Kotlin 实例化不可变列表 [英] Kotlin Instantiate Immutable List

问题描述

我已经开始使用 Kotlin 作为 Java 的替代品并且非常喜欢它.但是，如果不跳回 java-land，我一直无法找到解决方案:

I've started using Kotlin as a substitute for java and quite like it. However, I've been unable to find a solution to this without jumping back into java-land:

我有一个 Iterable 并且需要将它转换为一个列表，以便我可以多次迭代它.这是一个不可变列表的明显应用，因为我需要做的就是多次阅读它.但是，我如何在开始时将该数据实际放入列表中?(我知道它是一个接口，但我一直无法在文档中找到它的实现)

I have an Iterable<SomeObject> and need to convert it to a list so I can iterate through it more than once. This is an obvious application of an immutable list, as all I need to do is read it several times. How do I actually put that data in the list at the beginning though? (I know it's an interface, but I've been unable to find an implementation of it in documentation)

可能的(如果不满意)解决方案:

Possible (if unsatisfactory) solutions:

val valueList = arrayListOf(values)
// iterate through valuelist

或

fun copyIterableToList(values: Iterable<SomeObject>) : List<SomeObject> {
    var outList = ArrayList<SomeObject>()
    for (value in values) {
        outList.add(value)
    }
    return outList
}

除非我误解了，否则这些最终会得到 MutableLists，它可以工作但感觉像是一种解决方法.是否有类似的 immutableListOf(Iterable) 方法可以实例化一个不可变的列表对象?

Unless I'm misunderstanding, these end up with MutableLists, which works but feels like a workaround. Is there a similar immutableListOf(Iterable<SomeObject>) method that will instantiate an immutable list object?

推荐答案

在 Kotlin 中，List 是一个只读列表界面，它没有改变内容的功能，不像MutableList.

In Kotlin, List<T> is a read-only list interface, it has no functions for changing the content, unlike MutableList<T>.

一般来说，List实现可能是一个可变列表(例如ArrayList)，但是如果你传递它作为 List，不进行强制转换就不会暴露任何变异函数.这种列表引用称为只读，表明该列表不打算更改.这是通过接口的不变性，它被选为不变性的方法用于 Kotlin 标准库.

In general, List<T> implementation may be a mutable list (e.g. ArrayList<T>), but if you pass it as a List<T>, no mutating functions will be exposed without casting. Such a list reference is called read-only, stating that the list is not meant to be changed. This is immutability through interfaces which was chosen as the approach to immutability for Kotlin stdlib.

更接近问题，toList() stdlib 中 Iterable 的扩展函数将适合:它返回只读 List.

Closer to the question, toList() extension function for Iterable<T> in stdlib will fit: it returns read-only List<T>.

示例:

val iterable: Iterable<Int> = listOf(1, 2, 3)
val list: List<Int> = iterable.toList()

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