Java: How do I group all java dates in the corresponding week?

Question

Problem:

I have a long list of dates, and I need to sort them by week.

Question: How do I group dates by the week of the year in which they are located?

Data set example

Date date = new SimpleDateFormat.parse("04/01/2015") Date date = new SimpleDateFormat.parse("04/02/2015") Date date = new SimpleDateFormat.parse("04/03/2015") Date date = new SimpleDateFormat.parse("04/04/2015") Date date = new SimpleDateFormat.parse("04/05/2015") Date date = new SimpleDateFormat.parse("04/06/2015") Date date = new SimpleDateFormat.parse("04/07/2015") Date date = new SimpleDateFormat.parse("04/08/2015") Date date = new SimpleDateFormat.parse("04/09/2015") 

Desired Result

 HashMap<Date, Date> hashMap = groupByWeek(ArrayList<Date> dates); printWeeklyGroupedDates(); Week 1: 04/01/2015 04/02/2015 04/03/2015 04/04/2015 04/05/2015 04/06/2015 04/07/2015 Week 2: 04/08/2015 04/09/2015 

What i tried

 public Date getWhichYearlyWeek(Date date){ Calendar cal = Calendar.getInstance(); cal.setTime(date); cal.get(Calendear.WEEK_OF_YEAR); //Need to return this as 'Date' but indicate which week this is for the TreeMap functionality below return cal.getTime(); } public TreeMap<Date, ArrayList<Date>> getWeeklyMappedDates(ArrayList<Date> dateArray){ TreeMap<Date, ArrayList<Date>> treeMap = new TreeMap<Date, ArrayList<Date>> for(Date i : dateArray){ Date date = getWhichYearlyWeek(date); if(!treeMap.containsKey(date)) treeMap.get(date).add(date); else treeMap.put(date, new ArrayList<Date>()); } return treeMap; } 

Answer 1

You can simply sort the List from Date using a custom Comparator . The following example sorts the date by week of the year, and then by date, I don’t know if you want the second part, but this is a good demonstration ...

 import java.text.ParseException; import java.text.SimpleDateFormat; import java.util.ArrayList; import java.util.Calendar; import java.util.Collections; import java.util.Comparator; import java.util.Date; import java.util.List; public class Test { public static void main(String[] args) { try { SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy"); List<Date> dates = new ArrayList<Date>(25); dates.add(sdf.parse("04/01/2015")); dates.add(sdf.parse("04/02/2015")); dates.add(sdf.parse("04/03/2015")); dates.add(sdf.parse("04/04/2015")); dates.add(sdf.parse("04/05/2015")); dates.add(sdf.parse("04/06/2015")); dates.add(sdf.parse("04/07/2015")); dates.add(sdf.parse("04/08/2015")); dates.add(sdf.parse("04/09/2015")); int week = 0; int woy = -1; Collections.sort(dates, new WeekComparator()); for (Date date : dates) { if (woy != getWeekOfYear(date)) { woy = getWeekOfYear(date); week++; System.out.println("Week " + week + ":"); } System.out.println(date); } } catch (ParseException exp) { exp.printStackTrace(); } } public static class WeekComparator implements Comparator<Date> { @Override public int compare(Date o1, Date o2) { int result = getWeekOfYear(o1) - getWeekOfYear(o2); if (result == 0) { result = o1.compareTo(o2); } return result; } } protected static int getWeekOfYear(Date date) { Calendar cal = Calendar.getInstance(); cal.setTime(date); return cal.get(Calendar.WEEK_OF_YEAR); } } 

What are the exits

 Week 1: Wed Apr 01 00:00:00 EST 2015 Thu Apr 02 00:00:00 EST 2015 Fri Apr 03 00:00:00 EST 2015 Sat Apr 04 00:00:00 EST 2015 Week 2: Sun Apr 05 00:00:00 EST 2015 Mon Apr 06 00:00:00 EST 2015 Tue Apr 07 00:00:00 EST 2015 Wed Apr 08 00:00:00 EST 2015 Thu Apr 09 00:00:00 EST 2015 

Java 8 Time API

And just because I need practice ...

