Collectors.groupingBy()でList<? extends String>をcollect

ExampleGroupBy.java

            Map<Integer, List<String>> list4 = Stream.of("a", "b")
                    .collect(Collectors.groupingBy(
                            String::length,
                            Collectors.toCollection(() -> new ArrayList<String>())
                    ));

            Map<Integer, List<? extends String>> list5 = Stream.of("a", "b")
                    .collect(Collectors.groupingBy(
                            String::length,
                            Collector.of(
                                    () -> new ArrayList<String>(),
                                    (list, x) -> list.add(x),
                                    (x, y) -> { x.addAll(y); return x; },
                                    list -> (List<? extends String>) list
                            )
                    ));

            Map<Integer, List<? extends String>> list6 = Stream.of("a", "b")
                    .collect(Collectors.groupingBy(
                            String::length,
                            Collectors.collectingAndThen(
                                    Collectors.toList(),
                                    l -> (List<? extends String>) l
                            )
                    ));