按流java 8中的多个字段分组

Question

我有一个类:

public class PublicationDTO {
final String publicationName;
final String publicationID;
final String locale;
final Integer views;
final Integer shares;
}

A需要获得视图和共享的总和。对于测试，我创建了一个列表:

PublicationDTO publicationDTO1 = new PublicationDTO("Name1", "name1", "CA", 5, 6);
PublicationDTO publicationDTO2 = new PublicationDTO("Name2", "name2", "US", 6, 3);
PublicationDTO publicationDTO3 = new PublicationDTO("Name1", "name1", "CA", 10, 1);
PublicationDTO publicationDTO4 = new PublicationDTO("Name2", "name2", "CA", 2, 3);
List<PublicationDTO> publicationDTOS = List.of(publicationDTO1, publicationDTO2, publicationDTO3, publicationDTO4);

我想按publicationName, publicationId和locale对列表中的对象进行分组，并获得如下结果列表:

List.of(new PublicationDTO("Name1", "name1", "CA", 15, 7),
new PublicationDTO("Name2", "name2", "CA", 2, 3),
new PublicationDTO("Name2", "name2", "US", 6, 3));

我找到了一个解决方案:

List<PublicationDTO> collect = publicationDTOS.stream()
.collect(groupingBy(PublicationDTO::getPublicationID))
.values().stream()
.map(dtos -> dtos.stream()
.reduce((f1, f2) -> new PublicationDTO(f1.publicationName, f1.publicationID, f1.locale, f1.views + f2.views, f1.shares + f2.shares)))
.map(Optional::get)
.collect(toList());

但结果不是按地区分组，我不确定它是否适用于publicationId。请告诉我在这种情况下如何正确使用收集器?

Answer 1

您只按getPublicationID分组:

.collect(groupingBy(PublicationDTO::getPublicationID))

由于您对分组感兴趣的字段都是字符串，因此您可以将它们连接起来并将其用作分组分类器:

.collect(Collectors.groupingBy(p -> p.getPublicationName() + p.getPublicationID() + p.getLocale()))