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j17 > Working with Date and Times > Flashcards

Working with Date and Times Flashcards

1
Q

What we need to import to work with the modern date and time classes?

A 
import java.time.*; // import time classes
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2
Q

modern date and time classes

A
  • LocalDate : Contains just a date—no time and no time zone.
    A good example of LocalDate is your birthday this year. It is your birthday for a full day, regardless of what time it is.
  • LocalTime : Contains just a time—no date and no time zone.
    A good example of LocalTime is midnight. It is midnight at the same time every day.
  • LocalDateTime : Contains both a date and time but no time zone.
    A good example of LocalDateTime is “the stroke of midnight on New Year’s Eve.” Midnight on January 2 isn’t nearly as special, making the date elatively unimportant, and clearly an hour after midnight isn’t as special either.
  • ZonedDateTime : Contains a date, time, and time zone.
    A good example of ZonedDateTime is “a conference call at 9:00 a.m. EST.” If you live in California, you’ll have to get up really early since the call is at 6:00 a.m. local time!
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3
Q

You obtain date and time instances using a static method now():

A 
System.out.println(LocalDate.now()); // 2024-05-07
System.out.println(LocalTime.now()); // 16:55:57.880929400
System.out.println(LocalDateTime.now()); // 2024-05-07T16:56:25.920557500
System.out.println(ZonedDateTime.now()); // 2024-05-07T16:56:51.549554300+08:00[Asia/Kuala_Lumpur]

The output uses T to separate the date and time when converting LocalDateTime to a String.

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4
Q

> [!Note]
Greenwich Mean Time is a time zone in Europe that is used as time zone zero when discussing offsets. You might have also heard of Coordinated Universal Time, which is a time zone standard. It is abbreviated as UTC, as a compromise between the English and French names. (That’s not a typo. UTC isn’t actually the proper acronym in either language!) UTC uses the same time zone zero as GMT.

A
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5
Q

Covert to GMT

2022–06–20T06:50+05:30[Asia/Kolkata]
2022–06–20T07:50-05:00[US/Eastern]
A

Remember that you need to add when subtracting a negative number. After converting to GMT, you can see that the U.S. Eastern time is 11 and a half hours behind the Kolkata time.

2022–06–20T06:50+05:30[Asia/Kolkata] // GMT 2022–06–20 01:20
2022–06–20T07:50-05:00[US/Eastern]   // GMT 2022–06–20 12:50
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6
Q

> [!NOTE]
Time Zone Rules
The time zone offset can be listed in different ways: +02:00, GMT+2, and UTC+2 all mean the same thing. You might see any of them on the exam.

A
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7
Q

LocalDate

A

of() method signature.

public static LocalDate of(int year, int month, int dayOfMonth)
public static LocalDate of(int year, Month month, int dayOfMonth)

EX :

var date1 = LocalDate.of(2022, Month.JANUARY, 20);
var date2 = LocalDate.of(2022, 1, 20);
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8
Q

> [!Note]
Up to now, we’ve been continually telling you that Java counts starting with 0. Well, months are an exception. For months in the new date and time methods, Java counts starting from 1, just as we humans do.

A
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9
Q

LocalTime

A

of() method signatures

public static LocalTime of(int hour, int minute)
public static LocalTime of(int hour, int minute, int second)
public static LocalTime of(int hour, int minute, int second, int nanos)

EX :

var time1 = LocalTime.of(6, 15);            // hour and minute
var time2 = LocalTime.of(6, 15, 30);        // + seconds
var time3 = LocalTime.of(6, 15, 30, 200);   // + nanoseconds
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10
Q

LocalDateTime

A

of() method signatures

public static LocalDateTime of(int year, int month, int dayOfMonth, int hour, int minute)
public static LocalDateTime of(int year, int month, int dayOfMonth, int hour, int minute, int second)
public static LocalDateTime of(int year, int month, int dayOfMonth, int hour, int minute, int second, int nanos)

take a Month reference:

public static LocalDateTime of(int year, Month month, int dayOfMonth, int hour, int minute)
public static LocalDateTime of(int year, Month month, int dayOfMonth, int hour, int minute, int second)
public static LocalDateTime of(int year, Month month, int dayOfMonth, int hour, int minute, int second, int nanos)
public static LocalDateTime of(LocalDate date, LocalTime time)

EX :

var dateTime1 = LocalDateTime.of(2022, Month.JANUARY, 20, 6, 15, 30);
var dateTime2 = LocalDateTime.of(date1, time1);
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11
Q

ZonedDateTime

A

In order to create a ZonedDateTime, we first need to get the desired time zone. We will use US/Eastern in our examples:

var zone = ZoneId.of("US/Eastern");
var zoned1 = ZonedDateTime.of(2022, 1, 20, 6, 15, 30, 200, zone);
var zoned2 = ZonedDateTime.of(date1, time1, zone);
var zoned3 = ZonedDateTime.of(dateTime1, zone);

Although there are other ways of creating a ZonedDateTime, you only need to know three for the exam:

public static ZonedDateTime of(int year, int month, int dayOfMonth, int hour, int minute, int second, int nanos, ZoneId zone)
public static ZonedDateTime of(LocalDate date, LocalTime time, ZoneId zone)
public static ZonedDateTime of(LocalDateTime dateTime, ZoneId zone)

Notice that there isn’t an option to pass in the Month enum.

