Kotlin elvis operator example

ElvisOperatorExample.kt

Elvis Operator
When we have a nullable reference r, we can say "if r is not null, use it, otherwise use some non-null value x":

val l: Int = if (b != null) b.length else -1
Along with the complete if-expression, this can be expressed with the Elvis operator, written ?::

val l = b?.length ?: -1
If the expression to the left of ?: is not null, the elvis operator returns it, otherwise it returns the expression to the right. Note that the right-hand side expression is evaluated only if the left-hand side is null.

Note that, since throw and return are expressions in Kotlin, they can also be used on the right hand side of the elvis operator. This can be very handy, for example, for checking function arguments:

fun foo(node: Node): String? {
    val parent = node.getParent() ?: return null
    val name = node.getName() ?: throw IllegalArgumentException("name expected")
    // ...
}
fun foo(node: Node): String? {
    val parent = node.getParent() ?: return null
    val name = node.getName() ?: throw IllegalArgumentException("name expected")
    // ...
}