Careeroza

val vs var

Declaring Variables

Kotlin has two keywords for variables:

  • val (value) — read-only. Assigned once, cannot be reassigned. Like final in Java.
  • var (variable) — mutable. Can be reassigned later.

Example

Use val for things that do not change and var for things that do.

fun main() {
    val name = "Asha"      // never changes
    var cartItems = 2      // changes as the user shops
    println("$name has $cartItems items")

    cartItems = 3
    cartItems += 1
    println("$name now has $cartItems items")
}

Output

Asha has 2 items
Asha now has 4 items

Reassigning a val is an Error

val city = "Pune"
city = "Mumbai"   // ERROR: Val cannot be reassigned

Tip: Rule of thumb used by Android and backend teams: always start with val. Change it to var only when you really need to reassign. Fewer changing values means fewer bugs.

Declare Now, Assign Later

Example

A val can be declared first and assigned exactly once later, as long as the compiler can see it is assigned before use.

fun main() {
    val discount: Int
    val isPrimeMember = true
    if (isPrimeMember) {
        discount = 20
    } else {
        discount = 0
    }
    println("Discount: $discount%")
}

Output

Discount: 20%