android - 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

!!在运行时评估，它只是一个运算符。

表达方式(x!!)

• 抛出一个KotlinNullPointerExceptionif x == null，
• 否则，它会将强制类型转换返回x到相应的不可为空的类型（例如，String当在类型为 的变量上调用时，它会以 a 的形式返回String?）。

这当然null!!是throw KotlinNullPointerException().

---

如果它有帮助，您可以认为!!与这样的功能相同：

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
}

这样做x!!会给你同样的结果x.toNonNullable()。