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12
Q

The date and time classes have private constructors along with static methods that return instances. This is known as the factory pattern. The exam creators may throw something like this at you:

var d = new LocalDate(); // DOES NOT COMPILE
A

You are not allowed to construct a date or time object directly.

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13
Q 
var d = LocalDate.of(2022, Month.JANUARY, 32)
A

When you pass invalid numbers to of(), it will throw DateTimeException

java.time.DateTimeException: Invalid value for DayOfMonth (valid values 1-28/31): 32
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14
Q

Manipulating Dates and Times

A

The date and time classes are immutable. Remember to assign the results of these methods to a reference variable so they are not lost.

12: var date = LocalDate.of(2022, Month.JANUARY, 20);
13: System.out.println(date); // 2022–01–20
14: date = date.plusDays(2);
15: System.out.println(date); // 2022–01–22
16: date = date.plusWeeks(1);
17: System.out.println(date); // 2022–01–29
18: date = date.plusMonths(1);
19: System.out.println(date); // 2022–02–28
20: date = date.plusYears(5);
21: System.out.println(date); // 2027–02–28
22: var date = LocalDate.of(2024, Month.JANUARY, 20);
23: var time = LocalTime.of(5, 15);
24: var dateTime = LocalDateTime.of(date, time);
25: System.out.println(dateTime); // 2024–01–20T05:15
26: dateTime = dateTime.minusDays(1);
27: System.out.println(dateTime); // 2024–01–19T05:15
28: dateTime = dateTime.minusHours(10);
29: System.out.println(dateTime); // 2024–01–18T19:15
30: dateTime = dateTime.minusSeconds(30);
31: System.out.println(dateTime); // 2024–01–18T19:14:30
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15
Q 
var date = LocalDate.of(2024, Month.JANUARY, 20);
var time = LocalTime.of(5, 15);
var dateTime = LocalDateTime.of(date, time)
        .minusDays(1).minusHours(10).minusSeconds(30);
A

It is common for date and time methods to be chained.

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16
Q

What do you think this prints?

var date = LocalDate.of(2024, Month.JANUARY, 20);
date.plusDays(10);
System.out.println(date);
A

It prints January 20, 2024.
Whenever you see immutable types , pay attention to make sure that the return value of a method call isn’t ignored.

17
Q

Do you see what is wrong here?

var date = LocalDate.of(2024, Month.JANUARY, 20);
date = date.plusMinutes(1);     // DOES NOT COMPILE
A

LocalDate does not contain time. This means that you cannot add minutes to it.

18
Q

Which method LocalDate can call?

A 
plusYears()
minusYears()

plusMonths()
minusMonths()

plusWeeks()
minusWeeks()

plusDays()
minusDays()
19
Q

Which method LocalTime can call?

A 
plusHours()
minusHours()

plusMinutes()
minusMinutes()

plusSeconds()
minusSeconds()

plusNanos()
minusNanos()
20
Q

Which method LocalDateTime or ZonedDateTime can call?

A 
plusYears()
minusYears()

plusMonths()
minusMonths()

plusWeeks()
minusWeeks()

plusDays()
minusDays()

plusHours()
minusHours()

plusMinutes()
minusMinutes()

plusSeconds()
minusSeconds()

plusNanos()
minusNanos()
21
Q

> [!Note]
LocalDate and LocalDateTime have a method to convert themselves into long values, equivalent to the number of milliseconds that have passed since January 1, 1970, referred to as the epoch.
What’s special about this date?
That’s what Unix started using for date standards, so Java reused it.

