Why Kotlin sortBy() seems to operate in reverse order?

Question

When I perform:

val array = arrayListOf<String?>(null, "hello", null)
array.sortBy { it == null }
println(array)

I expect it would print null values first as that's the selector I specified. However, println(array) returns [hello, null, null].

Why is this?

Answer 1

The expression:

it == null

returns a Boolean result true or false and this is what you use to sort the array.
The value true is greater than false, you can see it by executing:

println(false < true)

which will print

true

With your code:

array.sortBy { it == null }

for every item that the expression it == null returns false it will be placed before any item for which it will return true.
So do the opposite:

array.sortBy { it != null }

Result:

[null, null, hello]