将不可为空的字符串数组作为可为空的字符串数组传递 [英] Pass array of non-nullable strings as array of nullable strings

问题描述

我有一个接受Array<String?>的函数:

fun doStuff(words: Array<String?>) {
    // ...
}

是否可以将Array<String>传递给此函数?编译器照常给我一个类型不匹配"错误.

Is there a way I can pass in a Array<String> to this function? As-is the compiler is giving me a "type mismatch" error.

private val SOME_WORDS = arrayOf("I", "want", "to", "use", "these")

doStuff(SOME_WORDS) // throws a type-mismatch error

最好尽量避免将SOME_WORDS设置为arrayOf<String?>(...).

Preferably I'd like to avoid making SOME_WORDS an arrayOf<String?>(...) if possible.

推荐答案

使用 out-投射 Array<out String?>:

fun doStuff(words: Array<out String?>) { /* ... */ }

Kotlin中的数组是不变的，这意味着<对于任何不同的A和B(包括String和String?)，c6>和Array<B>都不是彼此的子类型，并且不能将它们分配并作为参数代替彼此传递.

Arrays in Kotlin are invariant, meaning that Array<A> and Array<B> are not subtypes of each other for any different A and B, including String and String?, and they cannot be assigned and passed as arguments in place of each other.

使用out-投影Array<out String?>使函数不仅接受Array<String?>，而且接受具有String?子类型的数组.基本上，由于类型是final，所以只有一个这样的子类型，它是String(非空类型，没有?).

Using an out-projection Array<out String?> makes the function accept not only Array<String?> but also arrays with subtypes of String?. Basically, since the type is final, there's only one such subtype, and it is String (the non-null one, without ?).

^{图片摄于:科特林式等级制度的旋风之旅 )}

因此，该函数也将接受Arrray<String>.但是您将无法将可为空的值放入数组中(这就是投影的工作方式)，因此将保留类型安全性(和null-safety).

So, the function will accept Arrray<String> as well. But you won't be able to put nullable values into the array (that's how the projection works), so the type safety (and null-safety) are preserved.

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