流分组按枚举类型列表 [英] Stream groupingBy a list of enum types

问题描述

我有一个Product类:

I have a Product class:

class Product {
    String name;
    List<Group> group;
    //more fields, getters, setters
    public Product(String name, Group... group) {
        this.name = name;
        this.group = Arrays.asList(group);
    }
}

其中Group是一个枚举

where Group is an enum

public enum Group {
    LEISURE,
    SPORT,
    FORMALATTIRE,
    BABY,
    MATERNITY
    //...
}

我要从产品列表中创建一个Map<Group,List<Product>>

From a list of products I want to create a Map<Group,List<Product>>

示例输入:

List<Product> productList = new ArrayList<>();

productList.add(new Product("A", Group.BABY, Group.MATERNITY));
productList.add(new Product("B", Group.BABY, Group.LEISURE, Group.SPORT));
productList.add(new Product("C", Group.SPORT, Group.LEISURE));
productList.add(new Product("D", Group.LEISURE, Group.SPORT, Group.FORMALATTIRE));
productList.add(new Product("E", Group.SPORT, Group.LEISURE));
productList.add(new Product("F", Group.FORMALATTIRE, Group.LEISURE));

如果组像名称一样是一个字段，我可以这样做:

If group was a single field just like name I could do:

productList.stream().collect(Collectors.groupingBy(Product::getName));

如何使用List<Group>来做到这一点?

How can I do it with a List<Group> ?

预期结果如下所示，其中对productList中存在的每个组，映射到在其字段group

Expected result is something like below, where for each group which exists in the productList a mapping to a list of products having this group in their field group

{MATERNITY=[A], FORMALATTIRE=[D, F], LEISURE=[B, C, D, E, F], SPORT=[B, C, D, E], BABY=[A, B]}

推荐答案

您可以将每个Product中的组flatMap映射到产品的name，然后通过Group映射相应的 s作为值.如:

You can flatMap the group within each Product to the name of the product and then group it by Group mapping the corresponding names as value. Such as:

Map<Group, List<String>> groupToNameMapping = productList.stream()
        .flatMap(product -> product.getGroup().stream()
                .map(group -> new AbstractMap.SimpleEntry<>(group, product.getName())))
        .collect(Collectors.groupingBy(Map.Entry::getKey,
                Collectors.mapping(Map.Entry::getValue, Collectors.toList())));

或者要获得组到产品列表的映射，可以用以下公式表示:

or to get a mapping of the group to list of product, you can formulate the same as:

Map<Group, List<Product>> groupToProductMapping = productList.stream()
        .flatMap(product -> product.getGroup().stream()
                .map(group -> new AbstractMap.SimpleEntry<>(group, product)))
        .collect(Collectors.groupingBy(Map.Entry::getKey,
                Collectors.mapping(Map.Entry::getValue, Collectors.toList())));

这篇关于流分组按枚举类型列表的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！