Language cheatsheet

Kotlin 2 Cheatsheet

Kotlin syntax and idioms: null safety, data classes, collections, extension and scope functions, sealed types and coroutines.

Tooling

Check the compiler version — Print the Kotlin compiler version.
Compile and run a file — Compile to a jar and run it with the Kotlin runtime included.
Run a script — Run a Kotlin script file (.kts).
Build and test with Gradle — Build, run the tests and run the application in a Gradle project.

Declare variables — val is read-only and var is mutable. Types are inferred.
Insert values in strings — Use $name or ${expression} inside a string.
Handle nulls — A type with ? can be null. Use ?. to call safely, ?: for a default, and !! only when you are certain.
Branch with when — when is a flexible switch that can be an expression.
Define functions — Parameters can have defaults and be passed by name. Single-expression functions drop the braces.
Check a type and use it — After an is check, Kotlin smart-casts the value to that type.
Validate arguments and state — require checks arguments and check verifies state, both throwing on failure.

Data classes — A data class gets equals, hashCode, toString, copy and destructuring for free.
Classes and constructors — The primary constructor is part of the class header. Properties can have getters.
Sealed types for fixed alternatives — A sealed interface limits its implementations, so when must handle every case.
Singletons and companion objects — object declares a singleton, and a companion object holds class-level members.
Add a function to an existing type — An extension function is called like a member but defined outside the class.
Interfaces and delegation — Implement an interface by delegating to another object with by.
Compute a value on first use — lazy runs its block once, the first time the property is read.

Create collections — listOf, setOf and mapOf are read-only. Use mutableListOf and friends to change them.
Transform and filter — Chain operations on collections with lambdas. it names a single parameter.
Group, sort and count — Group items by a key, sort them and count matches.
Process lazily — Sequences compute items on demand, which avoids intermediate lists for large pipelines.
Use scope functions — let transforms an object, apply configures it, also adds a side effect, and run/with compute a result.
Loop over a map — Destructure entries and ranges in for loops.

Add coroutines — Coroutine support is a separate library. Add it to your Gradle dependencies.
Start coroutines — runBlocking bridges regular code to coroutines. launch starts concurrent work and returns a Job.
Run work in parallel — async starts work that returns a value, and await gets it. Both calls below run at the same time.
Switch dispatcher — Run blocking or CPU-heavy work on a suitable dispatcher without blocking the caller.
Stream values with Flow — A flow emits values over time and is collected from a coroutine.
Limit how long work can take — Cancel a block that takes too long.

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