baeldung - Coding and Testing Stuff: Introduction to the Java 8 Date/Time API

Datetime:2016-08-23 00:37:15          Topic: Java8  Test Engineer           Share

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1. Overview

Java 8 introduced new APIs for Date and Time to address the shortcomings of the older java.util.Date and java.util.Calendar .

As part of this article, let’s start with the issues in the existing Date and Calendar APIs and let’s discuss how the new Java 8 Date and Time APIs address them.

We will also look at some of the core classes of the new Java 8 project that are part of the java.time package like LocalDate , LocalTime, LocalDateTime, ZonedDateTime, Period, Duration and their supported APIs.

2. Issues with the Existing Date / Time  APIs

  • Thread Safety – The  Date and Calendar  classes are not thread safe, leaving developers to deal with the headache of hard to debug concurrency issues and to write additional code to handle thread safety. On the contrary the new Date and Time APIs introduced in Java 8 are immutable and thread safe, thus taking that concurrency headache away from developers.
  • APIs Design and Ease of Understanding – The Date and Calendar APIs are poorly designed with inadequate methods to perform day-to-day operations. The new Date/Time APIs is ISO centric and follows consistent domain models for date, time, duration and periods. There are a wide variety of utility methods that support the commonest operations.
  • ZonedDate and Time – Developers had to write additional logic to handle timezone logic with the old APIs, whereas with the new APIs, handling of timezone can be done with Local and ZonedDate / Time APIs.

3. Using LocalDate , LocalTime and LocalDateTime

The most commonly used classes are LocalDate , LocalTime and LocalDateTime . As their names indicate, they represent the local Date/Time from the context of the observer.

These classes are mainly used when timezone are not required to be explicitly specified in the context. As part of this section, we will cover the most commonly used APIs.

3.1. Working with LocalDate

The LocalDate represents a date in ISO format (yyyy-MM-dd) without time .

It can be used to store dates like birthdays and paydays.

An instance of current date can be created from the system clock as below:

LocalDate localDate = LocalDate.now();

The LocalDate representing a specific day, month and year can be obtained using the “ of ” method or by using the “ parse ” method. For example the below code snippets represents the  LocalDate for 20 February 2015:

LocalDate.of(2015, 02, 20);

LocalDate.parse("2015-02-20");

The LocalDate provides various utility methods to obtain a variety of information. Let’s have a quick peek at some of these APIs methods.

The following code snippet gets the current local date and adds one day:

LocalDate tomorrow = LocalDate.now().plusDays(1);

This example obtains the current date and subtracts one month. Note how it accepts an enum as the time unit:

LocalDate previousMonthSameDay = LocalDate.now().minus(1, ChronoUnit.MONTHS);

In the following two code examples we parse the date “2016-06-12” and get the day of the week and the day of the month respectively. Note the return values, the first is an object representing the DayOfWeek while the second in an int representing the ordinal value of the month:

DayOfWeek sunday = LocalDate.parse("2016-06-12").getDayOfWeek();

int twelve = LocalDate.parse("2016-06-12").getDayOfMonth();

We can test if a date occurs in a leap year. In this example we test if the current date occurs is a leap year:

boolean leapYear = LocalDate.now().isLeapYear();

The relationship of a date to another can be determined to occur before or after another date:

boolean notBefore = LocalDate.parse("2016-06-12")
  .isBefore(LocalDate.parse("2016-06-11"));

boolean isAfter = LocalDate.parse("2016-06-12").isAfter(LocalDate.parse("2016-06-11"));

Date boundaries can be obtained from a given date. In the following two examples we get the LocalDateTime that represents the beginning of the day (2016-06-12T00:00) of the given date and the LocalDate that represents the beginning of the month (2016-06-01) respectively:

LocalDateTime beginningOfDay = LocalDate.parse("2016-06-12").atStartOfDay();
LocalDate firstDayOfMonth = LocalDate.parse("2016-06-12")
  .with(TemporalAdjusters.firstDayOfMonth());

Now let’s have a look at how we work with local time.

3.2. Working with LocalTime

The LocalTime represents time without a date .

Similar to LocalDate an instance of  LocalTime can be created from system clock or by using “parse” and “of” method. Quick look at some of the commonly used APIs below.

