Why is

if (list?.isEmpty())

not allowed when

if (list?.isEmpty() == true)

is? You are still not specifying null behaviour...

Yes, you are. Only

true

is equal to

true

.

false

,

null

and any other value is not equal to

true

.

list?.isEmpty()

will return null if

list

is null

list?.isEmpty()

returns

Boolean?

and comparing it to

true

lets you check if it’s true.

false

and

null

are two other states the if statement can’t make assumptions about. “not specifying null behavior” is only important if you’re using the

if

as an assignment expression, otherwise you just need to give it a non-nullable

Boolean

I see. Feels a bit weird, if statement and comparing to true not being the same. I guess next question is: Why does equals operator accept nullable arguments? Java compatibility?

Why would it not? How would you do

if (a == null)

otherwise?

That would be an equals method on nullable types. Like

fun Int?.equals(other: Int?): Boolean

I am not sure I see why that is any better than

==

.

I guess better type safety? But yeah maybe it's not any better.

You can do

list != null && list.isEmpty()

or

list?.isEmpty() ?: false

if that's better for you.

You could also write an extension function like

Iterable?.isEmptyOrNull() = this == null || this.isEmpty()

or make it more specific to the classes you need to use it for