val vs var

Declaring with val

val creates a reference that can be assigned a value exactly once. Attempting to assign a new value to a val after that is a compile-time error, which helps prevent accidental changes.

Note: Prefer val everywhere possible; it makes code easier to reason about.

Example: Declaring with val

fun main() {
    val name = "Kotlin"
    println("Language: $name")
}

Declaring with var

var creates a mutable reference whose value can be changed later using =. It is used when a value genuinely needs to change over the program's lifetime, such as a running counter.

Example: Declaring with var

fun main() {
    var counter = 0
    counter = counter + 1
    counter += 1
    println("Counter is now $counter")
}

val Reference vs Mutable Contents

A val only locks the reference itself, not necessarily what it points to. A val holding a mutable list still lets you add or remove items -- you just cannot point that val at a different list afterward.

Note: If you need the contents to be unchangeable too, use an immutable collection type.

Example: val Reference vs Mutable Contents

fun main() {
    val items = mutableListOf("apple", "banana")
    items.add("cherry")
    println(items)
}

Choosing val Over var

Idiomatic Kotlin favors val by default and reaches for var only when a variable must be reassigned. This habit reduces bugs caused by unexpected mutation and makes code easier to follow.

Example: Choosing val Over var

fun main() {
    val pi = 3.14159
    var radius = 2.0
    radius = 3.0
    println("Area: ${pi * radius * radius}")
}