Java Date Time Legacy Interoperability

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Prior to the Java 8, the Java date and time was defined by the java.util.Date, java.util.Calendar, and java.util.TimeZone classes, as well as their subclasses, such as java.util.GregorianCalendar.

The legacy date time API is defined in java.util package while the new Java 8 date time API is defined in java.time package.

• java.util.Date
• java.util.Calendar
• java.util.GregorianCalendar
• java.util.TimeZone
• java.sql.Date
• java.sql.Time
• java.sql.Timestamp
• java.nio.file.attribute.FileTime

JDK 8 date time API defined several methods to convert between java.util and java.time objects.

• Calendar.toInstant() converts the Calendar object to an Instant.
• java.util.Date toInstant() converts Date object to an Instant
• java.util.Date from() methods create a Date object from an Instant.
• GregorianCalendar.toZonedDateTime() converts a GregorianCalendar instance to a ZonedDateTime.
• GregorianCalendar.from(ZonedDateTime) creates a GregorianCalendar object using the default locale from a ZonedDateTime instance.
• TimeZone.toZoneId() converts a TimeZone object to a ZoneId.
• java.sql.Date toLocalDate() converts to LocalDate
• java.sql.Date valueOf() creates java.sql.Date from LocalDate
• java.sql.Time toLocalTime() converts to LocalTime
• java.sql.Time valueOf() creates java.sql.Time from LocalTime
• java.sql.Timestamp toInstant() converts to Instant
• java.sql.Timestamp valueOf() converts from LocalDateTime
• java.nio.file.attribute.FileTime toInstant() converts to Instant
• java.nio.file.attribute.FileTime from() converts from Instant

Example

The following code shows how to convert Date to Instant back and forth.

import java.time.Instant;
import java.util.Date;
//from   www  .  ja v a 2s .c  o  m
public class Main {
  public static void main(String[] args) {
    Date  dt =  new Date(); 
    System.out.println("Date: "  + dt);
    
    Instant in = dt.toInstant(); 
    System.out.println("Instant: "  + in);

    Date  dt2  = Date.from(in); 
    System.out.println("Date: "  + dt2);
  }
}

The code above generates the following result.

Example 2

We can GregorianCalendar to a ZonedDateTime, which can be converted to any other classes in the new Date-Time API.

We can convert an Instant to a ZonedDateTime then convert ZonedDateTime to a GregorianCalendar with from() method from GregorianCalendar.

The following code shows how to convert a GregorianCalendar to a ZonedDateTime and vice versa.

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;
import java.time.OffsetDateTime;
import java.time.OffsetTime;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.util.GregorianCalendar;
import java.util.TimeZone;
//from  www . j ava 2  s  . co  m
public class Main {
  public static void main(String[] args) {
    GregorianCalendar gc = new GregorianCalendar(2014, 1, 11, 15, 45, 50);
    LocalDate ld = gc.toZonedDateTime().toLocalDate();
    System.out.println("Local  Date: " + ld);
 
    LocalTime lt = gc.toZonedDateTime().toLocalTime();
    System.out.println("Local Time:  " + lt);
    
    LocalDateTime ldt = gc.toZonedDateTime().toLocalDateTime();
    System.out.println("Local DateTime:  " + ldt);

    OffsetDateTime od = gc.toZonedDateTime().toOffsetDateTime();
    System.out.println("Offset  Date: " + od);

    OffsetTime ot = gc.toZonedDateTime().toOffsetDateTime().toOffsetTime();
    System.out.println("Offset Time:  " + ot);

    ZonedDateTime zdt = gc.toZonedDateTime();
    System.out.println("Zoned DateTime:  " + zdt);

    ZoneId zoneId = zdt.getZone();
    TimeZone timeZone = TimeZone.getTimeZone(zoneId);
    System.out.println("Zone ID:  " + zoneId);
    System.out.println("Time Zone ID:  " + timeZone.getID());

    GregorianCalendar gc2 = GregorianCalendar.from(zdt);
    System.out.println("Gregorian  Calendar: " + gc2.getTime());
  }
}

The code above generates the following result.

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