[java] Convert LocalDate to LocalDateTime or java.sql.Timestamp

tl;dr

The Joda-Time project is in maintenance-mode, now supplanted by java.time classes.

• Just use java.time.Instant class.
• No need for:
  • LocalDateTime
  • java.sql.Timestamp
  • Strings

Capture current moment in UTC.

Instant.now()  

To store that moment in database:

myPreparedStatement.setObject( … , Instant.now() )  // Writes an `Instant` to database.

To retrieve that moment from datbase:

myResultSet.getObject( … , Instant.class )  // Instantiates a `Instant`

To adjust the wall-clock time to that of a particular time zone.

instant.atZone( z )  // Instantiates a `ZonedDateTime`

LocalDateTime is the wrong class

Other Answers are correct, but they fail to point out that LocalDateTime is the wrong class for your purpose.

In both java.time and Joda-Time, a LocalDateTime purposely lacks any concept of time zone or offset-from-UTC. As such, it does not represent a moment, and is not a point on the timeline. A LocalDateTime represents a rough idea about potential moments along a range of about 26-27 hours.

Use a LocalDateTime for either when the zone/offset is unknown (not a good situation), or when the zone-offset is indeterminate. For example, “Christmas starts at first moment of December 25, 2018” would be represented as a LocalDateTime.

Use a ZonedDateTime to represent a moment in a particular time zone. For example, Christmas starting in any particular zone such as Pacific/Auckland or America/Montreal would be represented with a ZonedDateTime object.

For a moment always in UTC, use Instant.

Instant instant = Instant.now() ;  // Capture the current moment in UTC.

Apply a time zone. Same moment, same point on the timeline, but viewed with a different wall-clock time.

ZoneId z = ZoneId.of( "Africa/Tunis" ) ;
ZonedDateTime zdt = instant.atZone( z ) ;  // Same moment, different wall-clock time.

So, if I can just convert between LocalDate and LocalDateTime,

No, wrong strategy. If you have a date-only value, and you want a date-time value, you must specify a time-of-day. That time-of-day may not be valid on that date for a particular zone – in which case ZonedDateTime class automatically adjusts the time-of-day as needed.

LocalDate ld = LocalDate.of( 2018 , Month.JANUARY , 23 ) ;
LocalTime lt = LocalTime.of( 14 , 0 ) ;  // 14:00 = 2 PM.
ZonedDateTime zdt = ZonedDateTime.of( ld , lt , z ) ;

If you want the first moment of the day as your time-of-day, let java.time determine that moment. Do not assume the day starts at 00:00:00. Anomalies such as Daylight Saving Time (DST) mean the day may start at another time such as 01:00:00.

ZonedDateTime zdt = ld.atStartOfDay( z ) ;

java.sql.Timestamp is the wrong class

The java.sql.Timestamp is part of the troublesome old date-time classes that are now legacy, supplanted entirely by the java.time classes. That class was used to represent a moment in UTC with a resolution of nanoseconds. That purpose is now served with java.time.Instant.

JDBC 4.2 with getObject/setObject

As of JDBC 4.2 and later, your JDBC driver can directly exchange java.time objects with the database by calling:

• PreparedStatement::setObject
• ResultSet::getObject

For example:

myPreparedStatement.setObject( … , instant ) ;

… and …

Instant instant = myResultSet.getObject( … , Instant.class ) ;

Convert legacy ? modern

If you must interface with old code not yet updated to java.time, convert back and forth using new methods added to the old classes.

Instant instant = myJavaSqlTimestamp.toInstant() ;  // Going from legacy class to modern class.

…and…

java.sql.Timestamp myJavaSqlTimestamp = java.sql.Timestamp.from( instant ) ;  // Going from modern class to legacy class.

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.

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

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
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). 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.