Java 扁平化列表列表的 3 种方法。有理由喜欢其中之一吗？
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3 ways to flatten a list of lists. Is there a reason to prefer one of them?
提问by 3VYZkz7t
Assume we have a list as follows. CoreResulthas a field of type List<Double>.
假设我们有一个列表如下。CoreResult有一个类型的字段List<Double>。
final List<CoreResult> list = new LinkedList<>(SOME_DATA);
The objective is to flatten the list after extracting that specific field from each CoreResultobject.
Here are 3 possible options. Is any one of them preferable to the others?
目标是在从每个CoreResult对象中提取特定字段后展平列表。这里有 3 种可能的选择。它们中的任何一个比其他的更可取吗？
Option 1: extract a field via map()and flatten inside collector
选项 1：通过map()并在收集器内部展平提取字段
final List<Double> A = list.stream().map(CoreResult::getField)
.collect(ArrayList::new, ArrayList::addAll, ArrayList::addAll);
Option 2: extract a field via map(), flatten via flatMap(), simple collector
选项 2：提取字段通过map()，展平通过flatMap()，简单收集器
final List<Double> B = list.stream().map(CoreResult::getField)
.flatMap(Collection::stream).collect(Collectors.toList());
Option 3: extract a field and flatten in one go via flatMap(), simple collector
选项3：通过flatMap()简单的收集器一次性提取字段并展平
final List<Double> C = list.stream().flatMap(
x -> x.getField().stream()).collect(Collectors.toList());
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>>?
如果不需要从 CoreResult 中提取任何字段，而是想要简单地展平 a ，答案会有所不同List<List<Double>>吗？
采纳答案by Zircon
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.
我不确定每一个的性能，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 是最易读的。选项 3 也不错。我会避免选项一，因为它有点“欺骗”了集合的扁平化。
Put each method on its own line and determine which is the most intuitive to read. I rather like the second one:
将每种方法放在自己的行上，并确定哪种方法最易于阅读。我更喜欢第二个：
final List<Double> B = list.stream()
.map(CoreResult::getField)
.flatMap(Collection::stream)
.collect(Collectors.toList());
回答by ByeBye
Option 3 and 2 is same, so up to you if you want to map first and then flat map or just flatmap.
选项 3 和 2 是相同的，所以如果您想先映射然后平面地图或只是平面地图，则由您决定。
With option 1 you have to open another stream to perform further operations.
使用选项 1，您必须打开另一个流以执行进一步的操作。
回答by marstran
I would go for option 2 or 3. If you want to flatten a List<List<Double>>, you would do this:
我会选择选项 2 或 3。如果你想展平 a List<List<Double>>，你可以这样做：
List<Double> list = doubleList.stream()
.flatMap(List::stream)
.collect(Collectors.toList());
