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 modified to this

val list = if (mutableList != null) mutableList else mutableListOf()

Answer 1

TL;DR: If the resulting object reference [first operand] is not null, it is returned. Otherwise the value of the second operand (which may be null) is returned. Additionally, the operator can throw an exception if null is returned.

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The Elvis operator is part of many programming languages, e.g. Kotlin but also Groovy or C#. I find the Wikipedia definition pretty accurate:

In certain computer programming languages, the Elvis operator ?: is a binary operator that returns its first operand if that operand is true, and otherwise evaluates and returns its second operand. It is a variant of the ternary conditional operator, ? :, found in those languages (and many others): the Elvis operator is the ternary operator with its second operand omitted.

The following is especially true for Kotlin:

Some computer programming languages have different semantics for this operator. Instead of the first operand having to result in a boolean, it must result in an object reference. If the resulting object reference is not null, it is returned. Otherwise the value of the second operand (which may be null) is returned. If the second operand is null, the operator is also able to throw an exception.

An example:

x ?: y // yields `x` if `x` is not null, `y` otherwise.
x ?: throw SomeException() // yields `x` if `x` is not null, throws SomeException otherwise