A Deeper Look into the Java 8 Date and Time API

Michael Scharhag

Feb. 27, 14 · News

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Within this post we will have a deeper look into the new Date/Time API we get with Java 8 (JSR 310). Please note that this post is mainly driven by code examples that show the new API functionality. I think the examples are self-explanatory so I did not spent much time writing text around them :-)

Let's get started!

Working with Date and Time Objects

All classes of the Java 8 Date/Time API are located within the java.time package. The first class we want to look at is java.time.LocalDate. A LocalDate represents a year-month-day date without time. We start with creating new LocalDate instances:

// the current date
LocalDate currentDate = LocalDate.now();
// 2014-02-10
LocalDate tenthFeb2014 = LocalDate.of(2014, Month.FEBRUARY, 10);
// months values start at 1 (2014-08-01)
LocalDate firstAug2014 = LocalDate.of(2014, 8, 1);
// the 65th day of 2010 (2010-03-06)
LocalDate sixtyFifthDayOf2010 = LocalDate.ofYearDay(2010, 65);

LocalTime and LocalDateTime are the next classes we look at. Both work similar to LocalDate. ALocalTime works with time (without dates) while LocalDateTime combines date and time in one class:

LocalTime currentTime = LocalTime.now(); // current time
LocalTime midday = LocalTime.of(12, 0); // 12:00
LocalTime afterMidday = LocalTime.of(13, 30, 15); // 13:30:15
// 12345th second of day (03:25:45)
LocalTime fromSecondsOfDay = LocalTime.ofSecondOfDay(12345);
// dates with times, e.g. 2014-02-18 19:08:37.950
LocalDateTime currentDateTime = LocalDateTime.now();
// 2014-10-02 12:30
LocalDateTime secondAug2014 = LocalDateTime.of(2014, 10, 2, 12, 30);
// 2014-12-24 12:00
LocalDateTime christmas2014 = LocalDateTime.of(2014, Month.DECEMBER, 24, 12, 0);

By default LocalDate/Time classes will use the system clock in the default time zone. We can change this by providing a time zone or an alternative Clock implementation:

// current (local) time in Los Angeles
LocalTime currentTimeInLosAngeles = LocalTime.now(ZoneId.of("America/Los_Angeles"));
// current time in UTC time zone
LocalTime nowInUtc = LocalTime.now(Clock.systemUTC());

From LocalDate/Time objects we can get all sorts of useful information we might need. Some examples:

LocalDate date = LocalDate.of(2014, 2, 15); // 2014-06-15
boolean isBefore = LocalDate.now().isBefore(date); // false
// information about the month
Month february = date.getMonth(); // FEBRUARY
int februaryIntValue = february.getValue(); // 2
int minLength = february.minLength(); // 28
int maxLength = february.maxLength(); // 29
Month firstMonthOfQuarter = february.firstMonthOfQuarter(); // JANUARY
// information about the year
int year = date.getYear(); // 2014
int dayOfYear = date.getDayOfYear(); // 46
int lengthOfYear = date.lengthOfYear(); // 365
boolean isLeapYear = date.isLeapYear(); // false
DayOfWeek dayOfWeek = date.getDayOfWeek();
int dayOfWeekIntValue = dayOfWeek.getValue(); // 6
String dayOfWeekName = dayOfWeek.name(); // SATURDAY
int dayOfMonth = date.getDayOfMonth(); // 15
LocalDateTime startOfDay = date.atStartOfDay(); // 2014-02-15 00:00
// time information
LocalTime time = LocalTime.of(15, 30); // 15:30:00
int hour = time.getHour(); // 15
int second = time.getSecond(); // 0
int minute = time.getMinute(); // 30
int secondOfDay = time.toSecondOfDay(); // 55800

Some information can be obtained without providing a specific date. For example, we can use the Year class if we need information about a specific year:

Year currentYear = Year.now();
Year twoThousand = Year.of(2000);
boolean isLeap = currentYear.isLeap(); // false
int length = currentYear.length(); // 365
// sixtyFourth day of 2014 (2014-03-05)
LocalDate date = Year.of(2014).atDay(64);

We can use the plus and minus methods to add or subtract specific amounts of time. Note that these methods always return a new instance (Java 8 date/time classes are immutable).

