Java 8 lambdas 按多个字段分组
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Java 8 lambdas grouping by multiple fields
提问by Robert Bain
I have a list of pojos that I want to perform some grouping on. Something like:
我有一个要对其进行分组的 pojo 列表。就像是：
public class Pojo {
private final Category category;
private final BigDecimal someValue;
}
public class Category {
private final String majorCategory;
private final String minorCategory;
}
I want a Map<String, Map<String, List<Pojo>>>where the key is majorCategoryand the value is a Mapwith key minorCategoryand values is a Listof Pojoobjects for said minorCategory.
我想要一个Map<String, Map<String, List<Pojo>>>键在哪里，majorCategory值是一个Map，键minorCategory是一个List，值是一个Pojo对象minorCategory。
I intend to use Java 8 lambdas to achieve this. I can get the first level of grouping done with the following:
我打算使用 Java 8 lambdas 来实现这一点。我可以通过以下方式完成第一级分组：
Map<String, Pojo> result = list
.stream()
.collect(groupingBy(p -> p.getCategory().getMajorCategory()));
How can I now group again on minorCategoryand get the Map<String, Map<String, List<Pojo>>>I desire?
我现在怎样才能再次分组minorCategory并获得Map<String, Map<String, List<Pojo>>>我想要的？
Update
更新
The first answer provided is correct for the example provided initially, however I have since updated the question. Ruben's comment in the accepted answer, provides the final piece of the puzzle.
提供的第一个答案对于最初提供的示例是正确的，但是我已经更新了问题。鲁本在接受的答案中的评论提供了最后一块拼图。
回答by Louis Wasserman
groupingBy(Pojo::getMajorCategory, groupingBy(Pojo::getMinorCategory))
should work, I think?
应该工作，我想？
