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

The elvis operator in Kotlin is used for null safety.

x = a ?: b

In the above code, x will be assigned the value of a if a is not null and b if a is null.

The equivalent kotlin code without using the elvis operator is below:

x = if(a == null) b else a

Answer 2

A little addition though is this

X = A ?: B

X will still be null if both A and B evaluate to null

Therefore, if you want X to always be non-null, make sure B is always a non-null or that B always evaluates to non-null if it's a function or expression.