Elvis Operator

Providing a Default Value

The Elvis operator ?: returns its left operand if that is not null, or falls back to the right operand otherwise, giving a concise way to supply a default.

Example: Providing a Default Value

fun main() {
    val nickname: String? = null
    val displayName = nickname ?: "Anonymous"
    println("Display name: $displayName")
}

Combining with Safe Calls

?: pairs naturally with ?., letting you write value?.property ?: default to safely reach into a nullable chain and fall back cleanly if anything along the way is null.

Example: Combining with Safe Calls

data class User(val name: String?)

fun main() {
    val user: User? = User(null)
    val name = user?.name ?: "Unknown"
    println("Name: $name")
}

Throwing on Null with Elvis

Because the right side of ?: can be any expression, including throw, it is a common idiom to fail fast with a clear exception when a value that should never be null turns out to be.

Example: Throwing on Null with Elvis

fun requireName(name: String?): String {
    return name ?: throw IllegalArgumentException("Name must not be null")
}

fun main() {
    println(requireName("Kotlin"))
}

Lazy Evaluation of the Right Side

The expression after ?: is only evaluated if the left side is null, so an expensive computation or a function call placed there does not run unnecessarily.

Note: Notice "Computing default..." never prints, since value was not null.

Example: Lazy Evaluation of the Right Side

fun computeExpensiveDefault(): String {
    println("Computing default...")
    return "computed"
}

fun main() {
    val value: String? = "already set"
    val result = value ?: computeExpensiveDefault()
    println("Result: $result")
}