Java 8 group list of objects with list inside

I would rather do it like so,

private static final String DELIMITER = "-";

Map<String, List<Favorite>> favsByGroup = favGroups.stream()
       .collect(Collectors.groupingBy(g -> g.getId() + DELIMITER + g.getUserId() + DELIMITER + g.getName(),
            Collectors.flatMapping(fg -> fg.getFavorites().stream(), Collectors.toList())));
List<FavoriteGroup> mergedFavoriteGroups = favsByGroup.entrySet().stream()
    .map(e -> new FavoriteGroup(Integer.parseInt(e.getKey().split(DELIMITER)[0]),
        e.getKey().split(DELIMITER)[1], e.getKey().split(DELIMITER)[2], e.getValue()))
    .collect(Collectors.toList());

First create a map taking the concatenated Id, userId and name properties of FavoriteGroup as the key and the List of Favorite instances as the value. Then for each entry in the map, I'll create a new FavoriteGroup by decoding the key and passing the values as it is. As a final step collect the resulting objects to a container.

Try this:-

favoriteGroups.stream()
        .collect(Collectors.toMap(FavoriteGroup::getId, Function.identity(), (fg1, fg2) -> {
            //This will modify the original object(s). If you don't want that, then you'll have to clone the object and set favorites.
            fg1.setFavorites(Stream.concat(fg1.getFavorites().stream(), fg2.getFavorites().stream())
                    .collect(Collectors.toList()));
            return fg1;
        }))
        .values();

This will return an object of type Collection<FavoriteGroup>, which you can iterate over. If you want to convert it back to List, you can do List<FavoriteGroup> list = new ArrayList<>(favoriteGroupsCollection);.