how do i count no of dates from weeks on the given date list

Question

I want to find the number of dates from 1st week, 2nd week, 3 rd week and 4th week from the given dates list

[2017-11-22 21:41:39.0, 2017-11-27 23:15:26.0, 2017-11-27 23:26:23.0,
 2017-11-28 19:50:18.0, 2017-11-29 16:14:33.0] are dates

//By Using map  
Map<Integer, List<Date>> map;
map = inboxOutboxentityList.stream()
    .collect(Collectors.groupingBy(element -> 
         element.getDate().get(weekFields.weekOfWeekBasedYear())
    )
);

Answer 1

If you can, just forget about the long outdated Date class and solve your problem in pure java.time . Your code is not far from working.

    List<LocalDateTime> entities = Arrays.asList(
            LocalDateTime.of(2017, Month.NOVEMBER, 22, 21, 41, 39),
            LocalDateTime.of(2017, Month.NOVEMBER, 27, 23, 15, 26),
            LocalDateTime.of(2017, Month.NOVEMBER, 27, 23, 26, 23),
            LocalDateTime.of(2017, Month.NOVEMBER, 28, 19, 50, 18),
            LocalDateTime.of(2017, Month.NOVEMBER, 29, 16, 14, 33));
    Map<Integer, Long> countPerWeek = entities.stream()
            .collect(Collectors.groupingBy(
                    ldt -> ldt.get(WeekFields.ISO.weekOfWeekBasedYear()),
                    Collectors.counting()));
    System.out.println(countPerWeek);

This prints:

{48=4, 47=1}

So the count is 1 for week 47 and 4 for week 48. I believe this was what you intended. Please substitute a different WeekFields object if required.

If you are getting a list of Date objects, you will need to convert them into some modern class first. There are plenty of questions and answers about how to do that, happy searcing. As you have probably already realized, you can do it in the same series of stream operations.