Stream groupingBy a list of enum types

Question

I have a Product class:

class Product {
    String name;
    List group;
    //more fields, getters, setters
    public Product(String name, Group..         


        

Answer 1

You can create a stream of Entry<Group,String> by using flatMap and then collect them into Map<Group, List<String>> using Collectors.mapping

productList.stream()
               .flatMap(p->p.getGroup()
                            .stream()
                            .map(g->new AbstractMap.SimpleEntry<>(g,p.getName())))   // or from jdk 9 you can use Map.entry(g, p.getName());
               .collect(Collectors.groupingBy(Map.Entry::getKey, 
                       Collectors.mapping(Map.Entry::getValue, Collectors.toList())));

Answer 2

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())));