压缩列表列表的3种方法。有理由更喜欢其中之一吗？ [英] 3 ways to flatten a list of lists. Is there a reason to prefer one of them?

问题描述

假设我们有一个如下列表。 CoreResult 的字段类型为列表< Double> 。

Assume we have a list as follows. CoreResult has a field of type List<Double>.

final List<CoreResult> list = new LinkedList<>(SOME_DATA);

目标是在从每个 CoreResult中提取特定字段后展平列表对象。
以下是3种可能的选择。其中任何一个都优于其他人吗？

The objective is to flatten the list after extracting that specific field from each CoreResult object. Here are 3 possible options. Is any one of them preferable to the others?

选项1 ：通过 map（）提取字段并在收藏家内展平

Option 1: extract a field via map() and flatten inside collector

final List<Double> A = list.stream().map(CoreResult::getField)
            .collect(ArrayList::new, ArrayList::addAll, ArrayList::addAll);

选项2 ：通过地图提取字段（），通过 flatten（），简单收集器

Option 2: extract a field via map(), flatten via flatMap(), simple collector

final List<Double> B = list.stream().map(CoreResult::getField)
            .flatMap(Collection::stream).collect(Collectors.toList());

选项3 ：提取字段并通过<$一次拼合c $ c> flatMap（），简单收集器

Option 3: extract a field and flatten in one go via flatMap(), simple collector

final List<Double> C = list.stream().flatMap(
             x -> x.getField().stream()).collect(Collectors.toList());

如果不需要从CoreResult中提取任何字段，那么答案会有所不同吗简单地展平列表< List< Double>> ？

Would the answer be different if there was no need to extract any field from CoreResult, and instead one wanted to simply flatten a List<List<Double>>?

推荐答案

我不确定每个人的性能，但java流使用的构建器模式的一个重要方面是它允许可读性。我个人认为选项2是最具可读性的。选项3也很好。我会避免选项一，因为它会欺骗集合的扁平化。

I'm not sure about the performance of each one, but an important aspect of the builder pattern utilized by java streams is that it allows for readability. I personally find the option 2 to be the most readable. Option 3 is good, too. I would avoid option one because it kind of "cheats" the flattening of the collection.

将每个方法放在自己的行上，并确定哪个方法最直观易读。我更喜欢第二个：

Put each method on its own line and determine which is the most intuitive to read. I rather like the second one:

final List<Double> B = list.stream()
                           .map(CoreResult::getField)
                           .flatMap(Collection::stream)
                           .collect(Collectors.toList());

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