Data types

Kotlin provides numeric types to represent integer values of varying sizes.

Byte

The Byte type takes up 8 bits and can store values between -128 and 127:

val byteValue: Byte = 100

Short

The Short type takes up 16 bits and can store values between -32,768 and 32,767:

val shortValue: Short = 1000

Int

The Int type is the most commonly used for integers. It takes up 32 bits and can store values from −2^31 to 2^31-1:

val intValue: Int = 123456

Long

The Long type takes up 64 bits and can store values from -2^63 to 2^63-1:

val longValue: Long = 123456789L

Floating-point numbers in Kotlin can be represented by the Float and Double types.

Float

The Float type takes up 32 bits and is generally used when less precision is needed (with 6 to 7 decimal digits of precision):

val floatValue: Float = 3.14f

Double

The Double type is more precise, taking up 64 bits (15 to 16 decimal digits of precision):

val doubleValue: Double = 3.14159265359

The Char type is used to represent a single character, such as letters or symbols:

val letter: Char = 'A'

The Boolean type represents truth values, with possible values true or false:

val isValid: Boolean = true

The String type represents a sequence of characters. Strings are immutable in Kotlin:

val greeting: String = "Hello, Kotlin!"

In Kotlin, arrays are represented by the Array type. They can store values of primitive types or objects:

val numbers = arrayOf(1, 2, 3)
val strings = arrayOf("Kotlin", "Java", "Python")

Besides arrays, Kotlin supports collections, such as lists (List), sets (Set), and maps (Map). These are widely used in situations that call for dynamic data structures.

List

An immutable list (cannot be changed):

val languages: List<String> = listOf("Kotlin", "Java", "Python")

A mutable list (can be changed):

val mutableLanguages: MutableList<String> = mutableListOf("Kotlin", "Java")
mutableLanguages.add("Python")

Set

A set (Set) does not allow duplicate elements:

val uniqueNumbers: Set<Int> = setOf(1, 2, 3, 3)

Map

A map (Map) stores key-value pairs:

val userRoles: Map<String, String> = mapOf("admin" to "Administrator", "user" to "Regular User")

The Any type is the root of the type hierarchy in Kotlin. Every class or type in Kotlin inherits from Any:

val something: Any = "This can be anything"

To work with dates and times on the JVM, use the java.time API, which is immutable and thread safe. The old java.util.Date and java.util.Calendar classes are legacy and should be avoided in new code. In multiplatform projects, the kotlinx-datetime library plays the same role.

LocalDate

import java.time.LocalDate

val today: LocalDate = LocalDate.now()
val nextWeek: LocalDate = today.plusWeeks(1)
println("Today: $today, a week from now: $nextWeek")

LocalDateTime

import java.time.LocalDateTime

val currentDateTime: LocalDateTime = LocalDateTime.now()
println(currentDateTime)

Unit

The Unit type in Kotlin is equivalent to void in other languages, representing the absence of a return value.

fun printMessage(message: String): Unit {
    println(message)
}

Nothing

The Nothing type indicates that a function never returns (used for functions that throw exceptions or enter infinite loops):

fun fail(message: String): Nothing {
    throw IllegalStateException(message)
}