public static <T, K, U>
    Collector<T, ?, Map<K,U>> toLinkedMap(Function<? super T, ? extends K> keyMapper,
                                          Function<? super T, ? extends U> valueMapper) {
        return Collectors.toMap(keyMapper, valueMapper, throwingMerger(), LinkedHashMap::new);
    }
@Test
    public void linked_map_collector_returns_map_with_insertion_order() {
        List<String> stringList = numberList();
        Map<String, String> orderedMap = stringList.stream().collect(toLinkedMap(identity(), identity()));
        int i = 0;
        for (String val : orderedMap.keySet()) {
            assertEquals(stringList.get(i), val);
            i++;
        }
    }