part1

DateTime - part1

Introduction

• Java 8 introduced a new Date-Time API,
• Before this new API, there was Legacy Date-Time API,
• Legacy Date-Time API:
  • Includes classes like:
    • Date,
    • Calendar,
    • GregorianCalendar
  • Available in java.util and java.sql packages,
  • Some issues with the legacy Date-Time API are:
    • A date always had two components: a date and a time. No option for using one separately,
    • Manipulating dates were complex,
    • Mutable and therefore not thread-safe,

The New Date-Time API:

• Correct, powerful, and extensible Date-Time API,
• It consists of about 80 classes,
• 15 classes are enough for using effectively,
• Has a separate set of classes to deal with machine-based time and calendar-based human time,
• Lets you convert machine-based time to human-based time and vice versa,
• Most classes are immutable,
• Doesn't provide public constructors,
• They allow creating object via static factory methods like,
  • of(), ofXxx(), and from(),

Exploring the New Date-Time API

• Contains large number of classes,
• Naming convention of methods help understand the API tremendously,
• Methods starting with the same prefix do the similar work,
• All classes, interfaces, and enums for the Date-Time API are in java.time package,
• Some frequently used classes of java.time package are:
  • LocalDate: Only Date,
  • LocalTime: Only Time,
  • LocalDateTime: Date and Time,
  • ZonedDateTime: Date, Time and Zone,
  • Period: Deals with years, months, and days & their operation,
  • Duration: Represents a period of time in terms of seconds and nanoseconds,
  • Instant: Represents a specific point in time,
• 4 of its subpackages are:
  • java.time.chrono:
    • Contains classes supporting non-ISO calendar systems like Hijrah calendar, ThaiBuddhist calendar,
  • java.time.format:
    • Contains classes for formatting and parsing dates and times,
    • java.time.temporal:
      • Contains classes for accessing components of dates and times,
      • Also contains classes that act like date-time adjusters,
    • java.time.zone:
      • contains classes supporting time zones and zone rules
• Ex: See basicExample() in Test.java: (Will be explained later),

  private static void basicExample(){
      LocalDate ld = LocalDate.of(2023, Month.MARCH,23);
      System.out.println(ld); // 2023-03-23
  
      LocalDate newDate = ld.plusDays(9);
      System.out.println(newDate); // 2023-04-01
  
      System.out.println( newDate.getDayOfWeek() ); // SATURDAY
      System.out.println( newDate.getMonthValue() ); // 4
  }

The ofXXX() Methods

• Allow creating object,
• Ex:

  LocalDate date1 = LocalDate.now();
  System.out.println(date1); // 2023-08-16
  
  LocalDate date2 = LocalDate.of(2023,4,23);
  System.out.println(date2); // 2023-04-23
  
  LocalDate date3 = LocalDate.ofEpochDay(1000); // epoch means time from Midnight January 1, 1970 UTC
  System.out.println(date3); // 1972-09-27
  
  LocalDate date4 = LocalDate.ofYearDay(2023,140);
  System.out.println(date4); // 2023-05-20

The from() Methods

• A static factory method, similar to an of() method,
• Create object from another argument. argument should have necessary value for converting,
• Ex:

  LocalDate date6 = LocalDate.now();
  System.out.println(date6); // 2023-08-16
  
  LocalDate date7 = LocalDate.from(date6);
  System.out.println(date7); // 2023-08-16
  
  LocalDateTime ldt = LocalDateTime.now();
  System.out.println(ldt); // 2023-08-16T23:33:54.223464
  
  LocalDate date8 = LocalDate.from(ldt);
  System.out.println(date8); // 2023-08-16

  • Here ldt has Date component, so it can be used for creating LocalDate object,

The withXXX() Methods

• Most classes in the Date-Time API are immutable,
• For changing some field you need to use something like withXXX,
• A withXXX() method returns a copy of an object with the specified field changed,
• Ex:

  LocalDate date9 = LocalDate.of(2023,Month.AUGUST,16);
  LocalDate date10 = date9.withYear(2024);
  System.out.println(date9); // 2023-08-16
  System.out.println(date10);// 2024-08-16
  
  LocalDate date11 = date9.withMonth(3);
  System.out.println(date11); // 2023-03-16

The getXXX() Methods

• Returns the specified component of the object,
• Ex:

  LocalDate date = LocalDate.of(2023,Month.AUGUST,16);
  
  System.out.println( date.getYear() ); // 2023
  
  System.out.println( date.getMonth() ); // AUGUST
  System.out.println( date.getMonthValue() ); // 8
  
  System.out.println( date.getDayOfMonth() ); // 16
  System.out.println( date.getDayOfWeek() ); // WEDNESDAY
  
  System.out.println( date.get(ChronoField.DAY_OF_YEAR) ); // 228

The toXXX() Methods

• Converts an object to a related XXX type,
• Ex:

  LocalDateTime dateTime = LocalDateTime.of(2023,Month.AUGUST,16,23,10);
  System.out.println(dateTime); // 2023-08-16T23:10
  
  LocalDate date1 = dateTime.toLocalDate();
  System.out.println(date1); // 2023-08-16
  
  LocalTime time1 = dateTime.toLocalTime();
  System.out.println(time1); // 23:10
          
  System.out.println( date.toEpochDay() ); // 19585

The atXXX() Methods

• Lets you build a new datetime object from an existing datetime object by supplying some additional pieces of information,
• withXXX() changes field and return same type of object. But
• atXXX() may return different type by taking additional info,
• Ex:

  LocalDate date2 = LocalDate.of(2023,Month.AUGUST,16);
  System.out.println(date2); // 2023-08-16
  
  LocalDateTime dateTime2 = date2.atTime(23,10);
  System.out.println(dateTime2); // 2023-08-16T23:10
  
  ZonedDateTime zonedDateTime2 = dateTime2.atZone(ZoneOffset.UTC);
  System.out.println(zonedDateTime2); // 2023-08-16T23:10Z

The plusXXX() and minusXXX() Methods

• Returns a copy after adding or subtracting a component value,
• Ex:

  LocalDate date3 = LocalDate.of(2023,Month.AUGUST,16);
  System.out.println(date3); // 2023-08-16
  
  LocalDate date4 = date3.plusDays(15);
  System.out.println(date4); // 2023-08-31
  
  LocalDate date5 = date3.plusMonths(35);
  System.out.println(date5); // 2026-07-16

The multipliedBy(), dividedBy(), negated() Methods

• Don't make sense on dates and times,
• Applicable to Duration and Period,
• Ex:

  Duration d = Duration.ofSeconds(200);
  System.out.println(d); // PT3M20S <-- 3min 20sec
  
  Duration d1 = d.multipliedBy(2);
  System.out.println(d1); // PT6M40S
  
  Duration d2 = d.dividedBy(4);
  System.out.println(d2); // PT50S
  
  Duration d3 = d.negated();
  System.out.println(d3); // PT-3M-20S