Java8 Collectors.groupingBy

Question

I want to groupingBy a Collection based on the LocalDateTime, but I only want to get the Hour, not the minutes, seconds...

.collect(Collectors.groupingBy(cp -> getUpdateLocalDate())


public LocalDate getUpdateLocalDate() {
    return getUpdateDate().toInstant().atZone(ZoneId.systemDefault()).toLocalDate();
}

but I need also the Date, something like: 2018-10-12T09:00:00, not only the hour

Answer 1

note that LocalDate doesn't have a time-zone. instead, return a LocalDateTime from the method getUpdateLocalDate (might be worth renaming also) then truncate the LocalDateTime instance.

 .collect(Collectors.groupingBy(cp -> cp.getUpdateLocalDateTime().truncatedTo(ChronoUnit.HOURS));