英文:
Java 8 how to merge multiple lists by enum type
问题
如何在Java 8中按枚举类型合并多个列表？我有两个列表：List<StatusSummary> listA // 因为有4个枚举类型，所以有4个项List<StatusSummary> listB // 因为有4个枚举类型，所以有4个项class StatusSummary {private StatusEnum resourcesStatus; // 4个状态：TypeA、TypeB、TypeC、TypeDprivate Integer count;}我想要有一个列表，其中包含每个 `resourcesStatus: TypeA, TypeB, TypeC, TypeD` 的4个项，并且 `count` 字段中的元素求和，如下：List<StatusSummary> listA+B，有4个对象：(resourcesStatus: TypeA, count: 3), (resourcesStatus: TypeB, count: 99), (resourcesStatus: typeC, count: 322), (resourcesStatus: TypeD, count: 2)到目前为止，我实现了类似以下的代码：listA.getResourcesStatus().forEach(la -> listB.getResourcesStatus().stream().filter(lb -> lb.getResourcesStatus() == la.getResourcesStatus()).forEach(lb -> result.add(new StatusSummary().count(la.getCount() + lb.getCount()).status(la.getStatus()))));
英文:
How to merge multiple lists in Java 8 by enum type?
I have 2 lists:
List<StatusSummary> listA //4 items because we have 4 enum typesList<StatusSummary> listB //4 items because we have 4 enum typesclass StatusSummary {private StatusEnum resourcesStatus; //4 statuses: TypeA, TypeB, TypeC, TypeDprivate Integer count;}
I want to have one list consisting of 4 items for each resourcesStatus: TypeA, TypeB, TypeC, TypeD with the sum of elements in count field like:
List<StatusSummary> listA+B with 4 objects:(resourcesStatus: TypeA, count: 3), (resourcesStatus: TypeB, count:99), (resourcesStatus: typeC, count:322), (resourcesStatus: TypeD, count:2)
So far I implemented something like this:
listA.getResourcesStatus().forEach(la -> listB.getResourcesStatus().stream().filter(lb -> lb.getResourcesStatus() == la.getResourcesStatus()).forEach(lb -> result.add(new StatusSummary().count(la.getCount() + lb.getCount()).status(la.getStatus()))));
答案1
得分: 3
你可以使用 Collectors.toMap。以 ResourcesStatus 为键，使用合并函数将两个具有 count 字段的 StatusSummary 合并为相同的 ResourcesStatus，然后您将获得类型为 Map<StatusEnum, StatusSummary> 的映射，然后可以使用 .values() 在 ArrayList 中获取映射的值。
List<StatusSummary> newList =new ArrayList<>(Stream.of(listA, listB).flatMap(Collection::stream).collect(Collectors.toMap(e -> e.getResourcesStatus(),i -> i,(a, b) -> new StatusSummary(a.getResourcesStatus(),a.getCount() + b.getCount()))).values());
英文:
You can use Collectors.toMap. Map by ResourcesStatus and merge two StatusSummary with count field in merge-function for the same ResourcesStatus then you get Map<StatusEnum, StatusSummary> and then get values of the map using .values() in ArrayList.
List<StatusSummary> newList =new ArrayList<>(Stream.of(listA, listB).flatMap(Collection::stream).collect(Collectors.toMap(e -> e.getResourcesStatus(), i -> i,(a,b) -> new StatusSummary(a.getResourcesStatus(),a.getCount() + b.getCount()))).values());
答案2
得分: 2
问题使用 forEach
就我所见，您当前使用 forEach 的解决方案，将流式 API 视为简单的带有附加功能的 for 循环。您违反了 [tag:java-stream] 的 Side-effects 原则，简而言之，该原则指出流不应在执行管道操作时修改另一个集合。我尚未测试您的代码，但这不是您应该处理流的方式。
可能的解决方案之一
您可以使用 Collectors.groupingBy 与 Collectors.summingInt 来获得带有状态和计数总和的 Map<StatusEnum, Integer>。然后，可以将每个条目转换为 StatusSummary 并将其收集为 List：
List<StatusSummary> list = Stream.of(listA, listB).flatMap(List::stream).collect(Collectors.groupingBy( // Map<StatusSummary, Integer>StatusSummary::getResourcesStatus, // .. 将 StatusEnum 作为键Collectors.summingInt(StatusSummary::getCount))) // .. 将 Integer 作为值（总和）.entrySet().stream() // Stream<Entry<..>>.map(e -> new StatusSummary(e.getKey(), e.getValue())) // Stream<StatusSummary>.collect(Collectors.toList()); // List<StatusSummary>
... 或者使用 Collectors.collectingAndThen（不少 verbose）：
List<StatusSummary> list1 = Stream.of(listA, listB, listC).flatMap(List::stream).collect(Collectors.collectingAndThen(Collectors.groupingBy(StatusSummary::getResourcesStatus,Collectors.summingInt(StatusSummary::getCount)),map -> map.entrySet().stream().map(e -> new StatusSummary(e.getKey(), e.getValue())).collect(Collectors.toList())));
英文:
Problem using forEach
As far as I see your current solution using forEach, you approach to Stream API as it would be a simple for-loop with additional features. You violate the Side-effects principle of [tag:java-stream] which in a nutshell says that a stream shouldn't modify another collection while performing the actions through the pipelines. I haven't tested your code, however, this is not a way you should treat streams.
One of the possible solutions
You can use Collectors.groupingBy with Collectors.summingInt to get Map<StatusEnum, Integer> with the status and a sum of counts. Each entry then can be transformed into StatusSummary and colleted as List:
List<StatusSummary> list = Stream.of(listA, listB).flatMap(List::stream).collect(Collectors.groupingBy( // Map<StatusSummary, Integer>StatusSummary::getResourcesStatus, // .. StatusEnum as keyCollectors.summingInt(StatusSummary::getCount))) // .. Integer as value (sum).entrySet().stream() // Stream<Entry<..>>.map(e -> new StatusSummary(e.getKey(), e.getValue())) // Stream<StatusSummary>.collect(Collectors.toList()); // List<StatusSummary>
... or using Collectors.collectingAndThen (not less verbose):
List<StatusSummary> list1 = Stream.of(listA, listB, listC).flatMap(List::stream).collect(Collectors.collectingAndThen(Collectors.groupingBy(StatusSummary::getResourcesStatus,Collectors.summingInt(StatusSummary::getCount)),map -> map.entrySet().stream().map(e -> new StatusSummary(e.getKey(), e.getValue())).collect(Collectors.toList())));
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库，让每个人都能够通过互相帮助和分享经验来进步。
评论