I have a problem resetting hours in Java. For a given date I want to set the hours to 00:00:00.
This is my code :
/**
* Resets milliseconds, seconds, minutes and hours from the provided date
*
* @param date
* @return
*/
public static Date trim(Date date) {
Calendar calendar = Calendar.getInstance();
calendar.setTime(date);
calendar.set(Calendar.MILLISECOND, 0);
calendar.set(Calendar.SECOND, 0);
calendar.set(Calendar.MINUTE, 0);
calendar.set(Calendar.HOUR, 0);
return calendar.getTime();
}
The problem is that sometimes the time is 12:00:00 and sometimes it is 00:00:00 and when I query the database for an entity that was saved on 07.02.2013 00:00:00 and the actual entity time, that is stored, is 12:00:00 the query fails.
I know that 12:00:00 == 00:00:00!
I am using AppEngine. Is this an appengine bug, problem or some other issue? Or does it depend on something else?
Use another constant instead of Calendar.HOUR, use Calendar.HOUR_OF_DAY.
calendar.set(Calendar.HOUR_OF_DAY, 0);
Calendar.HOUR uses 0-11 (for use with AM/PM), and Calendar.HOUR_OF_DAY uses 0-23.
To quote the Javadocs:
public static final int HOUR
Field number for get and set indicating the hour of the morning or afternoon. HOUR is used for the 12-hour clock (0 - 11). Noon and midnight are represented by 0, not by 12. E.g., at 10:04:15.250 PM the HOUR is 10.
and
public static final int HOUR_OF_DAY
Field number for get and set indicating the hour of the day. HOUR_OF_DAY is used for the 24-hour clock. E.g., at 10:04:15.250 PM the HOUR_OF_DAY is 22.
Testing ("now" is currently c. 14:55 on July 23, 2013 Pacific Daylight Time):
public class Main
{
static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
public static void main(String[] args)
{
Calendar now = Calendar.getInstance();
now.set(Calendar.HOUR, 0);
now.set(Calendar.MINUTE, 0);
now.set(Calendar.SECOND, 0);
System.out.println(sdf.format(now.getTime()));
now.set(Calendar.HOUR_OF_DAY, 0);
System.out.println(sdf.format(now.getTime()));
}
}
Output:
$ javac Main.java
$ java Main
2013-07-23 12:00:00
2013-07-23 00:00:00
java.time
Using the java.time framework built into Java 8 and later. See Tutorial.
import java.time.LocalTime;
import java.time.LocalDateTime;
LocalDateTime now = LocalDateTime.now(); # 2015-11-19T19:42:19.224
# start of a day
now.with(LocalTime.MIN); # 2015-11-19T00:00
now.with(LocalTime.MIDNIGHT); # 2015-11-19T00:00
If you do not need time-of-day (hour, minute, second etc. parts) consider using LocalDate class.
LocalDate.now(); # 2015-11-19
Here are couple of utility functions I use to do just this.
/**
* sets all the time related fields to ZERO!
*
* @param date
*
* @return Date with hours, minutes, seconds and ms set to ZERO!
*/
public static Date zeroTime( final Date date )
{
return DateTimeUtil.setTime( date, 0, 0, 0, 0 );
}
/**
* Set the time of the given Date
*
* @param date
* @param hourOfDay
* @param minute
* @param second
* @param ms
*
* @return new instance of java.util.Date with the time set
*/
public static Date setTime( final Date date, final int hourOfDay, final int minute, final int second, final int ms )
{
final GregorianCalendar gc = new GregorianCalendar();
gc.setTime( date );
gc.set( Calendar.HOUR_OF_DAY, hourOfDay );
gc.set( Calendar.MINUTE, minute );
gc.set( Calendar.SECOND, second );
gc.set( Calendar.MILLISECOND, ms );
return gc.getTime();
}
You would better to primarily set time zone to the DateFormat component like this:
DateFormat dateFormat = new SimpleDateFormat("HH:mm:ss");
dateFormat.setTimeZone(TimeZone.getTimeZone("GMT"));
Then you can get "00:00:00" time by passing 0 milliseconds to formatter:
String time = dateFormat.format(0);
or you can create Date object:
Date date = new Date(0); // also pass milliseconds
String time = dateFormat.foramt(date);
or you be able to have more possibilities using Calendar component but you should also set timezone as GMT to calendar instance:
Calendar calendar = Calendar.getInstance(TimeZone.getTimeZone("GMT"), Locale.US);
calendar.set(Calendar.HOUR_OF_DAY, 5);
calendar.set(Calendar.MINUTE, 37);
calendar.set(Calendar.SECOND, 27);
dateFormat.format(calendar.getTime());
One more JAVA 8 way:
LocalDateTime localDateTime = LocalDateTime.now().truncatedTo(ChronoUnit.HOURS);
But it's a lot more useful to edit the date that already exists.
