Java 8 - Date API

Hi, I am Malathi Boggavarapu working at Volvo Group and i live in Gothenburg, Sweden. I have been working on Java since several years and had vast experience and knowledge across various technologies.

This course teach you about Date API in Java8.

Date API in java8

java.time - completly replaces java.util and Calendar classes and still keeps inter-operability with legacy API

Instant - It is a class and is a point on the timeline

Timeline - Assume it is line of time with dates on it and it has given precision and precision is the nanosecond.

Conventions of instant class

Instant 0 is the January the 1st, 1970 at midnight GMT
Instant.MIN is 1 billion years ago
Instant.MAX is Dec 31 of the year 1,000,000,000
Instant.now() is the current instant

The timeline begins from 1 billion years ago and end with 1 billion years from now.
Precision of the time would be in the nanosecond

Instant object is immutable

Instant start = Instant.now();
.......
Instant end = Instant.now();

Duration

Duration gives the elapsed time between two Instant objects
Duration elapsed = Duration.between(start, end);
long mills = elapsed.toMillis();

Methods
--------

toNanos(), toMillis(), toSeconds(), toMinues(), toHours(), toDays() - Used to convert duration between two instant objects to nano seconds, milli seconds, seconds, minuets, hours and days
minusNanos() -  Used to add  some nano seconds to the duration
plusNanos() - Used to add some nano seconds to the duration
multipliedBy(), dividedBy(), negated() - Used to multiply or divide the duration with a number
isZero(), isNegative() - Check whether the duration is zero or negative

There are many cases where date is not an Instant
Ex: Shakespere was born Apr 23, 1564
Let us meet at 1Pm and have lunch together

In the first example, it is just a date without nanosecond precision but Instant represents a date with nanoseconds precision. So Instant is not suitable for such dates without nanosecond precision

The same applies to Example 2. It just says at 1PM, the hour of the day.
So we need another concept for those dates. LocaDate is used for that.

LocalDate

LocalDate is used to create a date without requiring precision like Instant class. So the above examples could be addressed using the LocalDate class.

LocalDate now = LocalDate.now();
LocalDate dateOfBirth = LocalDate.of(1564, Month.APRIL, 23)

Here if you see we represent Month using Month.APRIL which is more readable. But in Java 7, it should represent month as 3 which is April.

Period

The Period is the amount of time between two LocalDate whereas Duration is the amount of time between two Instant objects

Example: When was shakesphere was born?

Period p = dateOfBirth.until(now);
System.out.println("# years "+ p.getYears());

long days = dateOfBirth.until(now, ChronoUnit.DAYS);
System.out.println("# days "+ days);

Example

People.txt

Malathi 1984 05 18
Naresh 1983 10 01
Neelima 1980 10 20

DateAndTime.java

List<Person> persons = new ArrayList();
try{
     BufferedReader reader = new BufferedReader(new InputStreamReader(DateAndTime.class.getResourceAsStream("People.txt")));
Stream<String> stream = reader.lines();
stream.map(line -> {
     String s[] = line.split(" ");
     String name = s[0]
     int year = Integer.parseInt(s[1]);
     Month month = Month.of(Integer.parseInt(s[2]));
    int day = Integer.parseInt(s[3]);
    Person p = new Person(name, LocalDate(year, month, day));
    persons.add(p);
   return p; 
}).forEach(System.out::println);
LocalDate now = LocalDate.of(12, Month.MARCH, 2014);
persons.stream().forEach(p -> {
      Period period = Period.between(p.getDateOfBirth(), now);
System.out.println(p.getName() + "was born " +
                             period.get(ChronoUnit.YEARS) + "years and " +
                             period.get(ChronoUnit.MONTHS) + "months  " +
                             "[" + p.getDateOfBirth().until(now, ChronoUnit.MONTHS) + " months]"
                           );
                   }
               );
}

Person.java

public class Person{
   private String name;
   private LocalDate dateOfBirth;
  public void Person(String name, LocalDate dateOfBirth){
        this.name = name;
       this.dateOfBirth = dateOfBirth;
  }
  // getter and setter methods for name and LocalDate
}

The above example reads the input from Persons.txt file which contains Person name, Year, Month, Day. Using Stream API , LocalDate and Period classes we compare the date of birth of a person and current date and extract number of years, months and number of months since the Person is born.

Hope the above example is useful to easily understand.

DateAdjuster

Useful to add or subtract an amount of time to an instant or a LocalDate

LocalDate now = LocalDate.now();
LocalDate nextSunday = now.with(TemporalAdjusters.next(DayOfWeek.SUNDAY));

See more in javadoc for more methods available for TemporalAdjusters class

LocalTime

It is just a time in the day.
Ex: 10:20

LocalTime now = LocalTime.now();
LocalTime time = LocalTime.of(10, 20) //10:20

we have set of methods available to manipulate that time

LocalTime time = LocalTime.of(10, 20)
LocalTime wakeupTime = time.plusHours(8); 10:20 + 8hours

Zoned Time

There are Time zones all over the earth
The zones are available from

Set<String> allZoneIds = ZoneId.getAvailableZoneIds();
String uKTZ = ZoneId.of("Europe/London");

Example

System.out.println(
     ZonedDateTime.of(
      1564, Month.APRIL.getValue(), 23, //year, month, day
      10, 0, 0, 0,                                         // h/mm/s/nanos
     ZoneId.of("Europe/London"))
); // prints 1564-04-23T10:00-00:01:15[Europe/London]

See more methods available in javadoc

DateTimeFormatter
---------------------------
check javadoc...

Bridges between legacy (java 1 API inherited to java7)

How to interoperate with the legacy Date API from java 7? We have certain methods available to provide inter-operability between Java 7 date API (legacy API ) and Java 8 API.

1) Instant and Date

Date date = Date.from(instant); // Java 8 to legacy code (old java version API)
Instant instant = date.toInstant(); // legacy code to Java 8

2) Instant and TimeStamp

TimeStamp time = TimeStamp.from(instant); // Java 8 to legacy code (old java version API)
Instant instant = time.toInstant();  // legacy code to Java 8

3) LocalDate and Date

Date date = Date.from(localDate); // Java 8 to legacy code (old java version API)
LocalDate localDate = date.toLocalDate();  // legacy code to Java 8

4) LocalDate and Time

Time time = Time.from(localTime); // Java 8 to legacy code (old java version API)
LocalTime localTime = time.toLocalTime();  // legacy code to Java 8