Java Programming II on YouTube:
https://www.youtube.com/playlist?list=PLLIqcoGpl73hdVsj_auS8gs2cTFG1pYZx

GregorianCalendar, Date, DateFormat

1. The GregorianCalendar class
2. Current date and time
3. GregorianCalendar constructor examples
4. The Calendar class
5. Calendar constants
6. Calendar methods
7. GregorianCalendar examples
8. GregorianCalendar date parts
9. The Date class
10. GregorianCalendar to Date
11. Days diff
12. The DateFormat class
13. DateFormat constants
14. DateFormat format( )
15. Date formatting example
16. GregorianCalendar formatting example
17. Specific Date formats
18. Date usage example
19. DateUtils demo class
20. DateUtils usage

1. The GregorianCalendar class

• Represents date/time with milliseconds accuracy

  
      java.util.GregorianCalendar;
  
  

• Common constructors of the GregorianCalendar class

  
      GregorianCalendar()
      GregorianCalendar( year, month, day )
      GregorianCalendar( year, month, day, hour, minute )
      GregorianCalendar( year, month, day, hour, minute, second )
  
  

   

2. Current date and time

• A statement that gets the current date:

  
      GregorianCalendar now = new GregorianCalendar();
  
  

   

3. GregorianCalendar constructor examples

• Statements that create specific dates:

  
      // hardcoded dates
      GregorianCalendar startDate =
          new GregorianCalendar( 2009, 0, 30 );
  
      GregorianCalendar startTime =
          new GregorianCalendar( 2012, 6, 20, 15, 30 );
  
      // using variables
      GregorianCalendar birthDate = 
          new GregorianCalendar( birthYear, birthMonth, birthDay );
  
  

   

4. The Calendar class

• The Calendar is a superclass of GregorianCalendar:

  
      java.util.Calendar;
  
  

   

5. Calendar constants

• Common fields of the Calendar class

  
  DATE     DAY_OF_MONTH   DAY_OF_WEEK     DAY_OF_YEAR
  HOUR     HOUR_OF_DAY    MINUTE          MONTH
  SECOND   YEAR           MONDAY…SUNDAY   JANUARY...DECEMBER
  
  

   

6. Calendar methods

• Common methods of the Calendar and GregorianCalendar classes

  
  set(intYear, intMonth, ...)
  set(intField, intValue)
  setTime(Date) // use Date object to update the date/time
  add(intField, intValue) // adds/subtracts the intValue
  roll(intField, intValue) // same as add() but does not affect other fields
  roll(intField, booleanValue) // +1 if true, -1 if false
  get(intField)
  getTime() // convert GregorianCalendar to Date
  
  

  where the intField is one of the Calendar class fields.

   

7. GregorianCalendar examples

• Code that changes a GregorianCalendar object

  
  GregorianCalendar endDate = new 
     GregorianCalendar(2010, 0, 1);      // January 1, 2010
  endDate.set(2010, 2, 30);              // March 30, 2010
  endDate.set(2010, Calendar.MARCH, 30); // March 30, 2010
  endDate.set(Calendar.MONTH, Calendar.JANUARY);
                                         // January 30, 2010
  endDate.add(Calendar.MONTH, 5);        // June 30, 2010
  endDate.add(Calendar.MONTH, 14);       // August 30, 2011
  endDate.roll(Calendar.MONTH, 14);      // October 30, 2011
  endDate.roll(Calendar.MONTH, true);    // November 30, 2011: +1 tick
  endDate.roll(Calendar.DAY_OF_MONTH, false);
                                         // November 29, 2011: -1 tick
  
  

   

8. GregorianCalendar date parts

• Code that accesses fields in a GregorianCalendar object

  
  GregorianCalendar birthday = new GregorianCalendar(
      2010, Calendar.FEBRUARY, 4);  // Thurs, Feb 4, 2010
  int year = birthday.get(
      Calendar.YEAR);               // year is 2010
  int month = birthday.get(
      Calendar.MONTH);              // month is 1
  int day = birthday.get(
      Calendar.DAY_OF_MONTH);       // day is 4
  int dayOfWeek = birthday.get(Calendar.DAY_OF_WEEK);
                                    // dayOfWeek is 5
  int dayOfYear = birthday.get(Calendar.DAY_OF_YEAR);
                                    // dayOfYear is 35
  
  

   

9. The Date class

• Defined as

  
      java.util.Date;
  
  

  the Date class stores date/time values with accuracy to a millisecond since midnight January 1, 1970 UTC.

