Since Java 8, Oracle completely rebuilt its Date/Time API. The new API is supposed to replace the old one.
The new API which is located in the
java.time package is thread-safe because most of the new classes are immutable, meaning that, after the object is constructed, it cannot be modified. This is especially useful when working in a multi-threaded environment where issues like thread interference and data corruption cannot happen thanks to immutability.
This article shows how to work with the new API.
Creating Dates and Times
Java lets us create dates and times using static factory methods. Note that you cannot create date and time objects using a constructor (you can do so with the old API, but you shouldn’t) because it is made private.
Creating dates and times is straightforward; you’ll notice a pattern for creating dates and times:
You create a
LocalDate by using one of its static factory methods:
LocalDate now = LocalDate.now(); System.out.println(now);// 2020-02-21 LocalDate date = LocalDate.of(2020, 1, 23); System.out.println(date);// 2020-01-23 LocalDate date2 = LocalDate.of(2020, Month.JANUARY, 23); System.out.println(date2);// 2020-01-23
Note that month indexes are one-based.
Similarly, you create a
LocalTime object like so:
LocalTime now = LocalTime.now(); System.out.println(now);// 12:31:56.186 LocalTime midnight = LocalTime.of(23, 0); System.out.println(midnight); // 23:00
The second line output a
toString representation of
LocalTime, which represents
LocalDateTime class represents date and time combined. You can create it with:
LocalDateTime now = LocalDateTime.now(); System.out.println(now);// 2020-02-21T12:46:09.950 LocalDateTime newYear = LocalDateTime.of(LocalDate.of(2021, 12, 31), LocalTime.of(11, 59)); System.out.println(newYear);// 2021-12-31T11:59
Note that in the output date and time are separated with a
Use this class if you want to express in a date and time in a specific timezone. for example:
ZonedDateTime usPacific = ZonedDateTime.now(ZoneId.of("US/Pacific")); System.out.println(usPacific);// 2020-02-22T02:53:46.774455-08:00[US/Pacific] ZonedDateTime now = ZonedDateTime.now();// 2020-02-22T11:53:46.778363+01:00[Africa/Casablanca] System.out.println(now);
The format of the output consists of
LocalDateTime followed by the
Manipulating Dates and Times
LocalDate date = LocalDate.of(2020, Month.JANUARY, 20); System.out.println(date);// 2020–01–20 date = date.plusDays(1); System.out.println(date);// 2020-01-21 date = date.plusWeeks(3); System.out.println(date);// 2020-02-11 date = date.plusMonths(4); System.out.println(date);// 2020-06-11 date = date.plusYears(10); System.out.println(date);// 2030-06-11
You these methods return a
LocalDate object. Therefore, you can chain them:
LocalDate date = LocalDate.of(2020, Month.JANUARY, 20) .plusDays(1).plusWeeks(3) .plusMonths(4).plusYears(10); System.out.println(date);// 2030-06-11
Using the same pattern, you can subtract dates/times from
Periods and Durations
You create a period from the
Period class. This class represents the amount of time in years, months, and days. These examples demonstrate the typical ways you would create a
Period threeDays = Period.ofDays(3); System.out.println(threeDays);// P3D Period threeWeeks = Period.ofWeeks(3); System.out.println(threeWeeks);// P21D Period threeMonths = Period.ofMonths(3); System.out.println(threeMonths);// P3M Period threeYears = Period.ofYears(3); System.out.println(threeYears);// P3Y Period threeYearsAndFourMonthsAndTwoDays = Period.of(3, 4, 2); System.out.println(threeYearsAndFourMonthsAndTwoDays);// P3Y4M2D
These static factory methods are self-explanatory; they create an immutable
In the output, the letter
P stands for Period,
Y for years,
M for months, and
D for days.
Note that you cannot chain methods as you’ve seen in the
LocalDate example when you create
Period because these methods are static, If you chain them you’ll get unexpected behavior:
Period oneWeekAndADay = Period.ofDays(1).ofWeeks(1); System.out.println(oneWeekAndADay); // unexpected result: P7D
Remember that a
Period cannot be be used with some objects. Let’s look at some code:
Period period = Period.ofDays(1); LocalTime time = LocalTime.of(6, 15); time.plus(period); // UnsupportedTemporalTypeException
You create a duration form the
Duration class. This class represents the amount of time in seconds and nanoseconds. It can also be expressed using other duration-based units, such as minutes and hours. These examples demonstrate the typical ways you would create a
Duration oneNano = Duration.ofNanos(1); System.out.println(oneNano);// PT0.000000001S Duration oneMilli = Duration.ofMillis(1); System.out.println(oneMilli);// PT0.001S Duration oneSeconds = Duration.ofSeconds(1); System.out.println(oneSeconds);// PT1S Duration oneMinute = Duration.ofMinutes(1); System.out.println(oneMinute);// PT1M Duration oneHour = Duration.ofHours(1); System.out.println(oneHour);// PT1H Duration oneDay = Duration.ofDays(1); System.out.println(oneDay);// PT24H
Alternatively, you can create a
Duration using the following method:
Duration fiveHours = Duration.of(5, ChronoUnit.HOURS); System.out.println(fiveHours);
This method takes 5 as an amount and a unit that the duration is measured in.
