使用Collectors.groupingBy（）时TreeMap排序不起作用

Question

我的列表就像

List<Map<String, Object>> sortedList = new ArrayList<>();

我的列表所在的位置

[{column_changed = first_name，sequence_number = 190001，change_date = 2019-01-01，customer_number = 3100}， {column_changed =姓氏，sequence_number = 190001，change_date = 2019-01-01，customer_number = 3100}， {column_changed = mail_id，sequence_number = 19002，change_date = 2019-01-05，customer_number = 3101}， {column_changed =出生日期，sequence_number = 190002，change_date = 2019-01-05，customer_number = 3101}， {column_changed =国籍，sequence_number = 190002，change_date = 2019-01-05，customer_number = 3101}， {column_changed = contact_number，sequence_number = 19003，change_date = 2019-01-10，customer_number = 3102}]

我按如下所示初始化Map的实例，

Map<String, List<Map<String, Object>>> mySortedMap = new TreeMap<>();

当我通过sequence_number使用Collectors.groupingBy（）时，

mySortedMap = sortedList.stream().collect(Collectors.groupingBy(change -> (String)change.get("sequence_number")));

输出如下，

{190001 = [{column_changed = first_name，sequence_number = 190001，change_date = 2019-01-01，customer_number = 3100}， {column_changed =姓氏，sequence_number = 190001，change_date = 2019-01-01，customer_number = 3100}]， 19003 = [{column_changed = contact_number，sequence_number = 19003，change_date = 2019-01-10，customer_number = 3102}]， 19002 = [{column_changed = mail_id，sequence_number = 19002，change_date = 2019-01-05，customer_number = 3101}， {column_changed =出生日期，sequence_number = 190002，change_date = 2019-01-05，customer_number = 3101}， {column_changed =国籍，sequence_number = 190002，change_date = 2019-01-05，customer_number = 3101}]}

但是我需要对键进行排序。尽管实例是TreeMap，但键未排序。我不知道为什么。谁能帮我这个忙。

Answer 1

在这里，您正在更改mySortedMap引用的对象。 groupingBy（）返回的地图可以是任何类型。从文档中，

  

无法保证返回的Map或List对象的类型，可变性，可序列化性或线程安全性。

如果您的列表已排序，请尝试使用

mySortedMap = sortedList.stream().collect(Collectors.groupingBy(
    change -> (String) change.get("sequence_number"), TreeMap::new, Collectors.toList()));