Date Classes in Standard Library

• Many programs manipulate dates such as
  "Saturday, February 3, 2001"
• Date class:
  

  Date now = new Date();
     // constructs current date/time
  System.out.println(now.toString());
     // prints date such as 
     // Sat Feb 03 16:34:10 PST 2001
  

• Date class encapsulates point in time

Methods of the Date class

Methods of the Date class

• Deprecated methods omitted
• Date class methods supply total ordering on Date objects
• Convert to scalar time measure
• Note that before/after not strictly necessary
  (Presumably introduced for convenience)

Points in Time

The GregorianCalendar Class

• The Date class doesn't measure months, weekdays, etc.
• That's the job of a calendar
• A calendar assigns a name to a point in time
• Many calendars in use:
  • Gregorian
  • Contemporary: Hebrew, Arabic, Chinese
    
  • Historical: French Revolutionary,  Mayan

Date Handling in the Java Library

Designing a Day Class

• Custom class, for teaching/learning purpose
• Use the standard library classes, not this class, in your own programs
• Day encapsulates a day in a fixed location
• No time, no time zone
• Use Gregorian calendar

• Answer questions such as
  • How many days are there between now and the end of the year?
  • What day is 100 days from now?
    

Designing a Day Class

• daysFrom computes number of days between two days:
  

  int n = today.daysFrom(birthday);
  

• addDays computes a day that is some days away from a given day:
  

  Day later = today.addDays(999);
  

• Mathematical relationship:
  

  d.addDays(n).daysFrom(d) == n
  d1.addDays(d2.daysFrom(d1)) == d2
  

• Clearer when written with "overloaded operators":
  

  (d + n) - d == n
  d1 + (d2 - d1) == d2
  

• Constructor Date(int year, int month, int date)
  
• getYear, getMonth, getDate acccesors
  

Implementing a Day Class

• Straightforward implementation:
  

  private int year
  private int month
  private int date
  

• addDays/daysBetween tedious to implement
  • April, June, September, November have 30 days
  • February has 28 days, except in leap years it has 29 days
  • All other months have 31 days
  • Leap years are divisible by 4, except after 1582, years divisible by 100 but not 400 are not leap years
  • There is no year 0; year 1 is preceded by year -1
  • In the switchover to the Gregorian calendar, ten days were dropped: October 15, 1582 is preceded by October 4

• Ch3/code/day1/Day.java   (actual Day.java file)
• Ch3/code/day1/DayTester.java   (actual DayTester.java file)
• Note private helper methods
• Computations are inefficient: a day at a time

Second Implementation

• For greater efficiency, use Julian day number
• Used in astronomy
• Number of days since Jan. 1, 4713 BCE
• May 23, 1968 = Julian Day 2,440,000
• Greatly simplifies date arithmetic
• Ch3/code/day2/Day.java   (actual Day.java file)

Third Implementation

• Now constructor, accessors are inefficient
• Best of both worlds: Cache known Julian, y/m/d values
• Ch3/code/day3/Day.java   (actual Day.java file)
• Which implementation is best?