listToMapGroupedAndBack.java

private static Map<String, List<SomeObject>> groupListToMapById(List<SomeObject> items) {
    return items.stream()
                .collect(groupingBy(SomeObject::getId, toList()))
                .entrySet()
                .stream()
                .collect(toMap(
                  Map.Entry::getKey,
                  Map.Entry::getValue,
                  (a, b) -> {throw new AssertionError();},
                  LinkedHashMap::new
                ));
  }

private static List<SomeObject> getListFromMap(Map<String, List<SomeObject>> items) {
    return items.values().stream()
                .filter(e -> !e.isEmpty())
                .flatMap(Collection::stream)
                .collect(toList());
  }