Kotlin basics — variables and types

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Goal

Learn the minimum Kotlin vocabulary needed to read every later page in this tier. After this page you will know how to declare variables, write a main function, and name the types you'll use to model the list items.

Prerequisites

• Android Studio installed

How to run the snippets

Every snippet on this page runs in the Kotlin Playground — paste, click Run. You don't need Android Studio for these.

A program

A Kotlin program starts at a function called main:

fun main() {
    println("Hello, Mei!")
}

• fun declares a function.
• main is the function name the runtime calls first.
• println writes a line to standard output.

Variables: val and var

Kotlin has two ways to declare a variable.

• val — the value cannot be reassigned (read-only).
• var — the value can be reassigned.

fun main() {
    val name = "Aarav"
    var count = 0
    count = count + 1
    println("$name has $count messages")
}

Run output:

Aarav has 1 messages

Reassigning a val is a compile error. Prefer val unless you have a reason to mutate.

Types

Kotlin infers types from the value. You can also write them explicitly:

val name: String = "Sofia"
val age: Int = 28
val height: Double = 1.65
val isAdmin: Boolean = false

The four types you'll meet in this tier:

Type	What it holds	Example
Int	whole number	42
Double	decimal number	3.14
Boolean	true or false	true
String	text	"hello"

For the full list, see the Kotlin basic types reference. The four above cover this tier.

String templates

A $variable inside a string interpolates its value. Use ${expression} when the expression is more than a name:

fun main() {
    val name = "Olu"
    val notifications = 5
    println("$name has $notifications notifications")
    println("Tomorrow $name will have ${notifications + 3}")
}

Output:

Olu has 5 notifications
Tomorrow Olu will have 8

For frontend developers

val and var in Kotlin map closely to const and let in modern JavaScript. String templates use $name and ${expr} instead of JavaScript's ${name} everywhere — the bare-name form is the difference.

Next → Kotlin operators