Recalculation of datetime in Java Thread

Question

What's the best way determining the datetime inside a Java Thread (in Android)?

I have the following snippet:

public void repeatedlyPrintTheTime() {
    final Handler handler = new Handler();

    mR = new Runnable() {
        public void run() {
            System.out.println("Time 1 is " + mCalendar.getTime().toString());
            System.out.println("Time 2 is " + GregorianCalendar.getInstance().getTime().toString());
            handler.postDelayed(this, 1000);
        }
    };

    handler.postDelayed(mR, 1000);
}

Time 1 repeatedly prints the same time, but Time 2 updates on every iteration.

Do I really have to get a new instance of GregorianCalendar every time I want to check what the time is?

What is your requirement? `GregorianCalendar.getInstance()` creates a `Calender` with current time and timezone. `mCalender` is already initialized and not initializing every time, so it is printing its time (either set or created time) — RP-, Feb 17 '15 at 00:41
That or LocalDateTime.now() (Java8 only) See http://stackoverflow.com/questions/5175728/how-to-get-the-current-date-time-in-java for detailed analysis. — Adrian Leonhard, Feb 17 '15 at 00:45
@AdrianLeonhard this is Android, therefore no Java 8... Heck, Android doesn't even have Java 7 — fge, Feb 17 '15 at 00:48
why not use `new java.util.Date()` or just `System.currentTimeMillis()`? — Brett Okken, Feb 17 '15 at 02:14
So GregorianCalendar.getInstance() doesn't provide an object which can *tell* the time (at any future point), but rather provides an object which can provide information about the time that it was instantiated? OK, got it, thank you (if you want to provide that as an answer I'll happily accept it) — scubbo, Feb 17 '15 at 16:36

score 2 · Accepted Answer · edited May 23 '17 at 12:00

tl;dr

Yes, you must get a fresh date-time object instance each time you want to capture the current moment. No automatic updating of existing instance.

Instant.now()

Single moment, not updated automatically

A GregorianCalendar holds a single moment, and is not automatically updated. If it were updating automatically, you would never be able to do any business logic as the value would constantly be changing as your code ran!

Using java.time

Also, GregorianCalendar and its sibling classes are an awful mess and should be avoided. They are now legacy, supplanted by the java.time classes.

Use Instant for the current moment in UTC. The Instant class represents a moment on the timeline in UTC with a resolution of nanoseconds (up to nine (9) digits of a decimal fraction).

Instant instant = Instant.now();

instant.toString(): 2017-02-27T14:48:58.664Z

To later capture the current time again, obtain another Instant instance.

Instant instantLater = Instant.now();

instantLater.toString(): 2017-02-27T14:49:02.164Z

You can even capture the delta between them, as a Duration object.

Duration duration = Duration.between( instant, instantLater );

duration.toString(): PT3.5S

To see that same moment through the lens of some particular region’s wall-clock time, adjust into a time zone. Apply a ZoneId to get a ZonedDateTime object.

Specify a proper time zone name in the format of continent/region, such as America/Montreal, Africa/Casablanca, or Pacific/Auckland. Never use the 3-4 letter abbreviation such as EST or IST as they are not true time zones, not standardized, and not even unique(!).

ZoneId z = ZoneId.of( "America/Montreal" );
ZonedDateTime zdt = instant.atZone( z );

zdt.toString(): 2017-02-27T09:48:58.664-05:00[America/Montreal]

See this code run live at IdeOne.com.

To represent a moment in the future or the past, specify inputs to get an OffsetDateTime or a ZonedDateTime.

JavaDoc…

ZonedDateTime.of( int year, int month, int dayOfMonth, int hour, int minute, int second, int nanoOfSecond, ZoneId zone )

ZonedDateTime zdt = ZonedDateTime.of( 2017 , 1 , 23 , 12 , 34 , 56 , 123456789 , ZoneId.of( "America/Montreal" );

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

Where to obtain the java.time classes?

Java SE 8 and SE 9 and later
- Built-in.
- Part of the standard Java API with a bundled implementation.
- Java 9 adds some minor features and fixes.
Java SE 6 and SE 7
- Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
Android
- The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
- See How to use ThreeTenABP….

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

score 0 · Answer 2 · answered Oct 30 '15 at 09:41

Some calendar classes are not thread-safe. You could insert that code in a synchronized block with a lock:

private Object calendarLock; --Class attribute

public void repeatedlyPrintTheTime() {
    final Handler handler = new Handler();

    mR = new Runnable() {
        public void run() {
            synchronized(calendarLock) {
                System.out.println("Time 1 is " + mCalendar.getTime().toString());
                System.out.println("Time 2 is " + GregorianCalendar.getInstance().getTime().toString());
                handler.postDelayed(this, 1000);
            }
        }
    };

    handler.postDelayed(mR, 1000);
}

Recalculation of datetime in Java Thread

2 Answers2

tl;dr

Single moment, not updated automatically

Using java.time

About java.time