java - Stream groupingBy a list of enum types

Question

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);
    }
}

where Group is an enum

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

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

Example input:

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));

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

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

How can I do it with a List<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]}

Answer 1

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())));

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())));