22
Q

Working with Periods

A 
public static void main(String[] args) {
    var start = LocalDate.of(2022, Month.JANUARY, 1);
    var end = LocalDate.of(2022, Month.MARCH, 30);
    var period = Period.ofMonths(1); // create a period
    performAnimalEnrichment(start, end, period);
}
private static void performAnimalEnrichment(LocalDate start, LocalDate end, Period period) { // uses the generic period
    var upTo = start;
    while (upTo.isBefore(end)) {
        System.out.println("give new toy: " + upTo);
        upTo = upTo.plus(period); // adds the period
    }
}
23
Q

Creating Period

A 
var annually = Period.ofYears(1);           // every 1 year, P1Y
var quarterly = Period.ofMonths(3);         // every 3 months, P3M
var everyThreeWeeks = Period.ofWeeks(3);    // every 3 weeks, P21D
var everyOtherDay = Period.ofDays(2);       // every 2 days, P2D
var everyYearAndAWeek = Period.of(1, 0, 7); // every year and 7 days, P1Y7D
System.out.println(Period.of(1,2,3));

// output: P1Y2M3D
// P is for period
// 1Y is 1 year
// 2M is 2 months
// 3D is 3 days
24
Q

How to get Period of everyYearAndAWeek?

A

Invalid ex: (Period is immutable)

var wrong = Period.ofYears(1).ofWeeks(1); // every week, output: P7D

var wrong = Period.ofYears(1);
wrong = Period.ofWeeks(1); // every week, get new Period, output:P7D

Valid ex:

jshell> Period.of(1,0,7)
$7 ==> P1Y7D
25
Q 
3: var date = LocalDate.of(2022, 1, 20);
4: var time = LocalTime.of(6, 15);
5: var dateTime = LocalDateTime.of(date, time);
6: var period = Period.ofMonths(1);
7: System.out.println(date.plus(period));       // 2022–02–20
8: System.out.println(dateTime.plus(period));   // 2022–02–20T06:15
9: System.out.println(time.plus(period));       // Exception
A

Line 9 attempts to add a month to an object that has only a time.
This won’t work.
Java throws an UnsupportedTemporalTypeException and complains that we attempted to use an Unsupported unit: Months.

26
Q

Creating Durations

A 
var daily = Duration.ofDays(1);                 // PT24H
var hourly = Duration.ofHours(1);               // PT1H
var everyMinute = Duration.ofMinutes(1);        // PT1M
var everyTenSeconds = Duration.ofSeconds(10);   // PT10S
var everyMilli = Duration.ofMillis(1);          // PT0.001S
var everyNano = Duration.ofNanos(1);            // PT0.000000001S
var daily = Duration.of(1, ChronoUnit.DAYS);
var hourly = Duration.of(1, ChronoUnit.HOURS);
var everyMinute = Duration.of(1, ChronoUnit.MINUTES);
var everyTenSeconds = Duration.of(10, ChronoUnit.SECONDS);
var everyMilli = Duration.of(1, ChronoUnit.MILLIS);
var everyNano = Duration.of(1, ChronoUnit.NANOS);
27
Q

> [!Note]
ChronoUnit for Differences
ChronoUnit is a great way to determine how far apart two Temporal values are. Temporal includes LocalDate, LocalTime, and so on.
ChronoUnit is in the java.time.temporal package.

A

ChronoUnit is in the java.time.temporal package.

28
Q 
var one = LocalTime.of(5, 15);
var two = LocalTime.of(6, 30);
var date = LocalDate.of(2016, 1, 20);
System.out.println(ChronoUnit.HOURS.between(one, two));    // 1
System.out.println(ChronoUnit.MINUTES.between(one, two));  // 75
System.out.println(ChronoUnit.MINUTES.between(one, date)); // DateTimeException
A
  • The first print statement shows that between truncates rather than rounds.
  • The second shows how easy it is to count in different units. Just change the ChronoUnit type.
  • The last reminds us that Java will throw an exception if we mix up what can be done on date vs. time objects.
29
Q

Alternatively, you can truncate any object with a time element. For example:

LocalTime time = LocalTime.of(3, 12, 45);
System.out.println(time);         // 03:12:45
LocalTime truncated = time.truncatedTo(ChronoUnit.MINUTES);
System.out.println(truncated);    // 03:12
A

This example zeroes out any fields smaller than minutes. In our case, it gets rid of the seconds.

LocalTime truncated = time.truncatedTo(ChronoUnit.MINUTES);

30
Q 
7: var date = LocalDate.of(2022, 1, 20);
8: var time = LocalTime.of(6, 15);
9: var dateTime = LocalDateTime.of(date, time);
10: var duration = Duration.ofHours(6);
11: System.out.println(dateTime.plus(duration)); // 2022–01–20T12:15
12: System.out.println(time.plus(duration)); // 12:15
13: System.out.println(
14:     date.plus(duration)); // UnsupportedTemporalTypeException
A
  • Line 11 shows that we can add hours to a LocalDateTime, since it contains a time.
  • Line 12 also works, since all we have is a time.
  • Line 13 fails because we cannot add hours to an object that does not contain a time.
31
Q 
7: var date = LocalDate.of(2022, 1, 20);
8: var time = LocalTime.of(6, 15);
9: var dateTime = LocalDateTime.of(date, time);
10: var duration = Duration.ofHours(23);
11: System.out.println(dateTime.plus(duration)); // 2022–01–21T05:15
12: System.out.println(time.plus(duration)); // 05:15
13: System.out.println(
14:     date.plus(duration)); // UnsupportedTemporalTypeException
A
  • This time we see that Java moves forward past the end of the day.
  • Line 11 goes to the next day since we pass midnight.
  • Line 12 doesn’t have a day, so the time just wraps around—just like on a real clock.
32
Q

