i have made an application in which i need to perform date conversion. Here is my code.

GregorianCalendar c = new GregorianCalendar(Locale.GERMANY);
            c.set(2011, 04, 29,0,0,0);
            String cdate = (String) DateFormat.format("yyyy-MM-dd HH:mm:ss", c.getTime());
            Log.i(tag,cdate);

now when i check my LOG here is the output:

04-22 12:44:15.956: INFO/GridCellAdapter(30248): 2011-04-29 HH:00:00

why is the hour field not getting set. i have explicitly passed 0 when i was making the calendar object, still it is display HH in the LOG. what could be the problem?

thank you in advance.

回答1:

use lower-case hh:

String cdate = (String) DateFormat.format("yyyy-MM-dd hh:mm:ss", c.getTime());

回答2:

set c.set(Calendar.HOUR_OF_DAY,0) and it should work. Have you tried like this?

c.set(Calendar.YEAR, 2009);
c.set(Calendar.MONTH,11);
c.set(Calendar.DAY_OF_MONTH,4);
c.set(Calendar.HOUR_OF_DAY,0);
c.set(Calendar.MINUTE,0);
c.set(Calendar.SECOND,0)

回答3:

tl;dr

LocalDate.of( 2011 , 4 , 29 )                             // Represent April 29, 2011.
         .atStartOfDay( ZoneId.of( "America/Montreal" ) ) // Determine the first moment of the day. Often 00:00:00 but not always.
         .format( DateTimeFormatter.ISO_LOCAL_DATE_TIME ) // Generate a String representing the value of this date, using standard ISO 8601 format.
                                   .replace( "T" , " " )  // Replace the `T` in the middle of standard ISO 8601 format with a space for readability.

Using java.time

The modern way is with the java.time classes.

If you are trying to get the first moment of the day, do not assume the time 00:00:00. Anomalies in some time zones mean the day may start at another time-of-day such as 01:00:00.

The LocalDate class represents a date-only value without time-of-day and without time zone.

A time zone is crucial in determining a date. For any given moment, the date varies around the globe by zone. For example, a few minutes after midnight in Paris France is a new day while still “yesterday” in Montréal Québec.

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" );
LocalDate today = LocalDate.now( z );

You want a specific date in your Question.

LocalDate localDate = LocalDate.of( 2011 , 4 , 29 ) ;

Apply the time zone again in determining the first moment of the day.

ZonedDateTime zdt = localDate.atStartOfDay( z );  // Determine the first moment of the day on this date for this zone.

I recommend always including an indicator of the time zone or offset-from-UTC with your date-time strings. But if you insist, you can use a DateTimeFormatter predefined in java.time that does not include zone/offset: DateTimeFormatter.ISO_LOCAL_DATE_TIME. Merely remove the T from the middle.

String output = zdt.format( DateTimeFormatter.ISO_LOCAL_DATE_TIME )
                   .replace( "T" , " " ) ;

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, Java 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 Java 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….