An instance of current LocalTime can be created from the system clock as below:

LocalTime now = LocalTime.now();

In the below code sample , we create a LocalTime representing 06:30 AM by parsing a string representation:

LocalTime sixThirty = LocalTime.parse("06:30");

The Factory method “of” can be used to create a LocalTime . For example the below code creates LocalTime representing 06:30 AM using the factory method:

LocalTime sixThirty = LocalTime.of(6, 30);

The below example creates a LocalTime by parsing a string and adds an hour to it by using the “plus” API. The result would be LocalTime representing 07:30 AM:

LocalTime sevenThirty = LocalTime.parse("06:30").plus(1, ChronoUnit.HOURS);

Various getter methods are available which can be used to get specific units of time like hour, min and secs like below:

int six = LocalTime.parse("06:30").getHour();

We can also check if a specific time is before or after another specific time. The below code sample compares two LocalTime for which the result would be true:

boolean isbefore = LocalTime.parse("06:30").isBefore(LocalTime.parse("07:30"));

The max, min and noon time of a day can be obtained by constants in LocalTime class. This is very useful when performing database queries to find records within a given span of time. For example, the below code represents 23:59:59.99:

LocalTime maxTime = LocalTime.MAX

Now let’s dive into LocalDateTime .

3.3. Working with LocalDateTime

The LocalDateTime is used to represent a combination of date and time .

This is the most commonly used class when we need a combination of date and time. The class offers a variety of APIs and we will look at some of the most commonly used ones.

An instance of LocalDateTime  can be obtained from the system clock similar to  LocalDate and LocalTime:

LocalDateTime.now();

The below code samples explain how to create an instance using the factory “of” and “parse” methods. The result would be a LocalDateTime instance representing 20 February 2015, 06:30 AM:

LocalDateTime.of(2015, Month.FEBRUARY, 20, 06, 30);
LocalDateTime.parse("2015-02-20T06:30:00");

There are utility APIs to support addition and subtraction of specific units of time like days, months, year and minutes are available. The below code samples demonstrates the usage of “plus” and “minus” methods. These APIs behave exactly like their counterparts in LocalDate and LocalTime:

localDateTime.plusDays(1);
localDateTime.minusHours(2);

Getter methods are available to extract specific units similar to the date and time classes. Given the above instance of LocalDateTime , the below code sample will return the month February:

localDateTime.getMonth();

4. Using ZonedDateTime API

Java 8 provides  ZonedDateTime when we need to deal with time zone specific date and time. The  ZoneId is an identifier used to represent different zones. There are about 40 different time zones and the  ZoneId are used to represent them as follows.

In this code snippet we create a Zone for Paris:

ZoneId zoneId = ZoneId.of("Europe/Paris");

A set of all zone ids can be obtained as below:

Set<String> allZoneIds = ZoneId.getAvailableZoneIds();

The LocalDateTime can be converted to a specific zone:

ZonedDateTime zonedDateTime = ZonedDateTime.of(localDateTime, zoneId);

The ZonedDateTime provides parse method to get time zone specific date time:

ZonedDateTime.parse("2015-05-03T10:15:30+01:00[Europe/Paris]");

Another way to work with time zone is by using OffsetDateTime . The  OffsetDateTime is an immutable representation of a date-time with an offset. This class stores all date and time fields, to a precision of nanoseconds, as well as the offset from UTC/Greenwich.

The OffSetDateTime instance   can be created as below using ZoneOffset . Here we create a LocalDateTime representing 6:30 am on 20th February 2015:

LocalDateTime localDateTime = LocalDateTime.of(2015, Month.FEBRUARY, 20, 06, 30);

Then we add two hours to the time by creating a ZoneOffset and setting for the localDateTime instance:

ZoneOffset offset = ZoneOffset.of("+02:00");

OffsetDateTime offSetByTwo = OffsetDateTime
  .of(localDateTime, offset);

We now have a localDateTime of 2015-02-20 06:30 +02:00. Now let’s move on to how to modify date and time values using the Period and Duration classes.

5. Using Period and Duration

The Period class   represents a quantity of time in terms of years, months and days and the  Duration class represents a quantity of time in terms of seconds and nano seconds.