Period vs. Duration

A

Remember that Period and Duration are not equivalent. This example shows a Period and Duration of the same length:

var date = LocalDate.of(2022, 5, 25);
var period = Period.ofDays(1);
var days = Duration.ofDays(1);

System.out.println(date.plus(period)); // 2022–05–26
System.out.println(date.plus(days)); // Unsupported unit: Seconds
33
Q

Which objects can use Period and Duration?

A
  1. LocalDate, can use Period
  2. LocalTime, can use Duration
  3. LocalDateTime, can use both Period and Duration
  4. ZonedDateTime, can use both Period and Duration
34
Q

Working with Instants

A

The Instant class represents a specific moment in time in the GMT time zone. Suppose that you want to run a timer:

var now = Instant.now();
// do something time consuming
var later = Instant.now();

var duration = Duration.between(now, later);
System.out.println(duration.toMillis()); // Returns number milliseconds
35
Q

ZonedDateTime to Instant

A 
var date = LocalDate.of(2022, 5, 25);
var time = LocalTime.of(11, 55, 00);
var zone = ZoneId.of("US/Eastern");
var zonedDateTime = ZonedDateTime.of(date, time, zone);
var instant = zonedDateTime.toInstant(); // 2022–05–25T15:55:00Z
System.out.println(zonedDateTime);       // 2022–05–25T11:55–04:00[US/Eastern]
System.out.println(instant);             // 2022–05–25T15:55:00Z

The last two lines represent the same moment in time. The ZonedDateTime includes a time zone. The Instant gets rid of the time zone and turns it into an Instant of time in GMT.

You cannot convert a LocalDateTime to an Instant. Remember that an Instant is a point in time. A LocalDateTime does not contain a time zone, and it is therefore not universally recognized around the world as the same moment in time

36
Q

Accounting for Daylight Saving Time

A

You only have to work with U.S. daylight saving time on the exam

37
Q 
var date = LocalDate.of(2022, Month.MARCH, 13);
var time = LocalTime.of(1, 30);
var zone = ZoneId.of("US/Eastern");
var dateTime = ZonedDateTime.of(date, time, zone);
System.out.println(dateTime);   // 2022–03-13T01:30-05:00[US/Eastern]
System.out.println(dateTime.getHour());  // 1
System.out.println(dateTime.getOffset());   // -05:00
dateTime = dateTime.plusHours(1);
System.out.println(dateTime);   // 2022–03-13T03:30-04:00[US/Eastern]
System.out.println(dateTime.getHour()); // 3
System.out.println(dateTime.getOffset());   // -04:00
A

Notice that two things change in this example.
The time jumps from 1:30 to 3:30. The UTC offset also changes. Remember when we calculated GMT time by subtracting the time zone from the time? You can see that we went from 6:30 GMT (1:30 minus –5:00) to 7:30 GMT (3:30 minus –4:00). This shows that the time really did change by one hour from GMT’s point of view. We printed the hour and offset fields separately for emphasis.

38
Q 
var date = LocalDate.of(2022, Month.NOVEMBER, 6);
var time = LocalTime.of(1, 30);
var zone = ZoneId.of("US/Eastern");
var dateTime = ZonedDateTime.of(date, time, zone);
System.out.println(dateTime); // 2022-11-06T01:30-04:00[US/Eastern]
        
dateTime = dateTime.plusHours(1);
System.out.println(dateTime); // 2022-11-06T01:30-05:00[US/Eastern]
        
dateTime = dateTime.plusHours(1);
System.out.println(dateTime); // 2022-11-06T02:30-05:00[US/Eastern]
A

We went from 5:30 GMT to 6:30 GMT, to 7:30 GMT.

39
Q 
var date = LocalDate.of(2022, Month.MARCH, 13);
var time = LocalTime.of(2, 30);
var zone = ZoneId.of("US/Eastern");
var dateTime = ZonedDateTime.of(date, time, zone);
System.out.println(dateTime); // 2022–03–13T03:30–04:00[US/Eastern]
A

Java is smart enough to know that there is no 2:30 a.m. that night and switches over to the appropriate GMT offset.

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