Java habits imported wholesale forfeit exactly that value.

Method

1. Design types non-null first. String? is a decision, not a default; push nullability to the edges and convert early (?: throw, requireNotNull with a message). Use ?. and ?: for genuine absence; every !! is a latent NPE with your name on it: acceptable in tests, a review flag in production code. Platform types from Java are unchecked: annotate your Java (@Nullable/@NotNull) or wrap it at the boundary (see null-handling).
2. Model values as data classes and sealed hierarchies. data class for value carriers (copy() for updates, val everywhere: see immutability-defaults); sealed interface plus when for closed alternatives, letting the compiler enforce exhaustiveness (the exhaustive-switches discipline, built in). Prefer immutable List/Map interfaces in signatures and toList() copies at boundaries; Kotlin's read-only views are not deep immutability (see java-collections).
3. Use scope functions sparingly and conventionally. let for null-guarded transforms, apply for object configuration, also for side effects in a chain: one level deep. Nested lets with shadowed its are write-only code; a local val and an if reads better (see cognitive-load).
4. Write coroutines with structured concurrency. Suspend functions for async work; launch inside a scope tied to a lifecycle (coroutineScope, supervisor scopes for independent children), never GlobalScope. Cancellation is cooperative: check it in loops, keep withContext(Dispatchers.IO) around blocking calls, and treat CancellationException as control flow to rethrow, not an error to log (the go-concurrency and python-asyncio contracts, Kotlin dialect).
5. Keep Java interop deliberate. Expose Kotlin APIs Java can call cleanly (@JvmStatic, @JvmOverloads where Java callers exist); default arguments replace builder/overload ladders for Kotlin consumers. Checked exceptions vanish in Kotlin: document throwing behavior (see rust-error-handling's audience thinking) since the compiler no longer forces callers to see it.
6. Prefer expressions and small functions. Single-expression functions, when as an expression, early returns over nested conditionals (see guard-clauses); extension functions to give existing types domain verbs, kept in the domain's package rather than a global Utils file (see python-project-structure's grab-bag rule).

Boundaries

• Kotlin does not fix the JVM's rules: the memory model (see jvm-memory-model), GC behavior, and collections contracts apply unchanged under the nicer syntax.
• Operator overloading and DSL builders are library-author tools; application code inventing clever operators trades a day of writing for years of "what does this plus mean" (see naming-things).
• Coroutines interop with blocking frameworks needs explicit bridges (runBlocking at the very top only); sprinkling runBlocking inside services deadlocks thread pools.