Doing this could be easier (In Java 8)
LocalTime.ofNanoOfDay(0)
We can set java.util.Date time part to 00:00:00 By using LocalDate class of Java 8/Joda-datetime api:
Date datewithTime = new Date() ; // ex: Sat Apr 21 01:30:44 IST 2018
LocalDate localDate = LocalDate.fromDateFields(datewithTime);
Date datewithoutTime = localDate.toDate(); // Sat Apr 21 00:00:00 IST 2018
tl;dr
myJavaUtilDate // The terrible `java.util.Date` class is now legacy. Use *java.time* instead.
.toInstant() // Convert this moment in UTC from the legacy class `Date` to the modern class `Instant`.
.atZone( ZoneId.of( "Africa/Tunis" ) ) // Adjust from UTC to the wall-clock time used by the people of a particular region (a time zone).
.toLocalDate() // Extract the date-only portion.
.atStartOfDay( ZoneId.of( "Africa/Tunis" ) ) // Determine the first moment of that date in that zone. The day does *not* always start at 00:00:00.
java.time
You are using terrible old date-time classes that were supplanted years ago by the modern java.time classes defined in JSR 310.
Date ➙ Instant
A java.util.Date represent a moment in UTC. Its replacement is Instant. Call the new conversion methods added to the old classes.
Instant instant = myJavaUtilDate.toInstant() ;
Time zone
Specify the time zone in which you want your new time-of-day to make sense.
Specify a proper time zone name in the format of Continent/Region, such as America/Montreal, Africa/Casablanca, or Pacific/Auckland. Never use the 2-4 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
Apply the ZoneId to the Instant to get a ZonedDateTime. Same moment, same point on the timeline, but different wall-clock time.
ZonedDateTime zdt = instant.atZone( z ) ;
Changing time-of-day
You asked to change the time-of-day. Apply a LocalTime to change all the time-of-day parts: hour, minute, second, fractional second. A new ZonedDateTime is instantiated, with values based on the original. The java.time classes use this immutable objects pattern to provide thread-safety.
LocalTime lt = LocalTime.of( 15 , 30 ) ; // 3:30 PM.
ZonedDateTime zdtAtThreeThirty = zdt.with( lt ) ;
First moment of day
But you asked specifically for 00:00. So apparently you want the first moment of the day. Beware: some days in some zones do not start at 00:00:00. They may start at another time such as 01:00:00 because of anomalies such as Daylight Saving Time (DST).
Let java.time determine the first moment. Extract the date-only portion. Then pass the time zone to get first moment.
LocalDate ld = zdt.toLocalDate() ;
ZonedDateTime zdtFirstMomentOfDay = ld.atStartOfDay( z ) ;
Adjust to UTC
If you need to go back to UTC, extract an Instant.
Instant instant = zdtFirstMomentOfDay.toInstant() ;
Instant ➙ Date
If you need a java.util.Date to interoperate with old code not yet updated to java.time, convert.
java.util.Date d = java.util.Date.from( instant ) ;
Another way to do this would be to use a DateFormat without any seconds:
public static Date trim(Date date) {
DateFormat format = new SimpleDateFormat("dd.MM.yyyy");
Date trimmed = null;
try {
trimmed = format.parse(format.format(date));
} catch (ParseException e) {} // will never happen
return trimmed;
}
You can either do this with the following:
Calendar cal = Calendar.getInstance();
cal.set(year, month, dayOfMonth, 0, 0, 0);
Date date = cal.getTime();
If you need format 00:00:00 in string, you should use SimpleDateFormat as below. Using "H "instead "h".
Date today = new Date();
SimpleDateFormat ft = new SimpleDateFormat("dd-MM-yyyy HH:mm:ss");
//not SimpleDateFormat("dd-MM-yyyy hh:mm:ss")
Calendar calendarDM = Calendar.getInstance();
calendarDM.setTime(today);
calendarDM.set(Calendar.HOUR, 0);
calendarDM.set(Calendar.MINUTE, 0);
calendarDM.set(Calendar.SECOND, 0);
System.out.println("Current Date: " + ft.format(calendarDM.getTime()));
//Result is: Current Date: 29-10-2018 00:00:00
As Java8 add new Date functions, we can do this easily.
// If you have instant, then:
Instant instant1 = Instant.now();
Instant day1 = instant1.truncatedTo(ChronoUnit.DAYS);
System.out.println(day1); //2019-01-14T00:00:00Z
// If you have Date, then:
Date date = new Date();
Instant instant2 = date.toInstant();
Instant day2 = instant2.truncatedTo(ChronoUnit.DAYS);
System.out.println(day2); //2019-01-14T00:00:00Z
// If you have LocalDateTime, then:
LocalDateTime dateTime = LocalDateTime.now();
LocalDateTime day3 = dateTime.truncatedTo(ChronoUnit.DAYS);
System.out.println(day3); //2019-01-14T00:00
String format = day3.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME);
System.out.println(format);//2019-01-14T00:00:00
来源：https://stackoverflow.com/questions/17821601/set-time-to-000000