Kotlin Variables

Learn how to store and manage data in Kotlin

📦 What are Kotlin Variables?

Variables in Kotlin are containers that store data values. Use 'val' for unchangeable values and 'var' for values that can change during program execution. 


// Immutable variable (cannot be changed)
val name = "John"

// Mutable variable (can be changed)
var age = 25
age = 26 // This is allowed

Output:

name: John

age: 26

Variable Types

🔒

val (Immutable)

Cannot be changed after assignment 

val pi = 3.14159
🔄

var (Mutable)

Can be changed after assignment 

var counter = 0
🏷️

Type Declaration

Explicitly specify variable type 

val score: Int = 100
🤖

Type Inference

Kotlin automatically detects type 

val message = "Hello"

🔹 Variable Declaration Examples

Here are different ways to declare variables in Kotlin: 

// Basic variable declarations
val studentName = "Alice"           // String (immutable)
var studentAge = 20                 // Int (mutable)
val isStudent = true                // Boolean (immutable)
var gpa = 3.85                      // Double (mutable)

// With explicit types
val courseName: String = "Kotlin Programming"
var enrollmentCount: Int = 150

// Changing mutable variables
var temperature = 25
temperature = 30                    // OK - var can be changed

// val cannot be reassigned
// studentName = "Bob"              // Error! val cannot be reassigned

Output:

studentName: Alice

studentAge: 20

isStudent: true

gpa: 3.85

temperature: 30

🔹 Variable Naming Rules

Follow these rules when naming variables in Kotlin:

  • Start with letter or underscore: name, _count, userName
  • Use camelCase: firstName, totalAmount, isReady
  • No spaces or special characters: except underscore
  • Case sensitive: Name and name are different
  • Avoid keywords: fun, class, if, etc. 
// Good variable names
val firstName = "John"
val totalPrice = 99.99
val isLoggedIn = false
val user_id = 12345

// Bad variable names (avoid these)
// val 2name = "John"        // Cannot start with number
// val first-name = "John"   // Cannot use hyphen
// val class = "Math"        // 'class' is a keyword

🔹 When to Use val vs var

Choose the right variable type for your needs:

🔸 Use val when:

  • Value won't change (constants, configuration)
  • Mathematical constants (pi, e)
  • User input that stays the same
  • Function results that don't change

🔸 Use var when:

  • Value needs to change (counters, accumulators)
  • User interface states
  • Loop variables
  • Temporary calculations 
// val examples (immutable)
val appName = "My Kotlin App"
val maxUsers = 1000
val pi = 3.14159

// var examples (mutable)
var currentUser = "guest"
var loginAttempts = 0
var isConnected = false

// Updating var variables
currentUser = "john_doe"
loginAttempts = loginAttempts + 1
isConnected = true

🧠 Test Your Knowledge

Which keyword creates a variable that cannot be changed?