I am trying to build a chat app and I want to store date for every message in Firebase RealTime-Database using ServerValue.TIMESTAMP which stores date and time in milliseconds and it is server sided. All I want to achieve is to convert milliseconds to a safe date format which will have a pattern like "Thursday, 11th November 2022, 14:42" but I read a lot of things about Java's unsafe date and format libraries. My app uses FirebaseUI which is like an asynchronous way to observe/notify/listen to changes of my db so I need something that will be safe and supported from API/SDK level 21

Here is my code that I currently have

Calendar calendar = Calendar.getInstance(); calendar.setTimeInMillis(model.sentOnDateInMilliseconds); String date = DateFormat.getDateInstance(DateFormat.FULL).format(calendar.getTime());

and I make attempts to preview the format that I want like this

textView.setText(MessageFormat.format("{0} {1}:{2} {3}", date, calendar.get(Calendar.HOUR), calendar.get(Calendar.MINUTE), calendar.get(Calendar.AM_PM)));

any suggestions?

CodePudding user response：

Avoid legacy classes

Never use SimpleDateFormat, Calendar, or either Date class. These legacy classes are terribly flawed with poor design decisions by people who did not understand date-time handling.

Their modern replacements are the java.time classes defined in JSR 310.

java.time

You are using terrible date-time classes that were years ago supplanted by the modern java.time classes defined in JSR 310.

Though not documented clearly, apparently a firebase.database.ServerValue.TIMESTAMP type represents a moment as a count of milliseconds since the epoch reference of the first moment of 1970 in UTC, 1970-01-01T00:00:00Z.

java.time.Instant

The matching class in Java is java.time.Instant.

Instant instant = Instant.ofEpochMilli( millisSinceEpoch ) ;

You can extract that millisecond count from an Instant. Beware of data loss: A Instant has a resolution of nanoseconds, so getting a count of milliseconds ignores the microseconds/nanoseconds.

Instant instant = Instant.now() ;  // Capture the current moment as seen with an offset of zero hours-minutes-seconds from UTC.
long millisSinceEpoch = instant.toEpochMilli() ;

java.time.ZonedDateTime

convert milliseconds to a safe date format which will have a pattern like "Thursday, 11th November 2022, 14:42"

To generate text in formats other than standard ISO 8601, we need a class more flexible than Instant. And we need to adjust that moment from an offset of zero hours-minutes-seconds from UTC to a time zone (or offset) expected by the user.

ZoneId z = ZoneId.of( "Asia/Tokyo" ) ;
ZonedDateTime zdt = instant.atZone( z ) ;

java.time.format.DateTimeFormatter

To generate text, we must specify a format. You can hard-code a format, or you can let java.time automatically localize. To localize, we specify a Locale to determine the human language and cultural norms needed for localization such as name of day of week.

Locale locale = Locale.UK ;
DateTimeFormatter f = DateTimeFormatter.ofLocalizedDateTime( FormatStyle.LONG ).withLocale( locale ) ;
String output = zdt.format( f ) ;

Dump to console.

System.out.println( "instant = "   instant );
System.out.println( "millisSinceEpoch = "   millisSinceEpoch );
System.out.println( "output = "   output );

When run.

instant = 2022-11-18T19:27:22.561715Z
millisSinceEpoch = 1668799642561
output = Saturday, November 19, 2022 at 4:27:22 AM Japan Standard Time

Android 26 carries an implementation of the java.time classes. For earlier Android, the latest tooling provides access to most of the java.time functionality via “API desugaring”.