Sort map in stream by LocalDateTime

Question

I have map Map<LocalDateTime, String>.

How can I sort it by key values? What comparator should I use?

someMap.entrySet().stream().sorted(Comparator.comparing(???))

or

someMap.entrySet().stream().sorted(??)

How can I resolve it? What should I write instead "??" ?

Sorted:

Key                      Value
2020-01-09 09:57:58.631  Some info
2020-01-09 09:57:59.224  Some info
2020-01-09 09:59:03.144  Info

Without sorting:

Key                     Value
2020-01-09 09:57:58.631  Some info
2020-01-09 09:59:03.144  Info
2020-01-09 09:57:59.224  Some info

Can you show an example of an unsorted `Map`, and the sorted version of the same map? — Sweeper, Nov 10 '20 at 10:36
Your example is confusing. You should show the _same_ map both sorted and unsorted. Also, show which of date or string should have a higher priority. Do you sort by the dates first, and if the dates are the same, sort by the strings? Or, do you sort by the strings first, and if the strings are the same, sort by the dates? — Sweeper, Nov 10 '20 at 10:44

score 5 · Accepted Answer · answered Nov 10 '20 at 10:41

5

You can use .sorted(Map.Entry.comparingByKey()) like this:

Map<LocalDateTime, String> collect = someMap.entrySet().stream()
        .sorted(Map.Entry.comparingByKey())
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (o1, o2) -> o1, LinkedHashMap::new));

Output

{2020-04-01T12:27:48.054=info, 2020-04-17T11:15:13.423=info, 2020-04-29T11:01:21.372=info}

answered Nov 10 '20 at 10:41

Youcef LAIDANI

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3

Didn't know about `Map.Entry.comparingByKey()`. Nice one! – MC Emperor Nov 10 '20 at 10:52
Yes it exist from [Java-8](https://docs.oracle.com/javase/8/docs/api/java/util/Map.Entry.html#comparingByKey--), thank you :) – Youcef LAIDANI Nov 10 '20 at 10:54

score 1 · Answer 2 · answered Nov 10 '20 at 10:50

Map<LocalDateTime, String> map = new HashMap<>();
map.put(LocalDateTime.of(2020, 4, 17, 11, 15), "Value A");
map.put(LocalDateTime.of(2020, 4, 1, 12, 27), "Value B");
map.put(LocalDateTime.of(2020, 4, 29, 11, 1), "Value C");
    
map.entrySet().stream().sorted(Map.Entry.comparingByKey()).forEach(entry -> {
    System.out.println(entry);
});

will result in

2020-04-01T12:27=Value B
2020-04-17T11:15=Value A
2020-04-29T11:01=Value C

Sort map in stream by LocalDateTime

2 Answers2