In Kotlin classes, val and var are used to declare properties.

• val means read-only after initialization
• var means mutable / can be reassigned

Basic example

class User {
    val id: Int = 1
    var name: String = "Alice"
}

Usage:

fun main() {
    val user = User()

    println(user.id)      // 1
    println(user.name)    // Alice

    user.name = "Bob"     // OK: name is var

    // user.id = 2        // Error: id is val
}

val inside a class

Use val when the property should not be reassigned after it gets a value.

class Product {
    val sku: String = "ABC-123"
}

You can read it:

val product = Product()
println(product.sku)

But you cannot assign a new value:

// product.sku = "XYZ-999" // Not allowed

var inside a class

Use var when the property can change.

class Counter {
    var count: Int = 0

    fun increment() {
        count++
    }
}

Usage:

fun main() {
    val counter = Counter()

    counter.increment()
    counter.increment()

    println(counter.count) // 2
}

Declaring properties in the constructor

A common Kotlin style is to put properties directly in the class constructor.

class User(
    val id: Int,
    var name: String
)

This creates a class with:

• a read-only id
• a mutable name

Usage:

fun main() {
    val user = User(1, "Alice")

    println(user.id)
    println(user.name)

    user.name = "Bob" // OK

    // user.id = 2    // Error
}

Important distinction

If you write this:

val user = User(1, "Alice")

The variable user itself cannot point to another User, because it is a val.

But if the object has var properties, those properties can still change:

val user = User(1, "Alice")

user.name = "Bob" // OK, because name is var

// user = User(2, "Charlie") // Error, because user is val

So:

val user

means the reference cannot be reassigned.

var name

inside the class means the property can be changed.

Rule of thumb

Use val by default, and only use var when the value really needs to change.

class Person(
    val birthYear: Int,
    var displayName: String
)

Here, birthYear probably should not change, but displayName might.