enum.values（）的复杂度 [英] Complexity of enum.values()

问题描述

我有一个非常简单的枚举，如下所示：

I have a very simple Enum as follows:

公共枚举颜色{
红色，蓝色，绿色；
}


在这里我放了三种颜色，但可能有 undefined

Here I've put three colours, but it may have a undefined size.

此枚举将用于可能具有 undefined 实例数量（数百，数千甚至数百万）的类中

This Enum will be used in a class which can have an undefined number of instances (hundreds, thousands or even millions).

在此类中，我有一个必须返回随机颜色的方法。

In this class I have a method that must return a random Colour.

我有两个选择

private Colour[] colours;

public Datastructure() {

    colours = Colour.values();
}

public Colour getRandomColour() {
     return colours[rand.nextInt() % colours.length];
}

或者我可以继续调用Colour.values（）而不是创建颜色列表

Or I can keep calling Colour.values() instead of creating the colours list.

public Colour getRandromColour() {
     return Colour.values()[rand.nexInt() % Colour.values().length]

在第一个选项中，将创建一个额外的数组。请记住，此类可能有很多实例，因此它可能被视为浪费内存，并且也可能影响运行时间（实例化数组）。

In the first option, an extra array is created. Keep in mind that this class may have many instances, so it could be considered a waste of memory, and may have a impact on running time as well (instantiating the array). Especially when there are a lot of class instances.

在第二个选项中，Colour.values（）被调用了几次（在这个简单的示例中，它仅被调用了几次）时间，但是在我的项目中，它有点复杂，调用次数也更多），所以这可以被认为是对CPU使用率的浪费。

In the second option, Colour.values() is called a few times (in this simple example it is only a few times but in my project it's bit more complex and has more calls) so this can be considered a waste of CPU usage.

我更喜欢使用第二种方法，但是我我对Colour.values（）方法的复杂性感到好奇，我担心它可能是线性O（n）。 n是枚举中颜色的数量。当颜色很多时，这将是可怕的。

I would prefer using the second option, but I'm curious about the complexity of the Colour.values() method, which I fear may be linear O(n). n being the number of Colours in the enum. This would be horrible when there are a lot of colours.

还是权衡利弊，选择两种邪恶中的最佳方法？

Or would it just be a weigh off and choose the best of two evils?

tl; dr Enum.values（）的复杂性是什么？

tl;dr What is the complexity of Enum.values()?

推荐答案

Colour.values（）返回的数组始终包含相同的元素，但是 values（）创建每次调用 new 数组（所以它是O（n），其中n是枚举值的数量）。而且您永远不会修改该数组。因此，每次需要时调用该方法都是浪费时间。并且在DataStructure类的每个实例中存储数组都是浪费时间和内存。我只需调用该方法一次，然后将该数组缓存为一个常量：

The array returned by Colour.values() always contains the same elements, but values() creates a new array every time it's called (so it's O(n) where n is the number of enum values). And you never modify that array. So calling the method every time you need it is a waste of time. And storing an array in every instance of your DataStructure class is a waste of time and memory. I would simply call the method once and cache that array in a constant:

private static final Colour[] COLOURS = colour.values();

public Colour getRandomColour() {
    return COLOURS[rand.nextInt(COLOURS.length)];
}

只需确保此数组永远不会暴露在课堂之外。

Just make sure this array is never exposed outside of the class.

还要注意 Random.nextInt（limit）的用法，它确实可以满足您的要求，可能更快，并且表示意向比使用模更清楚。

Also note the usage of Random.nextInt(limit) which does exactly what you want, is probably faster, and expressed the intent more clearly than using a modulo.

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