GroupByTest.java

// group by traceId, event fields and get the max item.
void testIt() {
    List<Event> events = new ArrayList<>();
    events.add(new Event("AAA", "ROOM_START", 1000L, 10));
    events.add(new Event("AAA", "ROOM_START", 1000L, 10));
    events.add(new Event("BBB", "ROOM_START", 1002L, 15));
    events.add(new Event("BBB", "ROOM_START", 1001L, 10));

    // method1
    Map<String, Map<String, List<Event>>> collect = events.stream()
            .collect(Collectors.groupingBy(Event::getTraceId, Collectors.groupingBy(Event::getEvent)));
            
    collect.forEach((traceId, mp) -> {
        mp.forEach((event, ls) -> {
            System.out.printf("\t traceId=%s, event=%s, mp=%s\n", traceId, event, JSON.toJSONString(ls));
            Event last = ls.stream().sorted(Comparator.comparing(Event::getLocalTs).reversed()).findFirst().orElse(null);
            System.out.printf("\t last: %s\n", last);
        });
    });

    // method 2
    Function<Event, List<String>> compositeKey = event -> Arrays.asList(event.getTraceId(), event.getEvent());
    Map<List<String>, List<Event>> listMap = events.stream().collect(Collectors.groupingBy(compositeKey, Collectors.toList()));
    listMap.forEach((g, ls) -> {
        Event last = ls.stream().sorted(Comparator.comparing(Event::getLocalTs).reversed()).findFirst().orElse(null);
        System.out.printf("\t last: %s\n", last);
    });

    // method 3
    List<Event> rs = new ArrayList<>();
    events.stream()
            .collect(Collectors.groupingBy(Event::getTraceId, Collectors.groupingBy(Event::getEvent)))
            .forEach((traceId, mp) -> {
                mp.forEach((event, ls) -> {
                    Optional<Event> lastOpt = ls.stream().max(Comparator.comparing(Event::getLocalTs));
                    lastOpt.ifPresent(rs::add);
                });
            }
    );

    System.out.println(JSON.toJSONString(rs));

    // method 4
    List<Event> rs2 = new ArrayList<>();
    Function<Event, List<String>> gKey = event -> Arrays.asList(event.getTraceId(), event.getEvent());
    events.stream().collect(Collectors.groupingBy(gKey, Collectors.toList()))
                    .forEach((g, ls) -> {
                        Optional<Event> lastOpt = ls.stream().max(Comparator.comparing(Event::getLocalTs));
                        lastOpt.ifPresent(rs2::add);
                    });

    System.out.println(JSON.toJSONString(rs2));
}

@Data
@AllArgsConstructor
static class Event {
    private String traceId;
    private String event;
    private Long localTs;
    private Integer duration;
}