我如何告诉 Kotlin 数组或集合不能包含空值？

Question

如果我创建一个数组，然后填充它，Kotlin 认为数组中可能存在空值，并强制我考虑这一点

val strings = arrayOfNulls<String>(10000)
strings.fill("hello")
val upper = strings.map { it!!.toUpperCase() } // requires it!!
val lower = upper.map { it.toLowerCase() } // doesn't require !!

创建填充数组就不存在这个问题

val strings = Array(10000, {"string"})
val upper = strings.map { it.toUpperCase() } // doesn't require !!

如何告诉编译器结果strings.fill("hello")是一个 NonNull 数组？

Answer 1

经验法则：如果有疑问，请明确指定类型（对此有一个特殊的重构）：

val strings1: Array<String?> = arrayOfNulls<String>(10000)
val strings2: Array<String>  = Array(10000, {"string"})

所以你会看到strings1包含可空项，而不strings2包含。仅此一点决定了如何使用这些数组：

// You can simply use nullability in you code:
strings2[0] = strings1[0]?.toUpperCase ?: "KOTLIN"

//Or you can ALWAYS cast the type, if you are confident:
val casted = strings1 as Array<String>

//But to be sure I'd transform the items of the array:
val asserted = strings1.map{it!!}
val defaults = strings1.map{it ?: "DEFAULT"}