LocalDate tomorrow = LocalDate.now().plusDays(1);
// before 5 houres and 30 minutes
LocalDateTime dateTime = LocalDateTime.now().minusHours(5).minusMinutes(30);

TemporalAdjusters are another nice way for date manipulation. TemporalAdjuster is a single method interface that is used to separate the process of adjustment from actual date/time objects. A set of common TemporalAdjusters can be accessed using static methods of the TemporalAdjusters class.

LocalDate date = LocalDate.of(2014, Month.FEBRUARY, 25); // 2014-02-25
// first day of february 2014 (2014-02-01)
LocalDate firstDayOfMonth = date.with(TemporalAdjusters.firstDayOfMonth());
// last day of february 2014 (2014-02-28)
LocalDate lastDayOfMonth = date.with(TemporalAdjusters.lastDayOfMonth());

Static imports make this more fluent to read:

import static java.time.temporal.TemporalAdjusters.*;
...
// last day of 2014 (2014-12-31)
LocalDate lastDayOfYear = date.with(lastDayOfYear());
// first day of next month (2014-03-01)
LocalDate firstDayOfNextMonth = date.with(firstDayOfNextMonth());
// next sunday (2014-03-02)
LocalDate nextSunday = date.with(next(DayOfWeek.SUNDAY));

Time Zones

Working with time zones is another big topic that is simplified by the new API. The LocalDate/Time classes we have seen so far do not contain information about a time zone. If we want to work with a date/time in a certain time zone we can use ZonedDateTime or OffsetDateTime:

ZoneId losAngeles = ZoneId.of("America/Los_Angeles");
ZoneId berlin = ZoneId.of("Europe/Berlin");
// 2014-02-20 12:00
LocalDateTime dateTime = LocalDateTime.of(2014, 02, 20, 12, 0);
// 2014-02-20 12:00, Europe/Berlin (+01:00)
ZonedDateTime berlinDateTime = ZonedDateTime.of(dateTime, berlin);
// 2014-02-20 03:00, America/Los_Angeles (-08:00)
ZonedDateTime losAngelesDateTime = berlinDateTime.withZoneSameInstant(losAngeles);
int offsetInSeconds = losAngelesDateTime.getOffset().getTotalSeconds(); // -28800
// a collection of all available zones
Set<String> allZoneIds = ZoneId.getAvailableZoneIds();
// using offsets
LocalDateTime date = LocalDateTime.of(2013, Month.JULY, 20, 3, 30);
ZoneOffset offset = ZoneOffset.of("+05:00");
// 2013-07-20 03:30 +05:00
OffsetDateTime plusFive = OffsetDateTime.of(date, offset);
// 2013-07-19 20:30 -02:00
OffsetDateTime minusTwo = plusFive.withOffsetSameInstant(ZoneOffset.ofHours(-2));

Timestamps

Classes like LocalDate and ZonedDateTime provide a human view on time. However, often we need to work with time viewed from a machine perspective. For this we can use the Instant class which represents timestamps. An Instant counts the time beginning from the first second of January 1, 1970 (1970-01-01 00:00:00) also called the EPOCH. Instant values can be negative if they occured before the epoch. They followISO 8601 the standard for representing date and time.

// current time
Instant now = Instant.now();
// from unix timestamp, 2010-01-01 12:00:00
Instant fromUnixTimestamp = Instant.ofEpochSecond(1262347200);
// same time in millis
Instant fromEpochMilli = Instant.ofEpochMilli(1262347200000l);
// parsing from ISO 8601
Instant fromIso8601 = Instant.parse("2010-01-01T12:00:00Z");
// toString() returns ISO 8601 format, e.g. 2014-02-15T01:02:03Z
String toIso8601 = now.toString();
// as unix timestamp
long toUnixTimestamp = now.getEpochSecond();
// in millis
long toEpochMillis = now.toEpochMilli();
// plus/minus methods are available too
Instant nowPlusTenSeconds = now.plusSeconds(10);

Periods and Durations

Period and Duration are two other important classes. Like the names suggest they represent a quantity or amount of time. A Period uses date based values (years, months, days) while a Duration uses seconds or nanoseconds to define an amount of time. Duration is most suitable when working with Instants and machine time. Periods and Durations can contain negative values if the end point occurs before the starting point.

