Learning Java

Dates

Working with dates and times without the proper tools can be a chore.^[32] In SDK 1.1 and later, you get three classes that do all the hard work for you. The java.util.Date class encapsulates a point in time. The java.util.GregorianCalendar class, which descends from the abstract java.util.Calendar, translates between a point in time and calendar fields like month, day, and year. Finally, the java.text.DateFormat class knows how to generate and parse string representations of dates and times.^[33]

The separation of the Date class and the GregorianCalendar class is analogous to having a class representing temperature and a class that translates that temperature to Celsius units. Conceivably, we could define other subclasses of Calendar, say JulianCalendar or LunarCalendar.

The default GregorianCalendar constructor creates an object that represents the current time, as determined by the system clock:

GregorianCalendar now = new GregorianCalendar( );

Other constructors accept values to specify the point in time. In the first statement in the following code, we construct an object representing August 9, 1996; the second statement specifies both a date and a time, yielding an object that represents 9:01 a.m., April 8, 1997.

GregorianCalendar daphne =  
    new GregorianCalendar(1996, Calendar.AUGUST, 9); 
GregorianCalendar sometime =  
    new GregorianCalendar(1997, Calendar.APRIL, 8, 9, 1); // 9:01 AM

We can also create a GregorianCalendar by setting specific fields using the set( ) method. The Calendar class contains a torrent of constants representing both calendar fields and field values. The first argument to the set( ) method is a field constant; the second argument is the new value for the field.

GregorianCalendar kristen = new GregorianCalendar( ); 
kristen.set(Calendar.YEAR, 1972); 
kristen.set(Calendar.MONTH, Calendar.MAY); 
kristen.set(Calendar.DATE, 20);

A GregorianCalendar is created in the default time zone. Setting the time zone of the calendar is as easy as obtaining the desired TimeZone and giving it to the GregorianCalendar:

GregorianCalendar smokey = new GregorianCalendar( ); 
smokey.setTimeZone(TimeZone.getTimeZone("MST"));

To represent a GregorianCalendar’s date as a string, first create a Date object:

Date mydate = smokey.getTime( );

To create a string representing a point in time, create a DateFormat object and apply its format( ) method to a Date object. Although DateFormat itself is abstract, it has several static (“factory”) methods that return useful DateFormat subclass instances. To get a default DateFormat, simply call getInstance( ):

DateFormat plain = DateFormat.getInstance( ); 
String now = plain.format(new Date( ));         // 4/12/00 6:06 AM

You can generate a date string or a time string, or both, using the getDateInstance(), getTimeInstance(), and getDateTimeInstance( ) factory methods. The argument to these methods describes what level of detail you’d like to see. DateFormat defines four constants representing detail levels: they are SHORT, MEDIUM, LONG, and FULL. There is also a DEFAULT, which is the same as MEDIUM. The following code creates three DateFormat instances: one to format a date, one to format a time, and one to format a date and time together. Note that getDateTimeInstance( ) requires two arguments: the first specifies how to format the date, the second how to format the time:

// 12-Apr-00
DateFormat df  = DateFormat.getDateInstance(DateFormat.DEFAULT);

// 9:18:27 AM
DateFormat tf  = DateFormat.getTimeInstance(DateFormat.DEFAULT);

// Wednesday, April 12, 2000 9:18:27 o'clock AM EDT
DateFormat dtf =
  DateFormat.getDateTimeInstance(DateFormat.FULL, DateFormat.FULL);

We’re showing only how to create the DateFormat objects here; to actually generate a String from a date, you’ll need to call the format( ) method of these objects.

Formatting dates and times for other countries is just as easy. Overloaded factory methods accept a Locale argument:

// 12 avr. 00
DateFormat df =
  DateFormat.getDateInstance(DateFormat.DEFAULT, Locale.FRANCE); 

// 9:27:49
DateFormat tf =
  DateFormat.getTimeInstance(DateFormat.DEFAULT, Locale.GERMANY);

// mercoledi 12 aprile 2000 9.27.49 GMT-04:00 
DateFormat dtf = 
    DateFormat.getDateTimeInstance(
        DateFormat.FULL, DateFormat.FULL, Locale. ITALY);

To parse a string representing a date, we use the parse( ) method of the DateFormat class. The result is a Date object. The parsing algorithms are finicky, so it’s safest to parse dates and times that are in the same format that is produced by the DateFormat. The parse( ) method throws a ParseException if it doesn’t understand the string you give it. All of the following calls to parse( ) succeed except the last; we don’t supply a time zone, but the format for the time is LONG. Other exceptions are occasionally thrown from the parse( ) method. To cover all the bases, catch NullPointerExceptions and StringIndexOutOfBoundsExceptions , also.

try { 
  Date d; 
  DateFormat df; 

  df = DateFormat.getDateTimeInstance(
           DateFormat.FULL, DateFormat.FULL);
  d = df.parse("Wednesday, April 12, 2000 2:22:22 o'clock PM EDT"); 

  df = DateFormat.getDateTimeInstance(
           DateFormat.MEDIUM, DateFormat.MEDIUM); 
  d = df.parse("12-Apr-00 2:22:22 PM"); 

  df = DateFormat.getDateTimeInstance(
           DateFormat.LONG, DateFormat.LONG); 
  d = df.parse("April 12, 2000 2:22:22 PM EDT"); 

  // throws a ParseException; detail level mismatch 
  d = df.parse("12-Apr-00 2:22:22 PM");
} 
catch (Exception e) { ... }

---

^[32] For a wealth of information about time and world time-keeping conventions, see http://tycho.usno.navy.mil, the U.S. Navy Directorate of Time. For a fascinating history of the Gregorian and Julian calendars, try http://www.magnet.ch/serendipity/hermetic/cal_stud/cal_art.htm.

^[33] In Java 1.0.2, the Date class performed all three functions. In Java 1.1 and later, most of its methods have been deprecated, so that the only purpose of the Date class is to represent a point in time.