How to get and set specified time in java.time.Instant?

Question

I have two java.time.Instant objects

Instant dt1; Instant dt2; 

I want to get time (only hours and minutes without date) from dt2 and set it to dt1. What is the best way to to this? Using

dt2.get(ChronoField.HOUR_OF_DAY)  

throws java.time.temporal.UnsupportedTemporalTypeException

Answer 1

You have to interpret the Instant at some time zone to get ZonedDateTime. As an Instant measures the ellapsed seconds and nano seconds from epoch 1970-01-01T00:00:00Z you should use UTC to get the same time as the Instant would print. (Z ≙ Zulu Time ≙ UTC)

Getting the time

Instant instant;  // get overall time LocalTime time = instant.atZone(ZoneOffset.UTC).toLocalTime(); // get hour int hour = instant.atZone(ZoneOffset.UTC).getHour(); // get minute int minute = instant.atZone(ZoneOffset.UTC).getMinute(); // get second int second = instant.atZone(ZoneOffset.UTC).getSecond(); // get nano int nano = instant.atZone(ZoneOffset.UTC).getNano(); 

There are also methods to get days, month and year (getX).

Setting the time

Instants are immutable so you can only "set" the time by creating a copy of your instant with the given time change.

instant = instant.atZone(ZoneOffset.UTC)         .withHour(hour)         .withMinute(minute)         .withSecond(second)         .withNano(nano)         .toInstant(); 

There are also methods to alter days, month and year (withX) as well as methods to add (plusX) or subtract (minusX) time or date values.

To set the time to a value given as a string use: .with(LocalTime.parse("12:45:30"))