Working with Instants
You create an instant from the
Instant class. This class represents a single instantaneous point on the time-line in the GMT since January 1, 1970 (1970–01–01T00:00:00Z), a.k.a the EPOCH. It may come in handy when you want to record event timestamps in the program. These examples demonstrate the typical ways you would create an
Instant now = Instant.now(); System.out.println(now);//2020-02-25T11:14:46.032856Z ZonedDateTime zonedDateTime = ZonedDateTime.now(ZoneId.of("US/Eastern")); System.out.println(zonedDateTime);// 2020-02-25T06:27:27.572624-05:00[US/Eastern] Instant now2 = Instant.from(zonedDateTime); System.out.println(now2);// 2020-02-25T11:14:46.055857Z Instant instant = Instant.parse("2010-01-20T11:33:45Z"); System.out.println(instant);// 2010-01-20T11:33:45Z Instant epoch = Instant.ofEpochMilli(0); System.out.println(epoch);// 1970-01-01T00:00:00Z
Note that the output of
toString follows the ISO-8601 standard.
As you can see, when Java invoked
Instant.from(zonedDateTime); it created an
Instant object from
ZonedDateTime object and converted the time from US/Eastern timezone to GMT.
Likewise, this class provides various ways to operate on instants. For example:
Instant tenMinutesLater = Instant.now().plus(10, ChronoUnit.MINUTES); System.out.println(tenMinutesLater);// print 10 minutes later from the current time
Parsing and Formatting
The JDK provides a new API to parse and format Temporal-based objects, using the
DateTimeFormatter from the
java.time.format package we can parse and format dates and times. Similar to most other new Date/Time API classes,
DateTimeFormatter is immutable thus thread-safe.
format method is provided by those classes for formatting temporal-based objects for display. For example, this snippet of code format date and time using a predefined formatter:
LocalDate date = LocalDate.of(2020, Month.MARCH, 17); LocalTime time = LocalTime.of(9, 15, 45); LocalDateTime dateTime = LocalDateTime.of(date, time); System.out.println( dateTime.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));// 2020-03-17T09:15:45 System.out.println( date.format(DateTimeFormatter.ISO_LOCAL_DATE));// 2020-03-17 System.out.println( time.format(DateTimeFormatter.ISO_LOCAL_TIME));// 09:15:45 DateTimeFormatter shortF = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT); System.out.println(shortF.format(dateTime));// 3/17/20, 9:15 AM DateTimeFormatter mediumF = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.MEDIUM); System.out.println(mediumF.format(dateTime));// Mar 17, 2020, 9:15:45 AM
We can also define a custom formatter object using
LocalDate date = LocalDate.of(2020, Month.MARCH, 17); LocalTime time = LocalTime.of(9, 15, 45); LocalDateTime dateTime = LocalDateTime.of(date, time); DateTimeFormatter f = DateTimeFormatter.ofPattern("dd-MMMM-yyyy | hh:mm"); System.out.println(dateTime.format(f));// 17-March-2020 | 09:15
Make sure to take a look at the reference documentation if you want to know more about the syntax used in the
ofPattern method argument.
Now you know how to convert Temporal-based classes into strings, let’s see how we can convert into the other direction using the
DateTimeFormatter f = DateTimeFormatter.ofPattern("dd MM yyyy"); LocalDate date = LocalDate.parse("25 03 2020", f); LocalTime time = LocalTime.parse("09:30"); System.out.println(date);// 2020-03-25 System.out.println(time);// 09:30
LocalDate.parse(“25 03 2020“, f) returns a
LocalDate by parsing the text string using a custom formatter object. While
LocalTime.parse(“09:30”) parses the text string and returns a
LocalTime using the default formatter (DateTimeFormatter.ISO_LOCAL_TIME)
A Peek on Dates and Times Before Java 8
Before Java 8, developers used the
Date class, which represents the date and time altogether. There was no way to get a date or time separately, and it was all bundled in the
Date class. Developers should not use this class anymore, as you’ve seen, there is a better way. This class exists to support backward compatibility.
Another disadvantage of the
Date class is that it’s mutable. Therefore, you need to synchronize access to instances of this class when they’re accessed from multiple threads to avoid data corruption and unexpected behavior.
You may encounter the old Date API in legacy projects. You create a date by calling its constructor:
Date date = new Date() ,
date refers to the current date/time. You can specify a specific date using the
Calendar cal = Calendar.getInstance(); cal.set(2020, 0, 23); Date d = cal.getTime(); System.out.println(d);// Wed Jan 01 11:36:37 WET 2020
You can see how much verbose this old API compared to the new one. Beware that Month indexes are zero-based instead of one-based, which is confusing. The new API’s indexes are one-based.
In this post, I attempted to demonstrate why you should work with the new Date/Time API, how to work with it, how to format and parse dates and times. And, peeked at how dates/times were handled before Java 8.
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