Kotlin non-null assertion on null

Question

As Kotlin have the non-null assertion, I found some funny stuff...

val myvar: String = null!!

It will crash.

But the point is, it doesn't check at compile time.

The app will crash at runtime.

Shouldn't it throw compile time error?

Answer 1

!! is evaluated at runtime, it's just an operator.

The expression (x!!)

• throws a KotlinNullPointerException if x == null,
• otherwise, it returns x cast to the corresponding non-nullable type (for example, it returns it as a String when called on a variable with type String?).

This, of course, makes null!! shorthand for throw KotlinNullPointerException().

---

If it helps, you can think of !! as doing the same as a function like this does:

fun <T> T?.toNonNullable() : T {
    if(this == null) {
        throw KotlinNullPointerException()
    }
    return this as T // this would actually get smart cast, but this 
                     // explicit cast demonstrates the point better
}

So doing x!! would give you the same result as x.toNonNullable().