• Common constructors of the Date class

  • Date( )

  • Date( longMilliseconds )

• Common methods of the Date class:

  • getTime( )

  • toString( )

   

10. GregorianCalendar to Date

• A statement that converts a GregorianCalendar object to a Date object

  
      Date endDate = gregEndDate.getTime();
  
  

• A statement that gets a Date object for the current date/time

  
      Date now = new Date();
  
  

• Statements that convert Date objects to String and long variables

  
      String nowAsString = now.toString();
                                  // converts to a string
      long nowInMS = now.getTime();
                                  // converts to milliseconds
  
  

   

11. Days diff

• Code that calculates the number of days between two dates

  
      Date startDate = gregStartDate.getTime();
      Date endDate = gregEndDate.getTime();
      long startDateMS = startDate.getTime();
      long endDateMS = endDate.getTime();
      long elapsedMS = endDateMS – startDateMS;
      long elapsedDays = elapsedMS / (24 * 60 * 60 * 1000);
  
  

   

12. The DateFormat class

• The DateFormat class converts Date object to String using various formats

  
      java.text.DateFormat;
  
  

• Common static methods of the DateFormat class

  
      getDateInstance()
      getTimeInstance()
      getDateTimeInstance()
      getDateInstance( intField )
      getTimeInstance( intField )
      getDateTimeInstance( intField, intField )
  
  

  where intField is DateFormat.SHORT, MEDIUM, LONG, or FULL

   

13. DateFormat constants

• Common fields of the DateFormat class

  
   Style    Date example                   Time example
   -------  ----------------------------   ----------------
   SHORT    12/31/10                       12:00 AM
   MEDIUM   Dec 31, 2010                   7:30:00 PM
   LONG     December 31, 2010              7:30:00 AM PST
   FULL     Saturday, December 31, 2010    7:30:00 AM PST
  
  

   

14. DateFormat format( )

• Common method of the DateFormat class is

  
      format( Date )
  
  

  which returns the formatted String

   

15. Date formatting example

• 
      Date now = new Date();
      DateFormat defaultDate = DateFormat.getDateTimeInstance();
      String nowString = defaultDate.format(now);
  
  

   

16. GregorianCalendar formatting example

• 
  GregorianCalendar gregEndDate = 
      new GregorianCalendar( 2010, 11, 31, 7, 30 );
  
  Date endDate = gregEndDate.getTime();
  
  DateFormat defaultDate = DateFormat.getDateInstance();
  String endDateString = defaultDate.format( endDate );
  
  

   

17. Specific Date formats

• Code that overrides the default date and time formats:

  
  DateFormat shortDate = 
      DateFormat.getDateInstance( DateFormat.SHORT );
  
  DateFormat shortTime = 
      DateFormat.getTimeInstance( DateFormat.SHORT );
  
  DateFormat shortDateTime = 
      DateFormat.getDateTimeInstance( DateFormat.SHORT, DateFormat.SHORT );
  
  

   

18. Date usage example

• 
  public class DateUsageDemo {
  
      Date invoiceDate = DateUtils.getCurrentDate();
  
      public Date getCurrentDate()
      {  
          return currentDate;
      }
  
      public String getFormattedDate()
      {
          DateFormat shortDate =
              DateFormat.getDateInstance( DateFormat.SHORT );
  
          return shortDate.format( currentDate );
      }
  }// class DateUsageDemo
  
  

   

19. DateUtils demo class

• The proposed DateUtils class represents common operations needed by business applications:

  
  import java.util.*;
  
  public class DateUtils {
      static final int MILLS_IN_DAY = 24 * 60 * 60 * 1000;
  
      // Returns Date with current date and removes time component
      public static Date getCurrentDate() {
          GregorianCalendar currentDate = 
              new GregorianCalendar();
          currentDate.set( Calendar.HOUR, 0 );
          currentDate.set( Calendar.MINUTE, 0 );
          currentDate.set( Calendar.SECOND, 0 );
          return currentDate.getTime();
      }
  
      // Returns Date for the specified date
      public static Date createDate(
          int year, int month, int day
          )
      {
          GregorianCalendar date = 
              new GregorianCalendar( year, month, day );
          return date.getTime();
      }
  
      // Returns Date with the time component removed
      public static Date stripTime( Date date )
      {
          GregorianCalendar currentDate = 
              new GregorianCalendar();
          currentDate.setTime( date );
          currentDate.set( Calendar.HOUR, 0 );
          currentDate.set( Calendar.MINUTE, 0 );
          currentDate.set( Calendar.SECOND, 0 );
          return currentDate.getTime();
      }
  
      // Calculates diff between two Date objects in days
      public static int daysDiff( Date date1, Date date2 )
      {
          date1 = stripTime( date1 );
          date2 = stripTime( date2 );
          long longDate1 = date1.getTime();
          long longDate2 = date2.getTime();
          long longDiff = longDate2 - longDate1;
          return ( int ) ( longDiff / MILLS_IN_DAY );
      }
  }// class DateUtils
  
  

   

20. DateUtils usage

• Code that uses some of the DateUtils methods:

  
  GregorianCalendar currentGC = new GregorianCalendar();
  int currentYear = currentGC.get( Calendar.YEAR );
  
  Date currentDate = DateUtils.getCurrentDate();
  Date christmas = DateUtils.createDate(
      currentYear, Calendar.DECEMBER, 25 );
  
  int daysToChristmas = DateUtils.daysDiff(
      currentDate, christmas );
  
  DateFormat dateFormat = 
      DateFormat.getDateInstance( DateFormat.LONG );
  String formattedDate = dateFormat.format( currentDate );
  System.out.println( "Today is " + formattedDate );
  System.out.println( "There are " + daysToChristmas + 
      " days until Christmas." );
  
  
  

  Resulting output:

  
  Today is July 11, 2011
  There are 166 days until Christmas.