8. Java 8: Use Java SE8 Date/Time API

Question 1

True/False

The new date/time API is not thread-safe because it resulted in performance problems in the past.

Answer 1

False

The new API is thread-safe and all objects are immutable (just like a String). This was the main problem with the old date / Calendar API.

Question 2

What are the 4 ways a Instant object be created?

Answer 2

Instant.now()
Instant.ofEpochSecond(999999)
Instant.ofEpochMilli(99999L)
Instant.parse(“2015-07-10T12:00:00Z”)

Question 3

How can you get the EPOCH in seconds from a instant object?

Answer 3

instant.getEpochSecond();

Question 4

How can you get the EPOCH in milliseconds from a instant object?

Answer 4

instant.getEpochMilli();

Question 5

What are the 3 ways to create a LocalDate object?

Answer 5

LocalDate.now();
LocalDate.of(1979, Month.FEBRUARY, 15);
LocalDate.ofYearDay(2015,256)

Question 6

What are the 4 ways of creating a LocalTime object?

Answer 6

LocalTime.now()
LocalTime.of(12, 0) // 12:00
LocalTime.of(13,33,45) // 13:33:45
LocalTime.ofSecondOfDay(121) // 12:02:01

Question 7

Name 2 ways of creating a LocalDateTime object?

Answer 7

LocalDateTime.now();

LocalDateTime.of(2015, 12, 31, 13, 45); //2015-12-31 13:45

Question 8

What is the difference between a LocalDateTime object and a ZonedDateTime object?

Answer 8

ZonedDateTime contains timezone information.

Question 9

How would one create a ZonedDateTime object using the current time and the timezone of Europe/Berlin? And a ZonedDateTime using a offset of 3 hours from GMT?

Answer 9

ZoneId zoneIdBerlin = ZoneId.of(“Europe/Berlin”);
ZoneId zoneIdOffset = ZoneId.offOffset(“GMT”, ZoneOffset.ofHours(“+3”));

LocalDateTime now = LocalDateTime.now();
ZonedDateTime berlin = ZonedDateTime.of(now, zoneIdBerlin);
ZonedDateTime plus3 = berlin.withZoneSameInstant(zonedIdOffset);

Question 10

Use the Period class to print the amount of years, months and days that two dates are apart.

Answer 10

LocalDate d1 = LocalDate.of(2015,1,1)
LocalDate d2 = LocalDate.of(2017,2,15)

Period period = Period.between(d1,d2);
System.out.println(period.getYears()); // 2
System.out.println(period.getMonths()); // 1
System.out.println(period.getDays()); // 14

Question 11

Use the ChronoUnit class to calculate the total amount of days between two dates.

Answer 11

LocalDate d1 = LocalDate.of(2017, 1, 1);
LocalDate d2 = LocalDate.of(2017,2,1);
ChronoUnit.DAYS.between(d1, d2);

Question 12

How can you create Instant of the current time?

Answer 12

Instant.now();

Question 13

What is the result of the following code:
Instant i1 = Instant.now();
Thread.sleep(1000);
Instant i2 = Instant.now();
System.out.println(Duration.between(i1,i2).getSeconds());
System.out.println(Duration.between(i2,i1).getSeconds());

Answer 13

1

-1

Question 14

Given two Instant objects (i1, i2) how would one get the number of nano seconds between those Instants?

Answer 14

Duration.between(i1, i2).toNanos();

Question 15

How to format a LocalDateTime object to the pattern “ yyyy-MM-dd”?

Answer 15

date.format(DateTimeFormatter.ofPattern(“yyyy-MM-dd”));

Question 16

How to format a LocalDateTime object to the Short date pattern, with the nl-NL locale?

Answer 16

DateTimeFormmater f = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT)
.withLocale(new Locale("nl-NL");
date.format(f);

Question 17

How to format a LocalDateTime using the predefined BASIC_ISO_DATE format?

Answer 17

date.format(DateTimeFormatter.BASIC_ISO_DATE);

Question 18

How to parse the date string 2017/05/01 to a LocalDate object?

Answer 18

LocalDate d = LocalDate.parse(“2017-05-01”, DateTimeFormatter.ofPattern(“yyyy/MM/dd”));

Question 19

How to parse the date string 2017/05/01 to a LocalDate object?

