Variables and Data Types

Jun 23, 2026Trushi Jasani

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Variables and Data Types

Declaring Variables

Kotlin has two keywords for declaring variables:

Keyword	Meaning	Mutable?
val	Value (constant)	No
var	Variable	Yes

fun main() {
    val name = "Alice"      // immutable — cannot be changed
    var age = 25            // mutable — can be changed

    age = 26                // OK
    // name = "Bob"         // ERROR: val cannot be reassigned

    println("$name is $age years old")
}

Type Inference

Kotlin can infer types automatically:

val city = "Mumbai"         // Kotlin infers String
val population = 20_000_000 // Kotlin infers Int
val temperature = 36.5      // Kotlin infers Double
val isCapital = false       // Kotlin infers Boolean

Explicit Type Declaration

You can also declare types explicitly:

val name: String = "Kotlin"
val version: Double = 1.9
val year: Int = 2024
val isActive: Boolean = true

Basic Data Types

Integer Types

val byte: Byte   = 127             // -128 to 127
val short: Short = 32767           // -32,768 to 32,767
val int: Int     = 2_147_483_647   // ~2.1 billion
val long: Long   = 9_223_372_036_854_775_807L

Floating-Point Types

val float: Float   = 3.14f     // 32-bit, note the 'f' suffix
val double: Double = 3.14159   // 64-bit (default for decimals)

Character Type

val letter: Char = 'K'
val digit: Char  = '9'
val symbol: Char = '@'

Boolean Type

val isKotlinFun: Boolean = true
val isJavaFaster: Boolean = false

String Type

val greeting: String = "Hello, Kotlin!"
val multiline: String = """
    This is
    a multiline
    string
""".trimIndent()

Type Conversion

Kotlin does not do implicit type conversions. You must convert explicitly:

fun main() {
    val intVal: Int = 42
    val longVal: Long = intVal.toLong()
    val doubleVal: Double = intVal.toDouble()
    val strVal: String = intVal.toString()
    val floatVal: Float = intVal.toFloat()

    println(longVal)   // 42
    println(doubleVal) // 42.0
    println(strVal)    // "42"
}

Conversion Functions

Function	Converts To
toByte()	Byte
toShort()	Short
toInt()	Int
toLong()	Long
toFloat()	Float
toDouble()	Double
toChar()	Char
toString()	String

Constants with const

const val PI = 3.14159       // compile-time constant (must be top-level or in object)
const val APP_NAME = "MyApp"

Nullable Variables

By default, variables in Kotlin cannot hold null. Use ? to allow null:

var name: String = "Alice"
// name = null  // ERROR!

var nullableName: String? = "Bob"
nullableName = null  // OK

Summary

fun main() {
    // val = immutable, var = mutable
    val language = "Kotlin"
    var count = 0

    // Explicit types
    val pi: Double = 3.14159
    val flag: Boolean = true

    // Type conversion
    val intNum = 10
    val dbl = intNum.toDouble()

    // Nullable
    var maybeNull: String? = null

    println("$language | $count | $pi | $flag | $dbl | $maybeNull")
}