In Kotlin, the Elvis operator ?: provides a fallback value when the expression on its left is null.

val result = nullableValue ?: defaultValue

If nullableValue is not null, result gets that value.
If nullableValue is null, result gets defaultValue.

Example:

val name: String? = null

val displayName = name ?: "Guest"

println(displayName) // Guest

With a non-null value:

val name: String? = "Alice"

val displayName = name ?: "Guest"

println(displayName) // Alice

It’s often used with safe calls:

val name: String? = null

val nameLength = name?.length ?: 0

println(nameLength) // 0

Here, name?.length returns null if name is null, so ?: 0 supplies the default.

You can also use it with functions:

fun getUsername(): String? {
    return null
}

val username = getUsername() ?: "anonymous"

And because throw and return are expressions in Kotlin, they can be used on the right side:

fun printName(name: String?) {
    val actualName = name ?: return
    println(actualName)
}

fun requireName(name: String?) {
    val actualName = name ?: throw IllegalArgumentException("Name is required")
    println(actualName)
}

So the basic pattern is:

val value = somethingNullable ?: fallbackValue