java - How to map a list using sublist as key and parent object as value

Question

This question already has answers here: question from:https://stackoverflow.com/questions/65950134/how-to-map-a-list-using-sublist-as-key-and-parent-object-as-value

Answer 1

You need to create a temporary pair that holds both the privilege (after flattening the list) and the related user, which you can then use in a grouping collector:

Map<String, List<User>> result = users.stream()
        .flatMap(user -> user.getPrivileges()
                .stream()
                .map(priv -> new SimpleEntry<>(priv, user)))
        .collect(
                Collectors.groupingBy(Entry::getKey, 
                    Collectors.mapping(Entry::getValue, Collectors.toList())));