Answer 19

LocalDate d = LocalDate.parse("2017-05-01");
// No pattern needed because it's de default ISO pattern.

Question 20

How to get the EPOCH from a instant?

Answer 20

Instant.EPOCH; // 0 (1970-1-1 00:00:00)

Question 21

How to create a Instant object of 10 seconds after EPOCH?

Answer 21

Instant.ofEpochSecond(10);

Question 22

How can create a Instant object based on a certain timezone?

Answer 22

Clock clock = Clock.system(ZoneId.of(“Europe/Berlin”));

Instant i = Instant.now(clock);

Question 23

Given a Instant add 2 hours.

Answer 23

Instant i = Instant.now();
Duration d = Duration.ofHours(2);
Instant i2 = i1.plus(d);

Question 24

What is the difference between Duration and Period?

Answer 24

Duration should be used for machine timings (using the Instant object). Period should be used for detemining date (or human) timings (use LocalDate, LocalDateTime etc.). It will take in account for example that a month can have 31, 30 or even less days.

Question 25

Using the ChronoUnit enumeration, add 2 weeks to the LocalDate object.

Answer 25

LocalDate d = LocalDate.now();

d.plus(2, ChronoUnit.WEEKS);

Question 26

Given: Daylight Savings Time ends on Nov 1 at 2 AM in US/Eastern time zone. As a result, 2 AM becomes 1 AM. What will the following code print ?
LocalDateTime ld = LocalDateTime.of(2015, Month.OCTOBER, 31, 10, 0);

ZonedDateTime date = ZonedDateTime.of(ld, ZoneId.of(“US/Eastern”));
date = date.plus(Duration.ofDays(1)); System.out.println(date);

date = ZonedDateTime.of(ld, ZoneId.of(“US/Eastern”)); date = date.plus(Period.ofDays(1)); System.out.println(date);

Answer 26

2015-11-01T09:00-05:00[US/Eastern]
2015-11-01T10:00-05:00[US/Eastern]

Important thing to remember here is that Period is used to manipulate dates in terms of days, months, and years, while Duration is used to manipulate dates in terms of hours, minutes, and seconds. Therefore, Period doesn’t mess with the time component of the date while Duration may change the time component if the date is close to the DST boundary.

Question 27

What is the result of the following:
LocalDateTime greatDay = LocalDateTime.parse(“2015-01-01”);
String greatDayStr = greatDay.format(DateTimeFormatter.ISO_DATE_TIME); System.out.println(greatDayStr);

Answer 27

It will throw a DateTimeException because it doesn’t have time component.

Question 28

What is the result of the following?
LocalDate d1 = LocalDate.parse(“2015-02-05”, DateTimeFormatter.ISO_DATE);//T17:13:50”);
LocalDate d2 = LocalDate.of(2015, 2, 5);
LocalDate d3 = LocalDate.now();
System.out.println(d1);
System.out.println(d2);
System.out.println(d3);

Answer 28

2015-02-05
2015-02-05
2015-02-05

Question 29

What is the result of the following?
Period p = Period.between(LocalDate.now(), LocalDate.of(2015, Month.SEPTEMBER, 1)); System.out.println(p); Duration d = Duration.between(LocalDateTime.now(), LocalDateTime.of(2015, Month.SEPTEMBER, 2, 10, 10)); System.out.println(d);

Answer 29

P-1D
PT9H10M
Note that if the second date is before the first date, a minus sign is included in the output.

Question 30

What is the result of the following?

System.out.println(LocalDate.of(2015, Month.JANUARY, 31).format(DateTimeFormatter.ISO_DATE_TIME));

Answer 30

Exception at run time.

Observe that you are creating a LocalDate and not a LocalDateTime. LocalDate doesn’t have time component and therefore, you cannot format it with a formatter that expects time component such as DateTimeFormatter.ISO_DATE_TIME.

Question 31

What is the result of the following?
Period p = Period.between(LocalDate.now(),
LocalDate.of(2015, Month.SEPTEMBER, 1)); System.out.println(p);
Duration d = Duration.between(LocalDate.now(), LocalDate.of(2015, Month.SEPTEMBER, 1)); System.out.println(d);

Answer 31

It will throw an exception at run time.

The call to Duration.between will throw java.time.temporal.UnsupportedTemporalTypeException because LocalDate.now() does not have a time component, while Duration.between method needs Temporal arguments that have a time component.