groupByMap.java

package de.fwidm.streams;

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class GroupByFun {
    public static void main(String[] args) {
        GroupByFun groupByFun = new GroupByFun();
        List<Event> events = groupByFun.init();
        List<String> group = groupByFun.group(events, GroupByFun::byYear);
        System.out.println(group);

        group = groupByFun.group(events, GroupByFun::byYearMonth);
        System.out.println(group);

        group = groupByFun.group(events, GroupByFun::byYearQuarter);
        System.out.println(group);
    }

    private List<Event> init() {
        LocalDateTime baseDate = LocalDateTime.of(2019, 1, 1, 0, 0, 0, 0);

        List<Event> events = Arrays.asList(
                new Event("0xBEEF", baseDate, 0L),
                new Event("0xBEEF", baseDate.plusMonths(1), 1L),
                new Event("0xBEEF", baseDate.plusMonths(1).plusDays(2), 3L),
                new Event("0xBEEF", baseDate.plusMonths(1).plusDays(4), 4L),
                new Event("0xBEEF", baseDate.plusMonths(1).plusDays(7), 5L),
                new Event("0xBEEF", baseDate.plusMonths(2), 2L),
                new Event("0xBEEF", baseDate.plusMonths(2).plusDays(4), 6L),
                new Event("0xBEEF", baseDate.plusMonths(2).plusDays(24), 7L),
                new Event("0xBEEF", baseDate.plusMonths(3).plusDays(12), 8L),
                new Event("0xBEEF", baseDate.plusMonths(3).plusDays(20), 9L),
                new Event("0xBEEF", baseDate.plusMonths(4).plusDays(4), 10L),
                new Event("0xBEEF", baseDate.plusMonths(5), 13L),
                new Event("0xBEEF", baseDate.plusMonths(6), 20L),
                new Event("0xBEEF", baseDate.plusMonths(7), 23L),
                new Event("0xBEEF", baseDate.plusMonths(8), 24L),
                new Event("0xBEEF", baseDate.plusMonths(9), 36L),
                new Event("0xBEEF", baseDate.plusMonths(10), 40L),
                new Event("0xBEEF", baseDate.plusMonths(11), 42L),
                new Event("0xBEEF", baseDate.plusMonths(12), 42L),
                new Event("0xBEEF", baseDate.plusMonths(13), 45L),
                new Event("0xBEEF", baseDate.plusMonths(13), 56L),
                new Event("0xBEEF", baseDate.plusMonths(14), 60L),
                new Event("0xBEEF", baseDate.plusMonths(15), 54L),
                new Event("0xBEEF", baseDate.plusMonths(16), 100L));
        return events;
    }

    private static String byYear(Event e) {
        return e.getDateTime().format(DateTimeFormatter.ofPattern("yyyy"));
    }

    private static String byYearMonth(Event e) {
        return e.getDateTime().format(DateTimeFormatter.ofPattern("yyyy-MM"));
    }

    private static String byYearQuarter(Event e) {
        int year = e.getDateTime().getYear();
        switch (e.getDateTime().format(DateTimeFormatter.ofPattern("Q"))) {
            case "1":
                return String.format("%s-I", year);
            case "2":
                return String.format("%s-II", year);
            case "3":
                return String.format("%s-III", year);
            default:
                return String.format("%s-IV", year);
        }
    }

    private List<String> group(List<Event> events, Function<Event, String> func) {
        Map<String, Event> stringEventMap = events.stream()
                .sorted(Comparator.comparing(Event::getDateTime))
                .collect(Collectors.groupingBy(func,
                        Collectors.collectingAndThen(
                                Collectors.toList(),
                                this::getLast)));
        return stringEventMap.entrySet().stream()
                .sorted(Comparator.comparing(Map.Entry::getKey))
                .map(e -> String.format("%s=%s", e.getKey(), e.getValue().value))
                .collect(Collectors.toList());

    }

    private <T> T getLast(List<T> list) {
        return list.get(list.size() - 1);
    }

    public class Event {
        String parentUuid;
        LocalDateTime dateTime;
        Long value;

        public Event(String parentUuid, LocalDateTime dateTime, Long value) {
            this.parentUuid = parentUuid;
            this.dateTime = dateTime;
            this.value = value;
        }

        public String getParentUuid() {
            return parentUuid;
        }

        public void setParentUuid(String parentUuid) {
            this.parentUuid = parentUuid;
        }

        public LocalDateTime getDateTime() {
            return dateTime;
        }

        public void setDateTime(LocalDateTime dateTime) {
            this.dateTime = dateTime;
        }

        public Long getValue() {
            return value;
        }

        public void setValue(Long value) {
            this.value = value;
        }

        @Override
        public String toString() {
            return "Event{" +
                    "parentUuid='" + parentUuid + '\'' +
                    ", dateTime=" + dateTime +
                    ", value=" + value +
                    '}';
        }
    }
}

out

[2019=42, 2020=100]
[2019-01=0, 2019-02=5, 2019-03=7, 2019-04=9, 2019-05=10, 2019-06=13, 2019-07=20, 2019-08=23, 2019-09=24, 2019-10=36, 2019-11=40, 2019-12=42, 2020-01=42, 2020-02=56, 2020-03=60, 2020-04=54, 2020-05=100]
[2019-I=7, 2019-II=13, 2019-III=24, 2019-IV=42, 2020-I=60, 2020-II=100]