What does ?: do in Kotlin? (Elvis Operator)

Question

I can\'t figure out what ?: does in for example this case

val list = mutableList ?: mutableListOf() 

and why can it be modifi

Answer 1

This is called the Elvis operator and it does... Exactly what you've described in your question. If its left hand side is a null value, it returns the right side instead, sort of as a fallback. Otherwise it just returns the value on the left hand side.

a ?: b is just shorthand for if (a != null) a else b.

Some more examples with types:

val x: String? = "foo"
val y: String = x ?: "bar"      // "foo", because x was non-null    

val a: String? = null
val b: String = a ?: "bar"      // "bar", because a was null