Java 8. Manual. 2 part.

Date API

Java 8 contains brand new date and time APIs in the java.time package. The new Date API is comparable to the Joda-Time library , however it is not the same. The following examples will cover the most important parts of the new API.

clock

Clock gives access to the current date and time. Clocks are aware of timezones and therefore can be used instead of System.currentTimeMillis() to return the current time in milliseconds. This time precision is also represented by the Instant class . Instants can be used to create inherited java.util.Date objects . Clock clock = Clock.systemDefaultZone(); long millis = clock.millis(); Instant instant = clock.instant(); Date legacyDate = Date.from(instant); // legacy java.util.Date

timezones

Timezones are represented by the abstract class ZoneId . They can easily be accessed using statistical factory methods. Timezones define offsets that are important for converting between instantaneous and local dates and times. System.out.println(ZoneId.getAvailableZoneIds()); // prints all available timezone ids ZoneId zone1 = ZoneId.of("Europe/Berlin"); ZoneId zone2 = ZoneId.of("Brazil/East"); System.out.println(zone1.getRules()); System.out.println(zone2.getRules()); // ZoneRules[currentStandardOffset=+01:00] // ZoneRules[currentStandardOffset=-03:00]

LocalTime

LocalTime displays the time without a time zone, such as 17:30:15. The following example creates two local time objects for the time zones defined above. Then we compare these two objects and calculate the difference between them in hours and minutes. LocalTime now1 = LocalTime.now(zone1); LocalTime now2 = LocalTime.now(zone2); System.out.println(now1.isBefore(now2)); // false long hoursBetween = ChronoUnit.HOURS.between(now1, now2); long minutesBetween = ChronoUnit.MINUTES.between(now1, now2); System.out.println(hoursBetween); // -3 System.out.println(minutesBetween); // -239 LocalTime comes with various factory methods to simplify instantiation, including string parsing. LocalTime late = LocalTime.of(23, 59, 59); System.out.println(late); // 23:59:59 DateTimeFormatter germanFormatter = DateTimeFormatter .ofLocalizedTime(FormatStyle.SHORT) .withLocale(Locale.GERMAN); LocalTime leetTime = LocalTime.parse("13:37", germanFormatter); System.out.println(leetTime); // 13:37

LocalDate

LocalDate displays a specific date, such as 2014-03-11. Instances are immutable and work similarly to LocalTime. The example demonstrates how to calculate a new date by adding or subtracting days, months or years. Keep in mind that every operation on an object returns a new object. LocalDate today = LocalDate.now(); LocalDate tomorrow = today.plus(1, ChronoUnit.DAYS); LocalDate yesterday = tomorrow.minusDays(2); LocalDate independenceDay = LocalDate.of(2014, Month.JULY, 4); DayOfWeek dayOfWeek = independenceDay.getDayOfWeek(); System.out.println(dayOfWeek); // FRIDAY Parsing a LocalDate from a string is as easy as parsing a LocalTime: DateTimeFormatter germanFormatter = DateTimeFormatter .ofLocalizedDate(FormatStyle.MEDIUM) .withLocale(Locale.GERMAN); LocalDate xmas = LocalDate.parse("24.12.2014", germanFormatter); System.out.println(xmas); // 2014-12-24

LocalDateTime

LocalDateTime displays the date-time. This is a combination of the date and time given above, in one instance. Instances of LocalDateTime are immutable and work similarly to LocalTime and LocalDate. We can use methods to retrieve the instance property values ​​we need: LocalDateTime sylvester = LocalDateTime.of(2014, Month.DECEMBER, 31, 23, 59, 59); DayOfWeek dayOfWeek = sylvester.getDayOfWeek(); System.out.println(dayOfWeek); // WEDNESDAY Month month = sylvester.getMonth(); System.out.println(month); // DECEMBER long minuteOfDay = sylvester.getLong(ChronoField.MINUTE_OF_DAY); System.out.println(minuteOfDay); // 1439 Together with additional information about the time zone, the instance can be converted to instant. Instants can be easily converted to the old java.util.Date-derived types. Instant instant = sylvester .atZone(ZoneId.systemDefault()) .toInstant(); Date legacyDate = Date.from(instant); System.out.println(legacyDate); // Wed Dec 31 23:59:59 CET 2014 Date-time formatting works just like date or time formatting. Instead of using predefined date formats, we can use manually defined formats. DateTimeFormatter formatter = DateTimeFormatter .ofPattern("MMM dd, yyyy - HH:mm"); LocalDateTime parsed = LocalDateTime.parse("Nov 03, 2014 - 07:13", formatter); String string = formatter.format(parsed); System.out.println(string); // Nov 03, 2014 - 07:13 Unlike java.text.NumberFormat, the new DateTimeFormatter is immutable and thread-safe. For more information on the syntax for writing formats, read here .

Annotations

Annotations in Java 8 can be iterable. Let's look at an example to see how it is. First, we'll define an annotation wrapper that stores an array of valid annotations: @interface Hints { Hint[] value(); } @Repeatable(Hints.class) @interface Hint { String value(); } Java 8 enables us to use multiple annotations of the same type by declaring the annotation @Repeatable. Option 1: Using the container annotation (old school) (Java 8 allows us to use multiple annotations of the same type by declaring the @Repeatable annotation .) option two, the java compiler implicitly sets the @Hint annotation. This is important for reading annotation information via reflection. @Hints({@Hint("hint1"), @Hint("hint2")}) class Person {} @Hint("hint1") @Hint("hint2") class Person {} Hint hint = Person.class.getAnnotation(Hint.class); System.out.println(hint); // null Hints hints1 = Person.class.getAnnotation(Hints.class); System.out.println(hints1.value().length); // 2 Hint[] hints2 = Person.class.getAnnotationsByType(Hint.class); System.out.println(hints2.length); // 2 Although we didn't declare the @Hints annotation on the Person class , it can be read using the getAnnotation(Hints.class) method . However, a more convenient method is getAnnotationsByType , which provides access to all annotations with the @Hint annotation . In addition, the use of annotations in Java 8 expands to two purposes: @Target({ElementType.TYPE_PARAMETER, ElementType.TYPE_USE}) @interface MyAnnotation {}

This is all

My Java 8 programming guide is over. If you want to learn more about all the new classes and features of the JDK 8 API, just read my next article. This will help you understand all the new classes and hidden features of JDK 8 like Arrays.parallelSort , StampedLock and CompletableFuture .