How to negate a boolean expression when using the elvis operator in kotlin?

I want to negate the following expression:

`return SpUtils.loadEMail()?.isEmpty() ?: false`

If i add a ! before the expression, like

`return !SpUtils.loadEMail()?.isEmpty() ?: false`

The IDE(Android Studio) tells me

> Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type kotlin.Boolean?

How do I negate this kinds of expressions?

You have problem with nullable reference.

SpUtils.loadEMail()?.isEmpty()

This code produces value of type **Boolean?** that's mean expression can return an instance of **Boolean** or **null**.

I suggest following code to solve your problem:

return !(SpUtils().loadEMail()?.isEmpty() ?: false);

You trying negate **Boolean?** instead of **Boolean**, that [elvis][1] operator returns!


[1]:

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`?.` is a [safe call operator][1].

In this case it returns you:

* boolean value if the result of `loadEmail()` invocation **is not** `null`
* `null` otherwise

`!` is a [built-in][2] boolean operation which invokes the `package.Boolean`'s operator called `not()` which works only on non-nullable references. The result of `?.` is `Boolean?`, thus you get your error.

If you want to negate the whole expression, you can stick to the iRus answer:

!(SpUtils().loadEMail()?.isEmpty() ?: false)

If you want to negate **just** the `SpUtils().loadEMail()?.isEmpty()` part then the correct variant will be:

!(SpUtils().loadEMail()?.isEmpty() ?: true)

If the result of `?.` will be null (there is no mail) then the elvis operator will return you `true` (no e-mail) and you'll negate it.

[1]:

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[2]:

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`!(SpUtils().loadEMail()?.isEmpty() ?: false)` would work, but it's really difficult to see when this would actually return `true`.

It returns `true` when the result of `SpUtils().loadEMail()` is null or not empty. With this knowledge, we can easily make something readable:

`return SpUtils().loadEMail()?.isNotEmpty() ?: true`

You can expand it for even better readability:

val email = SpUtils().loadEMail()
if (email == null || email.isNotEmpty()) {
return true
}
return false