Kotlin Variables and Data Types

val and var

Kotlin has two keywords for variable declaration: val (immutable, like Java's final) and var (mutable). Prefer val whenever possible.

val pi = 3.14159      // immutable — cannot reassign
var count = 0         // mutable — can reassign
count = 5             // OK
// pi = 3.14          // ❌ Error: val cannot be reassigned

Type Annotations

Kotlin infers types automatically, but you can be explicit:

val name: String = "Alice"
val age: Int = 25
val height: Double = 1.75
val isStudent: Boolean = true

// Type inference
val city = "London"    // String
val score = 100        // Int
val ratio = 0.85       // Double

Basic Data Types

val i: Int = 42
val l: Long = 10_000_000L
val d: Double = 3.14
val f: Float = 3.14f
val b: Boolean = true
val c: Char = 'A'
val s: String = "Kotlin"

String Templates

val name = "Alice"
val age = 25
println("My name is $name and I am $age years old.")
// Expression inside template:
println("Next year I will be ${age + 1}")

Nullable Types

var nonNull: String = "hello"    // cannot be null
var nullable: String? = null     // can be null

// Safe call operator
println(nullable?.length)        // null (no crash)
// Elvis operator
val len = nullable?.length ?: 0
println(len)                     // 0