Converting between java time LocalDateTime and java util Date

Question

Java 8 has a completely new API for date and time. One of the most useful classes in this API is LocalDateTime, for holding a timezone-independent date-with-time value.

There are probably millions of lines of code using the legacy class java.util.Date for this purpose. As such, when interfacing old and new code there will be a need for converting between the two. As there seems to be no direct methods for accomplishing this, how can it be done?

Answer 1

Here is what I came up with ( and like all Date Time conundrums it is probably going to be disproved based on some weird timezone-leapyear-daylight adjustment :D )

Round-tripping: Date <<->> LocalDateTime

Given: Date date = [some date]

(1) LocalDateTime << Instant<< Date

    Instant instant = Instant.ofEpochMilli(date.getTime());
    LocalDateTime ldt = LocalDateTime.ofInstant(instant, ZoneOffset.UTC);

(2) Date << Instant << LocalDateTime

    Instant instant = ldt.toInstant(ZoneOffset.UTC);
    Date date = Date.from(instant);

Example:

Given:

Date date = new Date();
System.out.println(date + " long: " + date.getTime());

(1) LocalDateTime << Instant<< Date:

Create Instant from Date:

Instant instant = Instant.ofEpochMilli(date.getTime());
System.out.println("Instant from Date:\n" + instant);

Create Date from Instant (not necessary,but for illustration):

date = Date.from(instant);
System.out.println("Date from Instant:\n" + date + " long: " + date.getTime());

Create LocalDateTime from Instant

LocalDateTime ldt = LocalDateTime.ofInstant(instant, ZoneOffset.UTC);
System.out.println("LocalDateTime from Instant:\n" + ldt);

(2) Date << Instant << LocalDateTime

Create Instant from LocalDateTime:

instant = ldt.toInstant(ZoneOffset.UTC);
System.out.println("Instant from LocalDateTime:\n" + instant);

Create Date from Instant:

date = Date.from(instant);
System.out.println("Date from Instant:\n" + date + " long: " + date.getTime());

The output is:

Fri Nov 01 07:13:04 PDT 2013 long: 1383315184574

Instant from Date:
2013-11-01T14:13:04.574Z

Date from Instant:
Fri Nov 01 07:13:04 PDT 2013 long: 1383315184574

LocalDateTime from Instant:
2013-11-01T14:13:04.574

Instant from LocalDateTime:
2013-11-01T14:13:04.574Z

Date from Instant:
Fri Nov 01 07:13:04 PDT 2013 long: 1383315184574

Answer 2

Try this:

Date in = new Date();
LocalDateTime ldt = LocalDateTime.ofInstant(in.toInstant(), ZoneId.systemDefault());
Date out = Date.from(ldt.atZone(ZoneId.systemDefault()).toInstant());