 import java.text.SimpleDateFormat; import java.time.LocalDate; import java.time.format.DateTimeFormatter; import java.time.temporal.WeekFields; import java.util.ArrayList; import java.util.Collections; import java.util.Comparator; import java.util.List; import java.util.Locale; public class Test { public static void main(String[] args) { SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy"); DateTimeFormatter dtf = DateTimeFormatter.ofPattern("MM/dd/yyyy"); List<LocalDate> dates = new ArrayList<LocalDate>(25); dates.add(LocalDate.parse("04/01/2015", dtf)); dates.add(LocalDate.parse("04/02/2015", dtf)); dates.add(LocalDate.parse("04/03/2015", dtf)); dates.add(LocalDate.parse("04/04/2015", dtf)); dates.add(LocalDate.parse("04/05/2015", dtf)); dates.add(LocalDate.parse("04/06/2015", dtf)); dates.add(LocalDate.parse("04/07/2015", dtf)); dates.add(LocalDate.parse("04/08/2015", dtf)); dates.add(LocalDate.parse("04/09/2015", dtf)); int week = 0; int woy = -1; Collections.sort(dates, new WeekComparator()); for (LocalDate date : dates) { if (woy != getWeekOfYear(date)) { woy = getWeekOfYear(date); week++; System.out.println("Week " + week + ":"); } System.out.println(date); } } public static class WeekComparator implements Comparator<LocalDate> { @Override public int compare(LocalDate o1, LocalDate o2) { int result = getWeekOfYear(o1) - getWeekOfYear(o2); if (result == 0) { result = o1.compareTo(o2); } return result; } } protected static int getWeekOfYear(LocalDate date) { WeekFields wf = WeekFields.of(Locale.getDefault()); return date.get(wf.weekOfYear()); } } 

What are the exits

 Week 1: 2015-04-01 2015-04-02 2015-04-03 2015-04-04 Week 2: 2015-04-05 2015-04-06 2015-04-07 2015-04-08 2015-04-09 

In any case, you can match each of the dates in Map with a key for a week, but it is up to you

Answer 2

If Java 8 is available:

 DateTimeFormatter formatter = DateTimeFormatter.ofPattern("MM/dd/yyyy"); TemporalField weekOfYear = WeekFields.of(Locale.getDefault()).weekOfYear(); Stream.of( "04/01/2015" , "04/02/2015" , "04/03/2015" , "04/04/2015" , "04/05/2015" , "04/06/2015" , "04/07/2015" , "04/08/2015" , "04/09/2015" ) .collect(Collectors.groupingBy( d -> LocalDate.parse(d, formatter).get(weekOfYear), LinkedHashMap::new, Collectors.toList() )) .forEach((week, dates) -> { System.out.println("Week " + week + ":"); dates.forEach(System.out::println); }); 

This prints (using the de_CH locale on my computer)

 Week 14: 04/01/2015 04/02/2015 04/03/2015 04/04/2015 04/05/2015 Week 15: 04/06/2015 04/07/2015 04/08/2015 04/09/2015 

Answer 3

Use the code below to get the week number:

 public static int getWeek(Date date) { Calendar cal = Calendar.getInstance(); cal.setTime(date); return cal.get(WEEK_OF_YEAR); } 

Answer 4

Since you indicate that you need to return the date, try using Calendar#clear() and then Calendar#setTime(Date) , and then use Calendar # get (Calendar.WEEK_OF_YEAR).

Since #getWhichYearlyWeek(Date) now redundant, you can skip it altogether.

Answer 5

For may not work the way you implemented it, here are some fixes:

 for (Date i : dateArray) { Date date = getWhichYearlyWeek(date); if(treeMap.containsKey(date)) treeMap.get(date).add(i); else { ArrayList<Date> al = new ArrayList<Date>(); al.add(i); treeMap.put(date, al); } } 

• If it weren’t, but he should not
• You added a week instead of a date
• You forgot to add a date to an empty ArrayList

Use GregorianCalendar :

 public Date getWhichYearlyWeek(Date date) { GregorianCalendar cal = new GregorianCalendar(); cal.setTime(date); // (*) = remove if you are only dealing with real dates and not dateTime cal.set(Calendar.HOUR_OF_DAY, 0); // (*) cal.clear(Calendar.MINUTE); // (*) cal.clear(Calendar.SECOND); // (*) cal.clear(Calendar.MILLISECOND); // (*) cal.set(Calendar.DAY_OF_WEEK, cal.getFirstDayOfWeek()); // or Calendar.MONDAY return cal.getTime(); } 

Answer 6

You can try this code to group dates by WEEK_OF_YEAR.

