Getting Weekly Max value rom

Question

Problem:

• I have a Map<LocalDate, Integer> and I want to group these map elements by week using LocalDate and find the max value of each grouped values by week using Java 8 Stream.

• I am currently having issues where I can't convert the Key back into LocalDate object in the stream due to the fact that I do not have the required data fields to create object of LocalDate into the Map.

• If I were to not convert this to LocalDate and keep it as String instead, I cant use the TreeMap to arrange the keys-values in natural order using the keys.

• If I do not include month into DateTimeFormatter.ofPattern when grouping the data, weeks may overlap between two years.

• If I only use week number Integer as output of the map from the stream, the grouped values by week will be mixed and overlapped with week numbers from another year, i.e. 2020-2021 with Week 1.

Question:

• How do I get the max value of values per week using java streams?

DataSet:

Map<LocalDate,Integer> CumulativeGarbageWasteDate = new HashMap<LocalDate,Integer>();
    CumulativeGarbageWasteDate.put(LocalDate.parse(("5/2/2020"),DateTimeFormatter.ofPattern("[M/d/yyyy][M/d/yy]")),2400);
    CumulativeGarbageWasteDate.put(LocalDate.parse(("12/24/20"),DateTimeFormatter.ofPattern("M/d/yy")),140);
    CumulativeGarbageWasteDate.put(LocalDate.parse(("5/2/20"),DateTimeFormatter.ofPattern("M/d/yy")),2400);
    
    CumulativeGarbageWasteDate.put(LocalDate.parse(("01/1/21"),DateTimeFormatter.ofPattern("M/d/yy")),182);
    CumulativeGarbageWasteDate.put(LocalDate.parse(("01/2/21"),DateTimeFormatter.ofPattern("M/d/yy")),203);
    CumulativeGarbageWasteDate.put(LocalDate.parse(("01/3/21"),DateTimeFormatter.ofPattern("M/d/yy")),321);
    CumulativeGarbageWasteDate.put(LocalDate.parse(("01/3/21"),DateTimeFormatter.ofPattern("M/d/yy")),421);
    CumulativeGarbageWasteDate.put(LocalDate.parse(("5/2/2021"),DateTimeFormatter.ofPattern("[M/d/yyyy][M/d/yy]")),2400);
    CumulativeGarbageWasteDate.put(LocalDate.parse(("01/6/21"),DateTimeFormatter.ofPattern("M/d/yy")),1200);
    CumulativeGarbageWasteDate.put(LocalDate.parse(("01/31/21"),DateTimeFormatter.ofPattern("M/d/yy")),2400);

Code:

private static Map<LocalDate, Integer> sumByWeek (Map<LocalDate,Integer> CumulativeGarbageWasteDate){
    
    DateTimeFormatter formatter3 = new DateTimeFormatterBuilder().appendPattern("w/M/YY").parseDefaulting(ChronoField.DAY_OF_WEEK,DayOfWeek.FRIDAY.getValue()).toFormatter();
    

    return CumulativeGarbageWasteDate
            .entrySet()
            .stream()
            .collect(Collectors.groupingBy(
                    row-> LocalDate.parse(row.getKey().format(DateTimeFormatter.ofPattern("w/M/YY")),formatter3),
                    TreeMap::new,
                    Collectors.reducing(0, x->x.getValue(),Math::max)));
}

Error:

Here is the error I encountered when running the code.

java.time.format.DateTimeParseException: Text '5/2/20' could not be parsed: Conflict found: Field MonthOfYear 1 differs from MonthOfYear 2 derived from 2020-01-31

Answer 1

The output map will be of the type Map<LocalDate, Integer>, where the key would be the date which had the highest value for that week. (I am splitting to solution to make it more understandable).

First I would suggest the creation of a class (may be an inner class) to hold the date and it's corresponding value. This would make the processing easy.

private static class MyData {
    private LocalDate date;
    private int value;
    
    // getters and a constructor
}

Note: Assume static import for Collectors methods like grouping, toMap, toList, mapping in the below codes.

---

Week of the year could be retrieved using WeekFields.(Refer)

NOTE: row.getKey().getYear() should not be used in the below code instead of row.getKey().get(weekFields.weekBasedYear()) to fetch the year as it could yield incorrect results in the scenarios like: 2022-01-01 and 2021-12-31, belongs to week 1 of 2022. So if we use getYear(), then the respective generated string would be 1/2022 and 1/2021, which is incorrect. The expected string in this case would be 1/2022.

WeekFields weekFields = WeekFields.of(Locale.getDefault());

// function to generate a key of the form: week~year. year included to avoid overlap
Function<Entry<LocalDate, Integer>, String> groupFn 
    = row -> String.format("%d/%d",
                           row.getKey().get(weekFields.weekOfWeekBasedYear()),
                           row.getKey().get(weekFields.weekBasedYear()));

Now convert the input into MyData objects:

Map<String, List<MyData>> temp = 
    input.entrySet()
         .stream()
         .collect(groupingBy(groupFn,
                             TreeMap::new, 
                             mapping(e -> new MyData(e.getKey(), e.getValue()), toList())));

Now we find the date with max value within a week:

Function<Entry<String, List<MyData>>, MyData> maxDataFn 
    = entry -> entry.getValue()
                    .stream()
                    .collect(collectingAndThen(maxBy(Comparator.comparingInt(MyData::getValue)),
                                               Optional::get));

TreeMap<LocalDate, Integer> result 
    = temp.entrySet()
          .stream()
          .map(maxDataFn)
          .collect(toMap(MyData::getDate, MyData::getValue, Integer::max, TreeMap::new));