// periods
LocalDate firstDate = LocalDate.of(2010, 5, 17); // 2010-05-17
LocalDate secondDate = LocalDate.of(2015, 3, 7); // 2015-03-07
Period period = Period.between(firstDate, secondDate);
int days = period.getDays(); // 18
int months = period.getMonths(); // 9
int years = period.getYears(); // 4
boolean isNegative = period.isNegative(); // false
Period twoMonthsAndFiveDays = Period.ofMonths(2).plusDays(5);
LocalDate sixthOfJanuary = LocalDate.of(2014, 1, 6);
// add two months and five days to 2014-01-06, result is 2014-03-11
LocalDate eleventhOfMarch = sixthOfJanuary.plus(twoMonthsAndFiveDays);
// durations
Instant firstInstant= Instant.ofEpochSecond( 1294881180 ); // 2011-01-13 01:13
Instant secondInstant = Instant.ofEpochSecond(1294708260); // 2011-01-11 01:11
Duration between = Duration.between(firstInstant, secondInstant);
// negative because firstInstant is after secondInstant (-172920)
long seconds = between.getSeconds();
// get absolute result in minutes (2882)
long absoluteResult = between.abs().toMinutes();
// two hours in seconds (7200)
long twoHoursInSeconds = Duration.ofHours(2).getSeconds();

Formatting and Parsing

Formatting and parsing is another big topic when working with dates and times. In Java 8 this can be accomplished by using the format() and parse() methods:

// 2014-04-01 10:45
LocalDateTime dateTime = LocalDateTime.of(2014, Month.APRIL, 1, 10, 45);
// format as basic ISO date format (20140220)
String asBasicIsoDate = dateTime.format(DateTimeFormatter.BASIC_ISO_DATE);
// format as ISO week date (2014-W08-4)
String asIsoWeekDate = dateTime.format(DateTimeFormatter.ISO_WEEK_DATE);
// format ISO date time (2014-02-20T20:04:05.867)
String asIsoDateTime = dateTime.format(DateTimeFormatter.ISO_DATE_TIME);
// using a custom pattern (01/04/2014)
String asCustomPattern = dateTime.format(DateTimeFormatter.ofPattern("dd/MM/yyyy"));
// french date formatting (1. avril 2014)
String frenchDate = dateTime.format(DateTimeFormatter.ofPattern("d. MMMM yyyy", new Locale("fr")));
// using short german date/time formatting (01.04.14 10:45)
DateTimeFormatter formatter = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT)
.withLocale(new Locale("de"));
String germanDateTime = dateTime.format(formatter);
// parsing date strings
LocalDate fromIsoDate = LocalDate.parse("2014-01-20");
LocalDate fromIsoWeekDate = LocalDate.parse("2014-W14-2", DateTimeFormatter.ISO_WEEK_DATE);
LocalDate fromCustomPattern = LocalDate.parse("20.01.2014", DateTimeFormatter.ofPattern("dd.MM.yyyy"));

Conversion

Of course we do not always have objects of the type we need. Therefore, we need an option to convert different date/time related objects between each other. The following examples show some of the possible conversion options:

// LocalDate/LocalTime <-> LocalDateTime
LocalDate date = LocalDate.now();
LocalTime time = LocalTime.now();
LocalDateTime dateTimeFromDateAndTime = LocalDateTime.of(date, time);
LocalDate dateFromDateTime = LocalDateTime.now().toLocalDate();
LocalTime timeFromDateTime = LocalDateTime.now().toLocalTime();
// Instant <-> LocalDateTime
Instant instant = Instant.now();
LocalDateTime dateTimeFromInstant = LocalDateTime.ofInstant(instant, ZoneId.of("America/Los_Angeles"));
Instant instantFromDateTime = LocalDateTime.now().toInstant(ZoneOffset.ofHours(-2));
// convert old date/calendar/timezone classes
Instant instantFromDate = new Date().toInstant();
Instant instantFromCalendar = Calendar.getInstance().toInstant();
ZoneId zoneId = TimeZone.getDefault().toZoneId();
ZonedDateTime zonedDateTimeFromGregorianCalendar = new GregorianCalendar().toZonedDateTime();
// convert to old classes
Date dateFromInstant = Date.from(Instant.now());
TimeZone timeZone = TimeZone.getTimeZone(ZoneId.of("America/Los_Angeles"));
GregorianCalendar gregorianCalendar = GregorianCalendar.from(ZonedDateTime.now());

Conclusion

With Java 8 we get a very rich API for working with date and time located in the java.time package. The API can completely replace old classes like java.util.Date or java.util.Calendar with newer, more flexible classes. Due to mostly immutable classes the new API helps in building thread safe systems.

The source of the examples can be found on GitHub.

API Java (programming language)

Published at DZone with permission of Michael Scharhag, DZone MVB. See the original article here.

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