Getting current time in Java 8

问题:

I am exploring the new java.time API of Java 8. I am particularly trying to retrieve the current time (my current time zone, of a different time zone, and of a different offset).

The code is:

public static void getCurrentLocalTime(){
    LocalTime time = LocalTime.now();
    System.out.println("Local Time Zone: "+ZoneId.systemDefault().toString());
    System.out.println("Current local time : " + time);
}

public static void getCurrentTimeWithTimeZone(){
    LocalDateTime localtDateAndTime = LocalDateTime.now();
    ZoneId zoneId = ZoneId.of("America/Los_Angeles");
    ZonedDateTime dateAndTimeInLA  = ZonedDateTime.of(localtDateAndTime, zoneId);
    String currentTimewithTimeZone =dateAndTimeInLA.getHour()+":"+dateAndTimeInLA.getMinute();
    System.out.println("Current time in Los Angeles: " + currentTimewithTimeZone);
}

public static void getCurrentTimeWithZoneOffset(){
    LocalTime localtTime = LocalTime.now();
    ZoneOffset offset = ZoneOffset.of("-08:00");
    OffsetTime  offsetTime  = OffsetTime.of(localtTime, offset);
    String currentTimewithZoneOffset =offsetTime.getHour()+":"+offsetTime.getMinute();
    System.out.println("Current time  with offset -08:00: " + currentTimewithZoneOffset);
}

But, when I call the methods I get the same time-of-day (my system time), which is obviously not what I am expecting.

The output of the method calls:

Current time in Los Angeles: 19:59
Local Time Zone: Asia/Calcutta
Current local time : 19:59:20.477
Current time  with offset -08:00: 19:59

Even after setting a different time zone and offset, why am I getting the same time?

回答1:

LocalDateTime.now() always returns the current date/time in your default timezone (say 13 October @ 11.20am in London). When you create a ZonedDateTime or OffsetTime from it with a specific ZoneId or ZoneOffset, you get the same date and time but in a different time zone (for example 13 october at 11.20am in Los Angeles), which represents a different instant in time.

You are probably looking for something like:

Instant now = Instant.now();
ZoneId zoneId = ZoneId.of("America/Los_Angeles");
ZonedDateTime dateAndTimeInLA = ZonedDateTime.ofInstant(now, zoneId);

This will calculate the current date and time in Los Angeles: 13 october, 3.20am.

回答2:

Consider the following fixed methods:

public static void getCurrentLocalTime() {
    LocalTime time = LocalTime.now();
    System.out.println("Local Time Zone: " + ZoneId.systemDefault().toString());
    System.out.println("Current local time : " + time);
}

public static void getCurrentTimeWithTimeZone() {
    LocalDateTime localDateAndTime = LocalDateTime.now(ZoneId.of("America/Los_Angeles"));
    System.out.println("Current time in Los Angeles: " + localDateAndTime.toLocalTime());
}

public static void getCurrentTimeWithZoneOffset() {
    LocalTime localTime = LocalTime.now(ZoneOffset.of("-08:00"));
    System.out.println("Current time  with offset -08:00: " + localTime);
}

What changed is that instead of calling now(), the call is made to now(zone). That is because now() always return the current system time in your time-zone. The calls to atZone, OffsetTime.of or ZoneDateTime.of do not change the date/time, it only tells Java Time that the date should be understood at a date/time in this timezone.

When calling these 3 methods, here's the output on my machine:

Local Time Zone: Europe/Paris
Current local time : 12:32:21.560
Current time in Los Angeles: 03:32:21.579
Current time  with offset -08:00: 02:32:21.580

To make this very clear: you're in Europe and call LocalDateTime.now().atZone(ZoneId.of("America/Los_Angeles")) - you are creating a date/time which represents the current-time in Europe as if you were located in Los Angeles, so you are creating a date/time which is actually in the future for Los Angeles residents (8 or 9 hours in the future, depending on DST).

回答3:

There is also another way of displaying time in another zone by using withZoneSameInstant:

// now, in current zone,
// 2016-04-13T14:24:57.618+02:00[Europe/Warsaw]
ZonedDateTime now = ZonedDateTime.now();

// 2016-04-13T05:24:57.618-07:00[America/Los_Angeles]
ZonedDateTime losAngelesDateTime = now.withZoneSameInstant(ZoneId.of("America/Los_Angeles"));