Basic Types and Type Inference

Every value in Kotlin has a type, which decides what you can do with it. The basic types:

Type inference

Kotlin works out the type from the value you assign:

fun main() {
    val count = 10
    val price = 9.99
    val initial = 'A'
    val title = "Kotlin"
    val ready = true
    val big = 3_000_000_000

    println(count::class.simpleName)
    println(price::class.simpleName)
    println(initial::class.simpleName)
    println(title::class.simpleName)
    println(ready::class.simpleName)
    println(big::class.simpleName)
}

Int
Double
Char
String
Boolean
Long

3_000_000_000 is too big for an Int, so Kotlin makes it a Long automatically. Underscores in numbers are just for readability.

Type annotations

You can write the type explicitly after a colon:

val score: Int = 100
val ratio: Double = 3.0
val id: Long = 42

Annotations are useful when you declare a variable before assigning it, or to make the intended type clear.

Types are fixed

A variable keeps its type; Kotlin never silently converts between types:

fun main() {
    var level = 1
    level = "two"
}

error: assignment type mismatch: actual type is 'String', but 'Int' was expected.

Common mistakes

• Writing val ratio: Double = 3 — a whole-number literal isn't automatically a Double. Write 3.0.