Sort a TreeMap if the key is date

Question

What comparator should I use to sort a TreeMap<Date, Long> if I want its keys to be "the same" if they hold the same DAY. What I'm trying to say is that I want "15.09.2022 at 12:00" and "15.09.2022 at 12:01" to be the same.

I came up with an idea

Map<Date, Long> map = new TreeMap<>((date1, date2) -> {
    SimpleDateFormat fmt = new SimpleDateFormat("yyyyMMdd");
    return fmt.format(date1).compareTo(fmt.format(date2));
});

But it isn't quite the best practice to cast Date s to String s every time. Is there a more elegant way to do it?

Answer 1

Convert it to java.time , then trim off the local time.

new TreeMap<>(Comparator.comparing(
  date -> date.toInstant()
    .atZone(APPROPRIATE_TIMEZONE)
    .toLocalDate()))

(Or, better, use java.time in the first place.)

Note that you absolutely need a timezone; two java.util.Date s can represent different days depending on which timezone you interpret them in. (Confused? Yes, it's confusing. This is why java.util.Date got replaced with something clearer.)

Answer 2

Call .getTime() and integer divide it by 1000 60 60*24 (1000 milliseconds, 60 seconds, 60 minutes, 24 hours):

Date date = new Date();
Long dateDayKey = date.getTime() / 86400000L;

This will "chop off" the part of less resolution than a day. ( getTime() gives you the number of milliseconds since 1970-01-01 00:00:00)