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Kotlin Elvis Operator

The Elvis operator gives a fallback value when the value on its left is null. The fallback keeps a program working with a plain value instead of null.

A fallback for a missing length

The name holds null. The safe call gives null, so the fallback 0 is used.

fun main() {
    val name: String? = null
    println(name?.length ?: 0)
}

No fallback needed for a real length

The name holds a word. Its length is used and the fallback is ignored.

fun main() {
    val name: String? = "Ana"
    println(name?.length ?: 0)
}

A fallback word on its own

The nickname holds null. Printing the Elvis expression shows the fallback word.

fun main() {
    val nickname: String? = null
    println(nickname ?: "Guest")
}

An Elvis value saved in a val

The size takes the fallback 0. A later print adds 10 to that saved value.

fun main() {
    val name: String? = null
    val size = name?.length ?: 0
    println(size + 10)
}

Example

fun main() {
    val nickname: String? = null
    val displayName = nickname ?: "Guest"
    println("Hello, $displayName")
    val name: String? = "Cy"
    val size = name?.length ?: 0
    println(size)
}

Key points

  • Elvis operator: the ?: written between a value and its fallback.
  • Use ?: to supply a fallback when the left side is null.
  • A non-null left side is used as it is.
  • A fallback can be a number or a word.

In this example, the nickname holds null, so displayName takes the fallback Guest. The greeting prints that fallback. The name holds Cy, so size takes its real length 2.

Note

?: works with a safe call, as in name?.length ?: 0.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Kotlin Playground

Exercise

When name holds null, what value does name?.length ?: 0 give?