5.1. Working with  Period

The Period class is widely used to modify values of given a date or to obtain the difference between two dates:

LocalDate initialDate = LocalDate.parse("2007-05-10");

The Date can be manipulated using Period as shown in the following code snippet:

LocalDate finalDate = initialDate.plus(Period.ofDays(5));

The Period class has various getter methods such as getYears, getMonths and getDays to get values from a Period object. The below code example returns an int value of 5 as we try to get difference in terms of days :

int five = Period.between(finalDate, initialDate).getDays();

The Period between two dates can be obtained in a specific unit such as days or month or years, using ChronoUnit.between:

int five = ChronoUnit.DAYS.between(initialDate , initialDate);

This code example returns five days. Let’s continue by taking a look at the Duration class.

5.2. Working with Duration

Similar to Period, the   Duration class  is use to deal with Time. In the following code we create a LocalTime of 6:30 am and then add a duration of 30 seconds to make a LocalTime of 06:30:30am:

LocalTime initialTime = LocalTime.of(6, 30, 0);

LocalTime finalTime = initialTime.plus(Duration.ofSeconds(30));

The Duration between two instants can be obtained either as a  Duration or as a specific unit. In the first code snippet we use the between() method of the Duration class to find the time difference between finalTime and initialTime and return the difference in seconds:

int thirty = Duration.between(finalTime, initialTime).getSeconds();

In the second example we use the between() method of the ChronoUnit class to perform the same operation:

int thirty = ChronoUnit.SECONDS.between(finalTime, initialTime);

Now we will look at how to convert existing Date and Calendar to new Date / Time.

6. Compatibility with Date and Calendar

Java 8 has added the toInstant() method which helps to convert existing Date and Calendar instance to new Date Time API as in the following code snippet:

LocalDateTime.ofInstant(date.toInstant(), ZoneId.systemDefault());
LocalDateTime.ofInstant(calendar.toInstant(), ZoneId.systemDefault());

The LocalDateTime can be constructed from epoch seconds as below. The result of the below code would be a LocalDateTime representing 2016-06-13T11:34:50:

LocalDateTime.ofEpochSecond(1465817690, 0, ZoneOffset.UTC);

Now let’s move on to Date and Time formatting.

7. Date and Time Formatting

Java 8 provides APIs for the easy formatting of Date and Time :

LocalDateTime localDateTime = LocalDateTime.of(2015, Month.JANUARY, 25, 6, 30);

The below code passes an ISO date format to format the local date. The result would be 2015-01-25 :

LocalDate localDate = localDateTime.format(DateTimeFormatter.ISO_DATE);

The DateTimeFormatter provides various standard formatting options. Custom patterns can be provided to format method as well, like below, which would return a  LocalDate as   2015/01/25:

localDateTime.format(DateTimeFormatter.ofPattern("yyyy/MM/dd"));

We can pass in formatting style either as SHORT , LONG or MEDIUM as part of the formatting option. The below code sample would give an output representing LocalDateTime in 25-Jan-2015 06:30:00:

localDateTime
  .format(DateTimeFormatter.ofLocalizedDateTime(FormatStyle.MEDIUM)
  .withLocale(Locale.UK);

Let us take a look at alternatives available to Java 8 Core Date / Time APIs.

8. Backport and Alternate Options

8.1. Using Project Threeten

For organization that are on the path of moving to Java 8 from Java 7 or Java 6 and want to use date and time API, project threeten provides the backport capability. Developers can use classes available in this project to achieve the same functionality as that of new Java 8 Date and Time API and once they move to Java 8, the packages can be switched. Artifact for the project threeten can be found in the maven central repository :

<dependency>
    <groupId>org.threeten</groupId>
    <artifactId>threetenbp</artifactId>
    <version>1.3.1</version>
</dependency>

8.2. Joda-Time Library

Another alternative for Java 8 Date and Time library is Joda-Time library. In fact Java 8 Date Time APIs has been led jointly by the author of Joda-Time library (Stephen Colebourne) and Oracle. This library provides pretty much all capabilities that is supported in Java 8 Date Time project. The Artifact can be found in the maven central by including the below pom dependency in your project:

<dependency>
    <groupId>joda-time</groupId>
    <artifactId>joda-time</artifactId>
    <version>2.9.4</version>
</dependency>

9. Conclusion

Java 8 provides a rich set of APIs with consistent API design for easier development.

The code samples for the above article can be found in the Java 8 Date/Time git repository.

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