Cheat Sheets

Kotlin Cheat Sheet (2026): Syntax, Null Safety & Android

By ArpaNeuro Team September 7, 2026
Kotlin cheat sheet Android

A Kotlin cheat sheet is a short map of the language Google recommends for Android. The features you will actually search for are null safety (?, ?:, !!.let), data classes, collection operations, and coroutine suspend functions.

This Kotlin cheat sheet is written for people searching for a fast, accurate reference: students, junior developers, and teams shipping production software. Pin it, copy the snippets, and come back when syntax slips.

Quick reference

  • Vars: val immutable, var mutable. Prefer val.
  • Types: Type inference: val name = "Arpa" is a String.
  • Null: String? can be null. String cannot. Use ?. and ?:.
  • When: Replaces most switch statements; it is an expression.
  • Data class: data class User(val id: Int, val email: String) gives equals/copy/toString.
  • Default args: fun greet(name: String = "there").
  • Named args: greet(name = "Humza") — readable at call sites.
  • Collections: listOf, mutableListOf, mapOf. map/filter/firstOrNull.
  • String tpl: \"Hello \$name\" and \"\$price\".
  • SAM: Lambdas implement Java single-method interfaces.

Copy-paste examples

Null-safe user label

Never !! unless a crash is genuinely better than a fallback.

data class User(val name: String?, val city: String?)

fun label(user: User?): String {
    val name = user?.name?.ifBlank { null } ?: "Guest"
    val city = user?.city ?: "Unknown"
    return "$name · $city"
}

Filter a product list

Collection operations are eager by default; use asSequence() for long chains.

data class Product(val title: String, val price: Int, val active: Boolean)

fun catalog(items: List<Product>): List<String> =
    items.filter { it.active && it.price > 0 }
        .sortedBy { it.price }
        .map { "${it.title} — $${it.price}" }

Common mistakes

  • Using !! everywhere “to make the compiler quiet” — that is a crash operator.
  • Declaring var for values that never change.
  • Updating UI from a coroutine started on Dispatchers.IO without switching to Main.
  • Comparing data classes with === (identity) instead of == (equals).
  • Copying Java getX() style into Kotlin instead of properties.

FAQ

  1. Kotlin vs Java for Android? New Android apps should be Kotlin. Java still appears in older modules. Kotlin interoperates with Java, so you can migrate file by file.
  1. Is Kotlin only for Android? No. Kotlin runs on JVM, JS, and Native. Most job posts still mean Android + backend Ktor/Spring.
  1. Do I need coroutines on day one? Learn val, null safety, and collections first. Add coroutines when you hit network calls.
  1. What is a companion object? A place for “static” members on a class. Prefer top-level functions when you do not need a namespace.

Related Kotlin cheat sheets

Build with this stack

When a cheat sheet is not enough — you need a production app, a student FYP, or a custom dashboard — ArpaNeuro builds mobile app development and also sells ready-made source code. Request a quote and tell us the stack.

Browse software development services or the source code marketplace if you want a working codebase instead of starting from a blank file.

← All Articles Get a Quote →