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来替代Java，并且非常喜欢它.但是，如果不返回到Java领域，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<SomeObject>，需要将其转换为列表，这样我才能多次遍历它.这是不可变列表的一个明显的应用，因为我需要做的就是多次读取它.我实际上如何将这些数据放在开头的列表中? (我知道这是一个接口，但是我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(如果不能令人满意)的解决方案:

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
}

除非我有误会，否则这些都以MutableLists结尾，该命令可以工作，但感觉像是一种解决方法.是否有类似的immutableListOf(Iterable<SomeObject>)方法将实例化不可变的列表对象?

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<T> 是一个只读列表界面，与

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

通常，List<T>实现可能是可变列表(例如ArrayList<T>)，但是如果将其作为List<T>传递，则不会在不进行强制转换的情况下公开任何变异函数.这样的列表引用称为只读，表示该列表不打算更改.这是通过接口的不变性，它被选为实现不变性的方法用于Kotlin stdlib.

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.

更接近问题的地方，Iterable<T>的> toList() 扩展功能将适合:它返回只读的List<T>.

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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