Java Date and Time
Introduction
Greetings!
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:
LocalDate
You create a LocalDate
by using one of its static factory methods:
LocalDate now = LocalDate.now();
System.out.println(now);// 2020-02-21LocalDate date = LocalDate.of(2020, 1, 23);
System.out.println(date);// 2020-01-23LocalDate date2 = LocalDate.of(2020, Month.JANUARY, 23);
System.out.println(date2);// 2020-01-23
Note that month indexes are one-based.
LocalTime
Similarly, you create a LocalTime
object like so:
LocalTime now = LocalTime.now();
System.out.println(now);// 12:31:56.186LocalTime midnight = LocalTime.of(23, 0);
System.out.println(midnight); // 23:00
The second line output a toString
representation of LocalTime
, which represents [hour:minutes:seconds.milliseconds]
LocalDateTime
The LocalDateTime
class represents date and time combined. You can create it with:
LocalDateTime now = LocalDateTime.now();
System.out.println(now);// 2020-02-21T12:46:09.950LocalDateTime 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 T
.
ZonedDateTime
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 ZoneOffset
.
Manipulating Dates and Times
LocalDate date = LocalDate.of(2020, Month.JANUARY, 20);
System.out.println(date);// 2020–01–20date = date.plusDays(1);
System.out.println(date);// 2020-01-21date = date.plusWeeks(3);
System.out.println(date);// 2020-02-11date = date.plusMonths(4);
System.out.println(date);// 2020-06-11date = 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 LocalDate
, LocalDateTime
, LocalTime
and ZonedDateTime
using minus###()
method.
Periods and Durations
Periods
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
:
Period threeDays = Period.ofDays(3);
System.out.println(threeDays);// P3DPeriod threeWeeks = Period.ofWeeks(3);
System.out.println(threeWeeks);// P21DPeriod threeMonths = Period.ofMonths(3);
System.out.println(threeMonths);// P3MPeriod threeYears = Period.ofYears(3);
System.out.println(threeYears);// P3YPeriod threeYearsAndFourMonthsAndTwoDays = Period.of(3, 4, 2);
System.out.println(threeYearsAndFourMonthsAndTwoDays);// P3Y4M2D
These static factory methods are self-explanatory; they create an immutable Period instance.
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
Durations
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
:
Duration oneNano = Duration.ofNanos(1);
System.out.println(oneNano);// PT0.000000001SDuration oneMilli = Duration.ofMillis(1);
System.out.println(oneMilli);// PT0.001SDuration oneSeconds = Duration.ofSeconds(1);
System.out.println(oneSeconds);// PT1SDuration oneMinute = Duration.ofMinutes(1);
System.out.println(oneMinute);// PT1MDuration oneHour = Duration.ofHours(1);
System.out.println(oneHour);// PT1HDuration 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
:
Instant now = Instant.now();
System.out.println(now);//2020-02-25T11:14:46.032856ZZonedDateTime 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);// 2020-02-25T11:14:46.055857ZInstant instant = Instant.parse("2010-01-20T11:33:45Z");
System.out.println(instant);// 2010-01-20T11:33:45ZInstant 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
Formating
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.
The 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));// 2020-03-17T09:15:45System.out.println(
date.format(DateTimeFormatter.ISO_LOCAL_DATE));// 2020-03-17System.out.println(
time.format(DateTimeFormatter.ISO_LOCAL_TIME));// 09:15:45DateTimeFormatter shortF = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT);
System.out.println(shortF.format(dateTime));// 3/17/20, 9:15 AMDateTimeFormatter 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 DateTimeFormatter.ofPattern
method:
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.
Parsing
Now you know how to convert Temporal-based classes into strings, let’s see how we can convert into the other direction using the parse
method:
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
class:
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.
Wrap Up
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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