Java - Collectors.groupingBy - the last 15 days

Question

is it possible to group by the last 15 days using Java's Collectors.groupingBy?

Input:

public class Tweet { 

 String id;
 LocalDate createdAt; 

 // Constructor Ignored

 List<Tweet> tweets = new ArrayList(){{
  add(new Tweet("1", "2021-11-17"));
  add(new Tweet("2", "2021-11-16"));
  add(new Tweet("3", "2021-11-14"));
  add(new Tweet("4", "2021-11-13"));
  add(new Tweet("5", "2021-11-12"));
  add(new Tweet("7", "2021-10-09"));
  add(new Tweet("8", "2021-10-08"));
  add(new Tweet("9", "2021-10-07"));
  add(new Tweet("10", "2021-09-02"));
  add(new Tweet("11", "2021-09-01"));
 ...

 }};

}

Expected Output:

2021-11-17:  (group must include 2021-11-17 + 15 days before 17th)
 Tweet("1", "2021-11-17");
 Tweet("2", "2021-11-16");
 Tweet("3", "2021-11-14");
 Tweet("4", "2021-11-13");
 Tweet("5", "2021-11-12");

2021-10-09: (group to include 2021-10-09 + 15 days before 9th)
 Tweet("7", "2021-10-09");
 Tweet("8", "2021-10-08");
 Tweet("9", "2021-10-07");

2021-09-01: (group to include 2021-09-01 + 15 days before 1st)
Tweet("10", "2021-09-02");
Tweet("11", "2021-09-01");

Current logic (Not working):

Main Method:


TemporalAdjusters adjuster = TemporalAdjusters.ofDateAdjuster(d -> d.minusDays(15));

Map<LocalDate, List<Tweets>> groupByLast15Days = tweets.stream()
                .collect(Collectors.groupingBy(item -> item.createdAt())
                  .with(adjuster)));

System.out.println(groupByLast15Days);  // provides incorrect results

any help is massively appreciated! Thanks!

@blurfus - I don't think `filter()` alone can do the trick. `Collectors.GroupBy` seems more elegant. — Dinesh, Nov 17 '21 at 22:01
Are you trying to group into buckets of length 15 days? Or filter out entries more than 15 days ago? — Louis Wasserman, Nov 17 '21 at 22:05
@LouisWasserman - I am trying to group them into buckets of the length of 15 days. — Dinesh, Nov 17 '21 at 22:07

dan1st · Accepted Answer · 2021-11-18T19:19:13.590

4

You can get the day difference between two LocalDateTimes using ChronoUnit.DAYS.between(from,to) as stated here.

You can then divide this by 15 (so you have the same number for all days in that range):

LocalDate origin=LocalDate.now();
Map<LocalDate, List<Tweets>> groupByLast15Days = tweets.stream()
    .collect(Collectors.groupingBy(item-> 
       Long.valueOf(ChronoUnit.DAYS.between(item.createdAt(),origin)/15));

If you want to group by the start date, you can reverse the calculation (while the difference in between the 15 days is eliminated by the integer division):

LocalDate origin=LocalDate.now();
Map<LocalDate, List<Tweets>> groupByLast15Days = tweets.stream()
    .collect(Collectors.groupingBy(item -> 
        origin.plusDays(
            (ChronoUnit.DAYS.between(origin, item.createdAt()) /15)*15
        )
    ));

If you want to use TemporalAdjusters, you can do it like that:

LocalDate origin=LocalDate.now();
TemporalAdjusters adjuster = TemporalAdjusters.ofDateAdjuster(d ->
    origin.plusDays(
        (ChronoUnit.DAYS.between(origin, d) /15)*15
    )
);

Explainations:

Dividing an integer by another integer always yields an integer, even if the quotient would have decimal places. Integer division just removes those decimal places. If you divide a number by 15, it will return rhe same result for 15 consecutive numbers. After multiplying it with 15 again, you would get the first of those 15 numbers. Dividing a number by 15 and multiplying it with 15 again has the purpose that every 15 consecutive numbers are set to the same number (the first of those 15).

ChronoUnit.DAYS.between(date1, date2) just calculates the number of days between date1 and date2.

edited Nov 18 '21 at 19:19

answered Nov 17 '21 at 22:02

dan1st

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Wow that was fast.. Is it possible to have the origin as `item.createdAt()` instead of `now()`? I want to group them by `item.createAt() - 15`. The `creationAt` date may not be today's date.. – Dinesh Nov 17 '21 at 22:20
1

`origin` can be any date but it needs to be the same date for all elements. – dan1st Nov 17 '21 at 22:22
1

The only difference is when each set of 15 days starts (shifting `origin` by one day means each block will start one day later, shifting it by 15 days does nothing) – dan1st Nov 17 '21 at 22:26
let me try it out.. is there a syntax error? getting compilation error at the line `item.createdAt()/15` in solution 2. - `ChronoUnit.DAYS.between(origin, item.createdAt()/15)` – Dinesh Nov 17 '21 at 22:33
1

I fixed it. See my edit. – dan1st Nov 17 '21 at 22:35
Solution 2 and 3 works like a charm! Are you able to explain how ChronoUnit works and why are we multiplying 15 and then dividing by 15? Say if I want to change the 15 days interval to 10. Do I simply change them both to 10? – Dinesh Nov 18 '21 at 16:58
1

I edited my answer. Yes, changing both to 10 would merge every 10 consecutive days. – dan1st Nov 18 '21 at 19:15

Java - Collectors.groupingBy - the last 15 days

1 Answers1