JDK8调整java.time.Instant的时间

Question

I'm trying to adjust the time of a java.time.Instant .

I tried this code:

Instant validFrom = //from my method parameter
validFrom = validFrom.with(ChronoField.HOUR_OF_DAY, 19).with(ChronoField.MINUTE_OF_DAY, 00)
                    .with(ChronoField.SECOND_OF_DAY, 00).with(ChronoField.MILLI_OF_DAY, 00);

But I've an exception:

java.time.temporal.UnsupportedTemporalTypeException: Unsupported field: HourOfDay
    at java.time.Instant.with(Instant.java:720)

This is quite expected reading the documentation and checking the source code. It's not really clear to me why I can't do that. Is there another way to to this operation without incurring in many conversions?

Answer 1

This is because Instant has no timezone information. 19:00 hours doesn't represent anything useful, unless you attach a timezone to it.

You can convert it to a ZonedDateTime object, and then convert it back to an Instant like this:

public void testInstant(){
    Instant now = Instant.now();
    ZonedDateTime utc = now.atZone(ZoneId.of("UTC"));

    ZonedDateTime zonedDateTime = utc.withHour(19)
            .withMinute(0)
            .withSecond(0);

    Instant instant = zonedDateTime.toInstant();
}