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Kotlin
JDBC with use { } — Auto-Close Connections
Plain JDBC is fine for simple cases. Wrap connections / statements / result sets in `use { }` for guaranteed cleanup even on exceptions.
@admin
Kotlin
Coroutine Testing with runTest
`runTest { }` from kotlinx-coroutines-test gives you a virtual time scheduler — `delay(1000)` finishes instantly while preserving ordering. No more flaky 1-second test sleeps.
Kotlin
Compose LaunchedEffect — Side Effects
`LaunchedEffect(key)` runs a coroutine when the key changes. Use for one-shot init, snapshot listening, data loading, or any suspend work tied to a composition.
Kotlin
Compose State Hoisting
Lift state UP to where it's needed. A composable becomes "stateless" (taking `value` + `onValueChange`) which makes it reusable and testable. The Compose mantra.
Kotlin
Ktor Server — Middleware (Plugins)
Plugins wrap every request — logging, CORS, auth, rate limiting. Install with `install(Plugin) { config }` at the application level.
Kotlin
kotlinx.serialization — JSON Basics
Compile-time code generation for JSON serialization. Annotate `@Serializable` on a data class; `Json.encodeToString` and `Json.decodeFromString` are typed.
Kotlin
Ktor Client — GET Request
Ktor is JetBrains' Kotlin-first HTTP client. Multiplatform (JVM, JS, Native). Coroutine-native — every call is a suspend function.
Kotlin
use { } — Auto-Close Resources
`use { }` calls `close()` on any `Closeable` (file, stream, JDBC connection, …) when the block exits — even on exception. Kotlin's try-with-resources.
Kotlin
Write to a File
`writeText` overwrites; `appendText` appends. For multiple writes, use `bufferedWriter().use { }` for efficiency + auto-close.
Kotlin
Read a File — Whole, Lines, or Stream
Stdlib extensions on `java.io.File` and `java.nio.file.Path`. Use `readText()` for small files, `useLines { }` for memory-efficient line-by-line.
Kotlin
Star Projections (List<*>, etc.)
`List<*>` means "list of something I don't need to know exactly" — read-only with Any? as element type. Useful when you genuinely don't care about the parameter.
Java
ReentrantLock — Beyond synchronized
`ReentrantLock` does what `synchronized` does, plus: tryLock with timeout, fair queueing, interruptible acquire, multiple condition variables. Use when synchronized's rigidity bites.
Kotlin
Flow — Cold Async Streams
`Flow<T>` is a coroutine-based reactive stream — like Sequence but async. Cold (each collector restarts the producer) and respects cancellation. The backbone of modern Kotlin/Android reactive code.
Kotlin
Coroutine Cancellation
Cancellation is cooperative — your coroutine must check via `ensureActive()`, `yield()`, or any other suspending call. CPU-busy loops without a suspend point are NOT cancellable.
Kotlin
launch and Jobs
`launch` starts a coroutine and returns a `Job` you can join, cancel, or check status on. Fire-and-forget or join-when-ready style.
Kotlin
distinct, distinctBy, intersect, union
Set-style ops on collections. `distinct` removes duplicates; `distinctBy` lets you key the dedupe by a derived value; `union/intersect/subtract` give you set operations.
Kotlin
zip and zipWithIndex
`zip` pairs elements positionally; the result is the length of the shorter list. `withIndex()` gives you `(index, value)` pairs without the manual `forEachIndexed`.