按枚举类型列表进行流分组

Question

我有一个产品类：

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);
    }
}

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

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group是一个单独的字段，就像name一样，我可以这样做：

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

如何使用列表 ？

预期的结果如下所示，对于productList中存在的每个组，映射到在其字段group中具有该组的产品列表

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

Answer 1

您可以将每个产品中的组映射到产品的名称，然后根据组将其分组，将相应的名称映射为值。例如：

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，您可以制定如下所示：

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())));