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显示目录

Java 8 – Convert Date to LocalDate and LocalDateTime

The java.util.Date has no concept of time zone, and only represents the number of seconds passed since the Unix epoch time – 1970-01-01T00:00:00Z (midnight at the start of January 1, 1970 GMT/UTC)

Note
The new Java 8 java.time.Instant is the equivalent class to the classic java.util.Date

1. Date -> java.time

The idea for the date conversion :

Date -> Instant + System default time zone = LocalDate
Date -> Instant + System default time zone = LocalDateTime
Date -> Instant + System default time zone = ZonedDateTime

This example shows you how to convert java.util.Date to the new Java 8 Date APIs – LocalDate, LocalDateTime and ZonedDateTime

DateToJavaTime.java

package com.mkyong.java8;

import java.time.*;
import java.util.Date;

public class DateToJavaTime {

    public static void main(String[] args) {

        //Asia/Kuala_Lumpur +8
        ZoneId defaultZoneId = ZoneId.systemDefault();
        System.out.println("System Default TimeZone : " + defaultZoneId);

        //toString() append +8 automatically.
        Date date = new Date();
        System.out.println("date : " + date);

        //1. Convert Date -> Instant
        Instant instant = date.toInstant();
        System.out.println("instant : " + instant); //Zone : UTC+0

        //2. Instant + system default time zone + toLocalDate() = LocalDate
        LocalDate localDate = instant.atZone(defaultZoneId).toLocalDate();
        System.out.println("localDate : " + localDate);

        //3. Instant + system default time zone + toLocalDateTime() = LocalDateTime
        LocalDateTime localDateTime = instant.atZone(defaultZoneId).toLocalDateTime();
        System.out.println("localDateTime : " + localDateTime);

        //4. Instant + system default time zone = ZonedDateTime
        ZonedDateTime zonedDateTime = instant.atZone(defaultZoneId);
        System.out.println("zonedDateTime : " + zonedDateTime);

    }

}

Output

System Default TimeZone : Asia/Kuala_Lumpur

date : Fri Aug 19 21:46:31 MYT 2016
instant : 2016-08-19T13:46:31.981Z

localDate : 2016-08-19
localDateTime : 2016-08-19T21:46:31.981
zonedDateTime : 2016-08-19T21:46:31.981+08:00[Asia/Kuala_Lumpur]

2. Explanation – Q&A

2.1 Question : If Date has no concept of time zone, why the time zone will be displayed while we print out the Date object? For example :

//Fri Aug 19 11:52:06 MYT 2016
System.out.println(new Date()); //MYT = my system default time zone

Answer : Check the java.uti.Date.toString() source code, if you print out the Date object, the system default time zone will be appended and display together.

java.util.Date

public String toString() {

        //...omitted...

        TimeZone zi = date.getZone();
        if (zi != null) {
            sb.append(zi.getDisplayName(date.isDaylightTime(), TimeZone.SHORT, Locale.US)); // zzz
        } else {
            sb.append("GMT");
        }
        sb.append(' ').append(date.getYear());  // yyyy
        return sb.toString();
}

Note
This behavior is a design flaw since JDK1.1, it makes a lot of confusion. Again, the java.util.Date doesn’t store any time zone info, but if you print it out, the system default time zone will be displayed together.

2.2 Question : For the Date conversion, why we need to add a system default time zone for java.time.instant?
Answer : Refer to the above 2.1 Q&A. Review another example :

1. Date = 19/08/2016T10:00:00
2. System default time zone = +08:00 [Asia/Kuala_Lumpur]
3. Date (Print) = 19/08/2016T10:00:00+08:00 = 19/08/2016T18:00:00

The goal of the conversion is make sure both print Date and print LocalDate will generates the same output.

// Assume 19/08/2016T10:00:00 = 1000
// System default time zone = +8

1. Date (1000) -> Print Date (1000) = 1000+08:00  
// we always see "1000+08:00" (but the Date is still 1000)

2. Date (1000) -> Instant (1000)
// instant has no time zone or zero offset (UTC+0/Z)

3. Instant(1000) -> LocalDate(1000) -> Print LocalDate(1000) = 1000
// The result is "1000", different with print date!

4. LocalDate(1000) + 08:00 -> LocalDate(1000+08:00)
// add default time zone +8

5. Print LocalDate(1000+08:00) = 1000+08:00

3. java.time -> Date

This example shows you how to convert LocalDate, LocalDateTime and ZonedDateTime back to the classic java.util.Date

JavaTimeToDate.java

package com.mkyong.java8;

import java.time.*;
import java.util.Date;

public class JavaTimeToDate {

    public static void main(String[] args) {

        //Asia/Kuala_Lumpur +8
        ZoneId defaultZoneId = ZoneId.systemDefault();
        System.out.println("System Default TimeZone : " + defaultZoneId);

        LocalDate localDate = LocalDate.of(2016, 8, 19);
        Date date = Date.from(localDate.atStartOfDay(defaultZoneId).toInstant());
        System.out.println("\n1. LocalDate -> Date");
        System.out.println("localDate : " + localDate);
        System.out.println("date : " + date);

        LocalDateTime localDateTime = LocalDateTime.of(2016,8,19,21,46,31);
        Date date2 = Date.from(localDateTime.atZone(defaultZoneId).toInstant());
        System.out.println("\n2. LocalDateTime -> Date");
        System.out.println("localDateTime : " + localDateTime);
        System.out.println("date2 : " + date2);

        ZonedDateTime zonedDateTime = localDateTime.atZone(defaultZoneId);
        Date date3 = Date.from(zonedDateTime.toInstant());
        System.out.println("\n3. ZonedDateTime -> Date");
        System.out.println("zonedDateTime : " + zonedDateTime);
        System.out.println("date3 : " + date3);

    }

}

Output

System Default TimeZone : Asia/Kuala_Lumpur

1. LocalDate -> Date
localDate : 2016-08-19
date : Fri Aug 19 00:00:00 MYT 2016

2. LocalDateTime -> Date
localDateTime : 2016-08-19T21:46:31
date2 : Fri Aug 19 21:46:31 MYT 2016

3. ZonedDateTime -> Date
zonedDateTime : 2016-08-19T21:46:31+08:00[Asia/Kuala_Lumpur]
date3 : Fri Aug 19 21:46:31 MYT 2016

References

  1. JSR 310: Date and Time API
  2. Unix time
  3. Instant JavaDoc
  4. Date JavaDoc
  5. Java – Convert date and time between timezone
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