16 September 2015

Java 8 date time API explained

Java developers have been used to java.util.Date for years, but with Java 8 a new API in java.time and its sub-packages like java.time.format has been added. Now it is time to learn this new way to deal with date and time on the JVM.

Drop the UNIX time

java.util.Date is designed around the UNIX time, which is still in mind of most developers. Creating a new object will use the current UNIX timestamp as the base

Date date = new Date();

java.time instead provides an API, that does not really care about UNIX time. It comes with four basic classes that provide you different levels of details:

They provide a similar API to deal with them

LocalDate date = LocalDate.now();
LocalTime time = LocalTime.now();
LocalDateTime dateTime = LocalDateTime.now();
ZonedDateTime zonedDateTime = ZonedDateTime.now();

The now() method is overloaded, so you can pass a java.time.Clock object to it. This is handy during unit tests, where you want to freeze time 😉:

// In production
Clock clock = Clock.systemDefaultZone();

// In tests
Clock clock = Clock.fixed(Instant.now(), ZoneId.systemDefault());

LocalDateTime dateTime = LocalDateTime.now(clock);

Use ISO 8601

The interesting about the java.time API is, it is designed around ISO 8601. This makes it great for dealing with Strings and human readable date and time representations. I highly recommend the usage of ISO 8601 based dates, since it makes dates much easier to read for developers. If you provide a RESTful API for example, a developer trying your API via curl can directly read a date like 2015-09-16.

The classes in the java.time API provide a parse method for this:

LocalDate date = LocalDate.parse("2015-09-16");
LocalTime time = LocalTime.parse("05:33:23");
LocalDateTime dateTime = LocalDateTime.parse("2015-09-16T05:33:23");
ZonedDateTime zonedDateTime = ZonedDateTime.parse("2015-09-16T05:33:23+02:00[Europe/Paris]");

If you call the toString method on the above objects, you will get the same representation back. Internally the classes use a java.time.format.DateTimeFormatter object to parse or format. If your dates and times are in a different format, you can pass your own formatter to it:

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("EEE, dd MMM yyyy HH:mm:ss zzz");
ZonedDateTime zonedDateTime = ZonedDateTime.parse("Wed, 16 Sep 2015 05:33:23 CEST", formatter);
String dateTime = formatter.format(zonedDateTime);

The above example will only work on machines with English as default locale. If it is running on a machine with German locale, it will fail due to the name Wed at the beginning. It is highly recommended to pass the right locale to the above formatter:

DateTimeFormatter enFormatter = DateTimeFormatter.ofPattern("EEE, dd MMM yyyy HH:mm:ss zzz", Locale:ENGLISH);
DateTimeFormatter deFormatter = DateTimeFormatter.ofPattern("EEE, dd MMM yyyy HH:mm:ss zzz", Locale:GERMAN);

ZonedDateTime.parse("Wed, 16 Sep 2015 05:33:23 CEST", enFormatter);
ZonedDateTime.parse("Mi, 16 Sep 2015 05:33:23 MESZ", deFormatter);

Dealing with UNIX time

Although the java.time API is not designed around it, it allows you to work without UNIX timestamps. Sometimes you will still need them. The simplest way to handle them is using java.time.Instant:

Instant instant = Instant.ofEpochSecond(1442374403);


There is much more to say about this API like how to convert between these different types, but this might go into another post. For the moment we have seen that Java now has a date and time API, that provides more powerful types than the old java.util.Date. I guess this should already be enough to start using it.