A beginner Kotlin cheat sheet is the language before architecture: val vs var, type inference, if/when as expressions, functions, lists, and the first null-safe call. On Android you will type these inside activities or composables; the grammar is the same on the JVM backend.
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
- val / var:
valread-only binding.varreassignable. Preferval. - Types:
Int,Long,Double,Boolean,String. Inference:val n = 3. - String tpl:
"Hello \$name"and"\${user.email}". - if / when: Both can return values:
val x = if (ok) 1 else 0. - Functions:
fun add(a: Int, b: Int): Int = a + b. - Lists:
listOf(1, 2),for (item in list),list.map { it * 2 }. - Null:
String?,?.,?:. Avoid!!while learning. - Run: Android Studio / IntelliJ scratch file, or
kotlincfor JVM. Android needs a device/emulator.
Copy-paste examples
When expression and a list transform
Type this until it is boring. Then add null safety on a String?.
fun grade(score: Int): String = when {
score >= 90 -> "A"
score >= 80 -> "B"
score >= 70 -> "C"
else -> "F"
}
fun activeTitles(rows: List<Pair<String, Boolean>>): List<String> =
rows.filter { it.second }.map { it.first }Common mistakes
- Starting with coroutines, Dagger, and Compose together on day one.
- Using
!!because the compiler complained. - Writing Java in Kotlin files (
System.out.printlneverywhere is fine, but skip getters). - Ignoring error messages in red — Kotlin’s compiler is trying to teach null safety.
FAQ
- Do I need Java first? Helpful, not required. You will still read Java docs. Learn Kotlin types and nulls first if Android is the goal.
- What is a package? A namespace matching folders:
package com.example.app. Imports bring types into a file.
- How do I print?
println("hi"). In Android,Log.d("TAG", "hi")shows in Logcat.
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.