如何从Java中的毫秒时间戳中提取一天中的时间段？

Question

Could somebody please shed some light on how can I get a period of the day (AM/PM) from the milliseconds timestamp value? This is currently being done by the following code:

Calendar cal = Calendar.getInstance();
cal.setTime(new Date(prevDate));
assertEquals(cal.get(Calendar.AM_PM), Calendar.PM));

I just need to get rid of java.util.Calendar .

My only idea is to convert long value to LocalDateTime and then to access DateTimeFormatter.ofPattern("a"); . Perhaps somebody could suggest another approach?

Answer 1

AmPm enum

I suggest you add the ThreeTen-Extra library to your project. This gives you a AmPm class to represent the AM or PM value that you seek. That AmPm class is an enum, defining two objects, one for AM and one for PM. The class carries methods, including the from method to determine the AM or PM of a given moment.

Using an object here makes your code more self-documenting , provides type-safety , and ensures valid values.

Note that a time zone is crucial here. For any given moment, the time-of-day (and even the date) varies around the globe by zone. So while a moment may be morning in Tokyo Japan, it may still be evening in Toledo Ohio US.

ZoneId z = ZoneId.of( "Africa/Tunis" );
ZonedDateTime zdt = ZonedDateTime.now( z );
AmPm amPm = AmPm.from( zdt );

You could then proceed to branch on which enum object was obtained.

switch ( amPm )
{
    case AM:
        System.out.println( "Good morning." );
        break ;
    case PM:
        System.out.println( "Good day." );
        break ;

    default:
        System.out.println( "ERROR Should never reach this point." );
}

Answer 2

You mentioned the formatting code, so it's not clear whether you are looking for a (localized?) string representation of AM/PM, but given a Clock in the appropriate zone, this will indicate the status:

boolean am = LocalTime.now(clock).isBefore(LocalTime.NOON);

If you have a specific Unix time as a number of milliseconds since the epoch, you'll need to specify the local time zone or offset in which you are interested to complete the conversion:

LocalTime when = Instant.ofEpochMilli(prevDate).atZone(zoneId).toLocalTime();
int meridiem = when.isBefore(LocalTime.NOON) ? Calendar.AM : Calendar.PM;
Assert.assertEquals(meridiem, Calendar.PM);