使用Stream API汇总对象的属性 [英] Sum attribute of object with Stream API

问题描述

我目前遇到以下情况：

我有一个报告对象，其中包含多个查询对象。 查询对象具有以下属性：可选<过滤器> comparisonFilter ，可选< String> filterChoice 和 int queryOutput 。

I have got a Report object which can contain multiple Query objects. The Query objects have properties: Optional<Filter> comparisonFilter, Optional<String> filterChoice and int queryOutput.

并非每个查询都有比较过滤器，所以我首先检查一下。然后，我确保我得到了特定过滤器的查询（这不是问题，所以我不会详细讨论）。每个过滤器都有一些选择，其中选项的数量是可变的。

Not every query has a comparison filter, so I first check on that. Then, I make sure I get the queries for a particular filter (which is not the problem here, so I will not discuss this in detail). Every filter has some choices, of which the number of choices is variable.

以下是输入的示例（这些查询对象都具有相同的comparisonFilter）：

Here is an example of the input (these Query objects all have the same comparisonFilter):

Query 1 -- Choice: 'First' -- Output: 10
Query 1 -- Choice: 'First' -- Output: 5
Query 1 -- Choice: 'Second' -- Output: 25
Query 1 -- Choice: 'Third' -- Output: 10

现在，我想对每个唯一选择的查询输出求和。我目前有这个代码：

Now, I would like to sum the query outputs for every unique choice. I currently have this code:

report
.getQueries()
.stream()
.filter(q -> q.getComparisonFilter().isPresent())
.filter(q -> q.getComparisonFilter().get().equals(view.getFilter().get()))
.forEach(query -> {

    //Sum the query outputs per choice

});

我可以通过创建 Map< String，Integer> ，其中键是选项，值是查询输入。但是我需要再次遍历 Map 来使用某些值（这里不重要）。

I could do this by creating a Map<String, Integer>, where the key is the choice and the value is the query input. But then I would need to loop through the Map again to use the value for something (which is not important here).

输出应该是这样的：

Choice: 'First' -- Summed Output: 15
Choice: 'Second' -- Summed Output: 25
Choice: 'Third' -- Summed Output: 10

但是我想直接在流上的 forEach 中使用这个'Summed Output'，但如果这不可能或不再实用，我没关系那个。

But I would like to use this 'Summed Output' directly in a forEach on the stream, but if this is not possible or practical anymore, I am okay with that.

我想用'Java 8'方式做这个，但我似乎无法弄清楚如何。

I would like to do this the 'Java 8'-way, but I can not seem to find out how.

所以我的问题是：是否可以使用新的Stream API缩短版本？

注意：如果有人有一些关于如何让这个问题更通用的想法（也许是一个更好的标题和一些概括），请告诉我！

Note: If anyone has some ideas about how I could make this question more general (maybe a better title and some generalizations), please let me know!

推荐答案

<如果我理解得很好，你确实在寻找 groupingBy ，然后你必须通过对它们的int属性求和来对这些值进行分组。

If I understand well, you are indeed looking for a groupingBy and then you have to group the values by summing their int property.

groupingBy 会给你一个 Map< String，List< Query>> 但是接下来的收集器（ Collectors.summingInt 在这种情况下）将汇总查询 int 值>列表中的实例，产生 Map< String，Integer> 。

The groupingBy will give you a Map<String, List<Query>> but then the downstream collector (Collectors.summingInt in this case) will sum all the int values of the Query instances in the list, resulting in a Map<String, Integer>.

import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.summingInt;
...

Map<String, Integer> map = 
    report.getQueries()
          .stream()
          .filter(q -> q.getComparisonFilter().isPresent())
          .filter(q -> q.getComparisonFilter().get().equals(view.getFilter().get()))
          .collect(groupingBy(q -> q.filterChoice.get(), summingInt(q -> q.queryOutput)));

请注意，您应该检查 filterChoice 可选不为空（可能添加另一个过滤器子句？）。您可以看到这个小的要点，以获得基本和简化的演示来说明原理。

Note that you should check whether the filterChoice Optional is not empty (maybe add another filter clause?). You can see this small gist for a basic and simplified demo to illustrate the principle.

此外可选类提供合理的等于的实现，因此过滤器子句可能如下所示：

Also the Optional class provide a sensible implementation of equals so the filter clause could looks like this:

.filter(q -> q.getComparisonFilter().equals(view.getFilter()))

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