Kotlin Variables and Data Types

Kotlin is a modern, concise, and type-safe programming language used for Android development, backend systems, desktop apps, and more. One of the core foundations of Kotlin is how it handles variables and data types. Understanding these is essential because they define how your program stores and manipulates information.

This post covers:

• What variables are
• Mutable vs immutable variables
• Type inference
• Kotlin’s built-in data types
• Strings, arrays, collections
• Null safety
• Examples for each concept

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1. What is a Variable in Kotlin?

A variable is a container that stores data in memory.

In Kotlin, you define a variable using two keywords:

• val → immutable (cannot be changed)
• var → mutable (can be changed)

Kotlin encourages immutability because it makes your code safer and more predictable.

2. Immutable Variables (val)

Use val when the value should not change after being assigned.

Example:

val name = "Luke"
val age = 30

Here:

• name and age cannot be reassigned.
• Trying to change them will produce a compilation error.

Immutability is best practice for most variables.

3. Mutable Variables (var)

Use var when the value needs to change.

Example:

var score = 10
score = 20   // ✔ allowed

Use var for variables like counters, states, and temporary values.

4. Type Inference in Kotlin

Kotlin is a statically typed language, meaning variable types are known at compile time.
However, you do not always need to specify the type because Kotlin can infer it.

Example:

val city = "Lagos"    // inferred as String
val temperature = 28  // inferred as Int

If you want, you can explicitly set the type:

val city: String = "Lagos"
val temperature: Int = 28

5. Kotlin Basic Data Types

Kotlin includes several built-in primitive data types, but unlike Java, they are all objects under the hood (no primitive vs wrapper distinction).

Numerical Types

Integer Types

Type	Range
Byte	-128 to 127
Short	-32,768 to 32,767
Int	-2^31 to 2^31-1
Long	-2^63 to 2^63-1

Example:

val count: Int = 100
val population: Long = 7_800_000_000

The underscore _ improves readability.

Floating-Point Types

Type	Precision
Float	6-7 decimal digits
Double	15-16 decimal digits

Example:

val percentage: Float = 99.5F
val pi: Double = 3.14159265358979

Note the F suffix for Float.

Character Type (Char)

Represents a single Unicode character.

val initial: Char = 'K'

Characters use single quotes.

Boolean Type (Boolean)

Stores logical values: true or false.

val isOnline: Boolean = true
val isSubscribed = false

6. Strings in Kotlin

Strings are sequences of characters enclosed in double quotes.

Example:

val name = "Lawson"

Kotlin also supports string templates, allowing you to insert variables directly:

val age = 25
println("My name is $name and I am $age years old.")

For expressions, use ${ }:

println("Next year I will be ${age + 1}")

Multi-line Strings

Use triple quotes """:

val message = """
    Hello Kotlin!
    Welcome to multiline strings.
""".trimIndent()

7. Arrays

An array stores multiple values of the same type.

Creating arrays:

val numbers = arrayOf(1, 2, 3, 4)
val names = arrayOf("Luke", "James", "Ada")

Access elements:

println(numbers[0])  // prints 1

Modify elements:

numbers[1] = 20

8. Kotlin Collections (Important)

Kotlin collections come in two major types:

Immutable Collections

• listOf()
• setOf()
• mapOf()

Mutable Collections

• mutableListOf()
• mutableSetOf()
• mutableMapOf()

Example:

val fruits = listOf("Apple", "Orange")         // cannot change
val numbers = mutableListOf(1, 2, 3)           // can change
numbers.add(4)

9. Kotlin Null Safety

Kotlin has built-in protection against null pointer exceptions.

By default, variables cannot hold null.

val name: String = "Luke"
// name = null  ❌ error

To allow null, you must add ?

Example:

var middleName: String? = null

You must safely handle nullable variables using:

• ?. (safe call)
• ?: (Elvis operator)
• !! (non-null assertion – risky)

Example:

println(middleName?.length)   // safe call (returns null)
println(middleName ?: "N/A")  // Elvis (default value)

10. Type Conversion

Kotlin does not automatically convert between different types.

To convert:

val num = "123".toInt()
val amount = 45.toDouble()

Common conversion functions:

• toInt()
• toLong()
• toFloat()
• toDouble()
• toString()

11. Putting It All Together (Example Program)

fun main() {

    val name: String = "Kotlin Student"
    var score: Int = 90

    val pi = 3.14      // inferred as Double
    val isActive = true

    var nickname: String? = null

    val numbers = mutableListOf(1, 2, 3)
    numbers.add(4)

    println("Name: $name")
    println("Score: $score")
    println("PI: $pi")
    println("Active: $isActive")
    println("Nickname: ${nickname ?: "No nickname"}")
    println("Numbers: $numbers")
}

Summary

Concept	Explanation
val	Immutable variable
var	Mutable variable
Type inference	Kotlin automatically detects types
Basic Types	Int, Double, Boolean, Char, String
Nullable types	Use ? to allow null
Collections	Immutable vs mutable lists, sets, maps
Type conversion	Manual conversions needed