[英]How to sort Map<YearMonth, List<LocalDate>> with java 8 lambda and streams
I have a list of sorted dates like this: 我有一个这样的排序日期列表:
2016-07-07
2016-07-08
2016-07-09
2016-07-10
2016-07-11
2016-07-12
2016-07-13
...
2016-07-31
2016-08-01
2016-08-02
2016-08-03
...
2017-01-01
2017-01-02
2017-01-03
...
From this list I generate a Map<YearMonth, List<LocalDate>>
with stream: 从这个列表中我生成一个
Map<YearMonth, List<LocalDate>>
with stream:
Map<YearMonth, List<LocalDate>> d = dates.stream().collect(Collectors.toList())
.stream().collect(Collectors.groupingBy(date -> YearMonth.from(date)));
The output of that map looks like this: 该映射的输出如下所示:
{2016-12=[2016-12-01, 2016-12-02,...2016-12-31], 2016-11=[2016-11-01, 2016-11-02,...]}
But my needed output should look be like this: 但我需要的输出应该是这样的:
{2016-07=[...], 2016-08=[...]}
{2016-07=[...], 2016-08=[...]}
{2016-07=[2016-07-01, 2016-07-02, ...], 2016-08=[2016-08-01, 2016-08-02, ...]}
{2016-07=[2016-07-01, 2016-07-02, ...], 2016-08=[2016-08-01, 2016-08-02, ...]}
I tried many options to get my expected result but I just get the correct sorting for the key or the value and not for both : 我尝试了很多选项来获得我的预期结果,但我只是得到了正确的键或值的排序, 而不是两者 :
Map<YearMonth, List<LocalDate>> m = stream().collect(Collectors.toList())
.stream().sorted((e1,e2) -> e2.compareTo(e1))
.collect(Collectors.groupingBy(date -> YearMonth.from(date)));
Result: 结果:
{2016-07=[2016-07-31, 2016-07-30, ...], 2016-08=[2016-08-31, 2016-08-30, ...]}
How can I sort both by key and value? 如何按键和值对它们进行排序?
Use a TreeMap as collector so the output is sorted by key. 使用TreeMap作为收集器,以便按键对输出进行排序。
Something like this: 像这样的东西:
dates.stream()
.sorted()
.collect(
Collectors.groupingBy(YearMonth::from, TreeMap::new, Collectors.toList())
);
Collectors.groupingBy(date -> YearMonth.from(date))
internally stores result in HashMap and key sorting is lost. Collectors.groupingBy(date -> YearMonth.from(date))
内部将结果存储在HashMap中,并且密钥排序丢失。
This implementation will preserve keys order: 此实现将保留键顺序:
Map<YearMonth, List<LocalDate>> d = dates
.stream()
.sorted((e1,e2) -> e2.compareTo(e1))
.collect(Collectors
.groupingBy(YearMonth::from,
LinkedHashMap::new,
Collectors.toList()));
You can use specific Collectors
that return a sorted Collection. 您可以使用返回已排序集合的特定
Collectors
。 In your case I would use a TreeMap
to sort the resulting Map by its keys, and explicitly sort the resulting value collections like that: 在您的情况下,我将使用
TreeMap
按其键对结果Map进行排序,并显式对结果值集合进行排序:
Map<YearMonth, List<LocalDate>> m = dates.stream()
.collect(Collectors.groupingBy(
date -> YearMonth.from(date),
TreeMap::new,
Collectors.collectingAndThen(
Collectors.toList(),
(list) -> { Collections.sort(list); return list; })));
You can sort them in the following way:- 您可以通过以下方式对它们进行排序: -
Map<YearMonth, List<LocalDate>> map = dates.stream()
.collect(Collectors.toList())
.stream()
.sorted((e1,e2) -> e1.compareTo(e2))
.collect(Collectors.groupingBy(date -> YearMonth.from(date), TreeMap::new, Collectors.toList()));
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