 import java.text.ParseException; import java.text.SimpleDateFormat; import java.util.ArrayList; import java.util.Calendar; import java.util.Date; import java.util.HashMap; import java.util.List; import java.util.Map; public class GroupByWeekSampleCode { /** * @param year * @param weekOfYear * @return a unique hashcode generated from year and week of year */ public static int getYearWeekOfYearHashCode(int year, int weekOfYear) { int result = (int) (((float) year + ((float) weekOfYear / 100)) * 100); return result; } /** * @param hashCode * @return the week of year from this hash code */ public static int getWeekOfYearFromHashCode(int hashCode) { int year = (int) hashCode / 100; int weekOfYear = (int) (hashCode % year); return weekOfYear; } /** * @param hashCode * @return year from this hash code */ public static int getYearFromHashCode(int hashCode) { int year = (int) hashCode / 100; return year; } public static void main(String[] args) { try { List<Date> datesToGroup = new ArrayList<Date>(); SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy"); datesToGroup.add(sdf.parse("04/01/2015")); datesToGroup.add(sdf.parse("04/02/2015")); datesToGroup.add(sdf.parse("04/03/2015")); datesToGroup.add(sdf.parse("04/04/2015")); datesToGroup.add(sdf.parse("04/05/2015")); datesToGroup.add(sdf.parse("04/06/2015")); datesToGroup.add(sdf.parse("04/07/2015")); datesToGroup.add(sdf.parse("04/08/2015")); datesToGroup.add(sdf.parse("04/09/2015")); datesToGroup.add(sdf.parse("12/27/2015")); datesToGroup.add(sdf.parse("12/28/2015")); datesToGroup.add(sdf.parse("12/29/2015")); datesToGroup.add(sdf.parse("12/30/2015")); datesToGroup.add(sdf.parse("12/31/2015")); datesToGroup.add(sdf.parse("01/01/2016")); datesToGroup.add(sdf.parse("01/02/2016")); datesToGroup.add(sdf.parse("01/03/2016")); datesToGroup.add(sdf.parse("01/04/2016")); final Map<Integer, List<Date>> groupedDatesByWeekOfYear = new HashMap<Integer, List<Date>>() { private static final long serialVersionUID = 1L; @Override public List<Date> get(Object key) { List<Date> list = super.get(key); if (list == null) { list = new ArrayList<Date>(); super.put((Integer) key, list); } return list; } }; for (Date date : datesToGroup) { Calendar cal = Calendar.getInstance(); cal.setTime(date); Calendar previousYearLastDate = Calendar.getInstance(); previousYearLastDate.set(Calendar.YEAR, cal.get(Calendar.YEAR) - 1); previousYearLastDate.set(Calendar.MONTH, 11); previousYearLastDate.set(Calendar.DAY_OF_MONTH, previousYearLastDate.getActualMaximum(Calendar.DAY_OF_MONTH)); // a hash code generated from year and week of year value int yearWeekOfYearHashCode = 0; // to check if some of the dates in this week of year comes // in previous year last month, if true then the previous year // should be taken to generate hash code if (cal.get(Calendar.MONTH) == 0 && cal.get(Calendar.WEEK_OF_YEAR) == previousYearLastDate.get(Calendar.WEEK_OF_YEAR)) { yearWeekOfYearHashCode = getYearWeekOfYearHashCode(previousYearLastDate.get(Calendar.YEAR), previousYearLastDate.get(Calendar.WEEK_OF_YEAR)); } else { yearWeekOfYearHashCode = getYearWeekOfYearHashCode(cal.get(Calendar.YEAR), cal.get(Calendar.WEEK_OF_YEAR)); } groupedDatesByWeekOfYear.get(yearWeekOfYearHashCode).add(date); } for (Integer yearWeekOfYearHashCode : groupedDatesByWeekOfYear.keySet()) { System.out.println("year=" + getYearFromHashCode(yearWeekOfYearHashCode) + ", weekOfYear=" + getWeekOfYearFromHashCode(yearWeekOfYearHashCode) + ", dates under this week "); List<Date> datesInThisWeek = groupedDatesByWeekOfYear.get(yearWeekOfYearHashCode); for (Date date : datesInThisWeek) { System.out.println(date); Calendar cal = Calendar.getInstance(); cal.setTime(date); } System.out.println(); } } catch (ParseException e) { e.printStackTrace(); } } } 

Will open

 year=2015, weekOfYear=1, dates under this week Sun Dec 27 00:00:00 IST 2015 Mon Dec 28 00:00:00 IST 2015 Tue Dec 29 00:00:00 IST 2015 Wed Dec 30 00:00:00 IST 2015 Thu Dec 31 00:00:00 IST 2015 Fri Jan 01 00:00:00 IST 2016 Sat Jan 02 00:00:00 IST 2016 year=2015, weekOfYear=14, dates under this week Wed Apr 01 00:00:00 IST 2015 Thu Apr 02 00:00:00 IST 2015 Fri Apr 03 00:00:00 IST 2015 Sat Apr 04 00:00:00 IST 2015 year=2015, weekOfYear=15, dates under this week Sun Apr 05 00:00:00 IST 2015 Mon Apr 06 00:00:00 IST 2015 Tue Apr 07 00:00:00 IST 2015 Wed Apr 08 00:00:00 IST 2015 Thu Apr 09 00:00:00 IST 2015 year=2016, weekOfYear=2, dates under this week Sun Jan 03 00:00:00 IST 2016 Mon Jan 04 00:00:00 IST 2016 

Answer 7

Use the functionality of functional programming with Java 8 threads and use the new Java Api time:

Assuming your dates are in a collection, for example:

 List<LocalDate> dates = getMyDates(); WeekFields weekFields = WeekFields.of(DayOfWeek.MONDAY,7); Map<Integer, List<LocalDate>> datesGroupedByWeekNumber = dates.stream() .collect(Collectors.groupingBy(date -> date.get(weekFields.weekOfYear()))); 

You are going to get a card where the key integer is the week number of the year, and the value for this key is a list of dates when these dates refer to this week. Room.