Type Inference

How Inference Works

When you write val age = 25, Kotlin looks at the value 25 and infers that age has type Int, without you writing Int anywhere. This happens at compile time, so the type is fixed and checked just as strictly as if you had written it.

Example: How Inference Works

fun main() {
    val age = 25
    val price = 19.99
    val name = "Kotlin"
    println("$name costs $price for someone aged $age")
}

Explicit Type Annotations

You can always write the type explicitly after a colon, such as val age: Int = 25. This is required when there is no initial value to infer from, and can also improve readability in some cases.

Example: Explicit Type Annotations

fun main() {
    val age: Int = 25
    val price: Double = 19.99
    println("Age: $age, Price: $price")
}

Inference and Static Typing

Even though the type is not always written, Kotlin remains statically typed: once inferred, a variable's type cannot change, and assigning a mismatched type is a compile-time error.

Note: Trying score = "high" after this would fail to compile -- the inferred type Int is locked in.

Example: Inference and Static Typing

fun main() {
    var score = 100 // inferred as Int
    score = 105     // fine, still an Int
    println("Score: $score")
}

When Inference Is Not Possible

A val or var declared without an initial value has no basis for inference, so an explicit type is required in that case, and the variable must be assigned before it is used.

Example: When Inference Is Not Possible

fun main() {
    val result: Int
    result = 10 * 4
    println("Result: $result")
}