Kotlin MutableList的初始容量 [英] Kotlin MutableList initial capacity

问题描述

我正在创建一个值列表，在这种情况下，尽管每次一次添加一个值，但最终的数字是事先知道的.该函数会被多次调用，因此它运行得越快越好.

I'm creating a list of values, in a context where it so happens that, though the values are being added one at a time, the eventual number is known in advance. This is in a function that will be called many times, so the faster it runs, the better.

在Java中，我将使用ArrayList构造函数来指定初始容量，因为从理论上讲，这会使其稍快一些，因为它避免了调整大小.

In Java, I would use the ArrayList constructor that specifies an initial capacity, because in theory this makes it slightly faster because it avoids resizing.

在Kotlin中，通常使用mutableListOf()，但这不允许初始容量.从理论上讲，这应该会导致代码变慢.

In Kotlin, one normally uses mutableListOf(), but this does not allow an initial capacity; in theory this should result in slightly slower code.

在这种情况下，是推荐的/惯用的Kotlin解决方案:

Is the recommended/idiomatic Kotlin solution in this case:

1. 继续使用ArrayList构造函数； ArrayList是一个完全有效的MutableList.
2. 忽略该问题；初始容量实际上不会对速度产生明显的影响.
3. 还有别的吗?

推荐答案

更新后的答案

我实际上对容量和大小感到困惑. Kotlin stdlib当前没有使用默认容量MutableList的实现.

Updated Answer

I was actually confused with capacity and size. There is no implementation of using a default capacity MutableList currently in Kotlin stdlib.

你可以自己做.

fun <T> mutableListWithCapacity(capacity: Int): MutableList<T> =
    ArrayList(capacity)

// creates a MutableList of Int with capacity of 5.
val mutableList = mutableListWithCapacity<Int>(5)

过时的答案

mutableListOf <的原因之一/a>不允许使用默认容量是因为kotlin中的默认值不为空.

Outdated Answer

One of the reason why mutableListOf does not allow for default capacity is because default values in kotlin is not null.

但是kotlin.collections软件包中有已定义的实用程序功能.

However there is a utility function defined in kotlin.collections package.

public inline fun <T> MutableList(size: Int, init: (index: Int) -> T): MutableList<T> {
    val list = ArrayList<T>(size)
    repeat(size) { index -> list.add(init(index)) }
    return list
}

您可以使用列表函数或 MutableList 函数与默认容量及其映射.

You can create a List with a List function or MutableList function with a default capacity and its mapping.

// creates a list of ints with default capacity of 10 and having nulls.
// But I highly doubt you should initialize it with a null since Kotlin is a null-safe language.
val list = MutableList<Int?>(10) { null }

但是，如果要使用非空列表，则Kotlin中不应有空值，否则您必须使用?. !!.之类的运算符进行空值检查.

But since there should not be nulls in Kotlin if it is intended use of non-null list else you have to do a null check using operators like ?. !!..

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