CodeBlanks Logo Icon CodeBlanks
Login Register

val and var

Learn to declare variables with val (immutable) and var (mutable).

val and var

In Kotlin you declare two kinds of variables. A variable declared with val (value) is assigned once and cannot be changed afterwards; this is called immutable. A variable declared with var (variable) can be reassigned later; this is called mutable.

Kotlin recommends using val by default and switching to var only when the value really needs to change.

val pi = 3.14    // immutable
var counter = 0   // mutable
counter = counter + 1

Practice Declare a counter

Write a fun main(). Declare var counter = 0. Then increment with counter = counter + 1 and print with println(counter).

Sample solution

Main.kt
fun main() {
    var counter = 0
    counter = counter + 1
    println(counter)
}

Expected output

1

When to use val?

The Kotlin community recommends using val by default. Variables declared with val are safer; since they don't change after assignment, code behavior is more predictable.

You switch to var only for loop counters, fresh user input, or running totals. This habit is especially valuable in concurrent code.

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Which cannot be reassigned?

  • const
  • var
  • val Correct answer
  • lateinit

Explanation: A variable declared with val is assigned once.

2 val and var: fill in the missing part

Question 
____ counter = 0
counter = counter + 1
Answer 
var counter = 0
counter = counter + 1

Explanation: A var variable can be changed later.

3 Which statement is true?

  • val is mutable, var is immutable
  • val is immutable, var is mutable Correct answer
  • Both are immutable
  • Both are mutable

Explanation: val is immutable, var is mutable.

All lessons in Beginner

1 Kotlin basics

  1. What is Kotlin?
  2. Your first program
  3. The main function
  4. Comments
  5. String templates
  6. Summary - Kotlin basics

2 Variables

  1. val and var
  2. Type inference
  3. const
  4. lateinit
  5. by lazy
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. Boolean and Char
  4. String
  5. Any and Unit
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Elvis operator
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String templates
  3. String methods
  4. substring
  5. split and replace
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. if expression
  4. when
  5. when expression
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Creating arrays
  2. Array access
  3. Array size
  4. Array loop
  5. Array methods
  6. Summary - Arrays

11 Lists

  1. Creating lists
  2. List operations
  3. List access
  4. List iteration
  5. List methods
  6. Summary - Lists

12 Maps and Sets

  1. Creating sets
  2. Set operations
  3. Creating maps
  4. Map operations
  5. Map iteration
  6. Summary - Maps and Sets

13 Functions

  1. Defining functions
  2. Return values
  3. Expression body
  4. Default parameters
  5. Named arguments
  6. Summary - Functions

14 Null safety

  1. Nullable types
  2. ?, !! and ?:
  3. let
  4. Null check
  5. lateinit
  6. Summary - Null safety

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. Regex
  2. matches and find
  3. replace
  4. StringBuilder
  5. trim and pad
  6. Summary - Advanced strings

17 Enums

  1. enum class
  2. Enum values
  3. Enum properties
  4. when with enum
  5. Enum and range
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. try expression
  5. Custom exceptions
  6. Summary - Exceptions

19 Class basics

  1. class
  2. Constructor
  3. Properties
  4. Methods
  5. this
  6. Summary - Class basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together