java 8 list Collectors.toMap 保持顺序的两种方案

1 年前 · 来自专栏 java技术

Miraitowa

Java 8 list 转 map 保持顺序

思路直接转换为LinkedHashMap。

 User user = new User();
        user.setAccountId("1");
        List<User> users = Lists.newArrayList();
        User user2 = new User();
        user2.setAccountId("2");
        User user3 = new User();
        user3.setAccountId("3");
        User user4 = new User();
        user4.setAccountId("4");
        users.add(user);
        users.add(user3);
        users.add(user2);
        users.add(user4);

常用的两个参数方法

 public static <T, K, U>
    Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
                                    Function<? super T, ? extends U> valueMapper) {
        return toMap(keyMapper, valueMapper, throwingMerger(), HashMap::new);
    }

Map<String, User> userMap2 = Future home of users.stream().collect(Collectors.toMap(a -> a.getAccountId(), a -> a));
        log.info("转回map：{}", JSON.toJSONString(userMap2));
        //转回map：{"1":{"account_id":"1","query":{}},"2":{"account_id":"2","query":{}},"3":{"account_id":"3","query":{}},"4":{"account_id":"4","query":{}}}

这个函数是转map,无法保证数据的顺序。而保留数据的顺序实际上就转为 LinkedHashMap

四个参数方法

  public static <T, K, U, M extends Map<K, U>>
    Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,
                                Function<? super T, ? extends U> valueMapper,
                                BinaryOperator<U> mergeFunction,
                                Supplier<M> mapSupplier) {
        BiConsumer<M, T> accumulator
                = (map, element) -> map.merge(keyMapper.apply(element),
                                              valueMapper.apply(element), mergeFunction);
        return new CollectorImpl<>(mapSupplier, accumulator, mapMerger(mergeFunction), CH_ID);