Is val the same as a constant in Kotlin?

val is read-only — you cannot reassign it after the first assignment. However, its content can still change if it references a mutable object. For true compile-time constants, use const val.

Should I use var or val by default?

Always prefer val. It communicates that the value will not change, making your code safer and easier to reason about. Use var only when the value genuinely needs to change.

Beginner 8 min readKotlin 2.0

Kotlin Variables — val, var, and Type Inference Explained

Kotlin has two ways to declare variables: val for read-only values and var for mutable ones. This tutorial explains both with practical examples.

What You Will Learn

How to declare variables with val and var
The difference between val and var
How type inference works in Kotlin
When to use explicit types
Common variable mistakes to avoid

Declaring Variables in Kotlin

In Kotlin, you declare variables using two keywords: - **val** — read-only (like a constant after assignment) - **var** — mutable (can be reassigned) Kotlin can infer the type automatically from the assigned value, so you rarely need to write the type explicitly.

Syntax

kotlin
val variableName = value
var variableName = value
val variableName: Type = value

Basic Variable Declaration

kotlin
fun main() {
    val language = "Kotlin"
    var year = 2026

    println(language)
    println(year)

    year = 2027
    println(year)
}

Output

Kotlin 2026 2027

language is a val — it cannot be reassigned after declaration. year is a var — it can be updated, and we change it from 2026 to 2027.

Beginner Tip: Default to val. Switch to var only when you genuinely need to change the value later. This habit makes your code safer and easier to understand.

Common Mistake: Trying to reassign a val causes a compile error: "Val cannot be reassigned". Always check which keyword you need before declaring.

Type Inference

Kotlin automatically infers the type from the assigned value. When you write val name = "Kotlin", Kotlin knows name is a String. You do not need to write val name: String = "Kotlin" unless you want to be explicit.

Inferred vs Explicit Types

kotlin
fun main() {
    val name = "Kotlin"          // inferred as String
    val age: Int = 25            // explicit Int
    val price = 9.99             // inferred as Double
    val isActive = true          // inferred as Boolean

    println(name)
    println(age)
    println(price)
    println(isActive)
}

Output

Kotlin 25 9.99 true

Kotlin infers the type from the right-hand side value. You can still write the type explicitly if it makes the code clearer — both styles are valid.

Beginner Tip: Use explicit types when the type is not obvious from the value, or when you want to declare a variable without assigning a value immediately.

Declaring Without Immediate Assignment

Sometimes you want to declare a variable and assign it later. In this case, you must write the type explicitly — Kotlin cannot infer the type from nothing.

Declare First, Assign Later

kotlin
fun main() {
    val result: Int
    result = 10 + 5
    println(result)
}

Output

result is declared as Int but assigned later. Kotlin ensures you cannot read result before it is assigned — this prevents uninitialized variable bugs.

For var, you must assign before reading. For val, you can declare first and assign once — but not twice.

Practice Exercise

Exercisefix bug

Fix the bug in this code: val score = 100 score = 200 println(score)

kotlin
val score = 100
score = 200
println(score)

Quick Quiz

Which keyword declares a read-only variable in Kotlin?

Kotlin Variables — val, var, and Type Inference Explained

What You Will Learn

Declaring Variables in Kotlin

Syntax

Basic Variable Declaration

Type Inference

Inferred vs Explicit Types

Declaring Without Immediate Assignment

Declare First, Assign Later

Practice Exercise

Quick Quiz

Frequently Asked Questions

Related Tutorials