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Exploring Java

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9.3 Dates

Working with dates and times without the proper tools can be a chore.[2] Java 1.1 gives you 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. In Java 1.0.2, the Date class performed all three functions. In Java 1.1, most of its methods have been deprecated, so that the only purpose of the Date class is to represent a point in time.

[2] For a wealth of information about time and world time-keeping conventions, see the web site 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.

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 initialize the calendar. In the first statement below, 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();

To create a string representing a point in time, use the DateFormat class. Although DateFormat itself is abstract, it has several 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());	// 4/9/97 6:06 AM

Those of you who don't live on the West coast of North America will notice that the example above produces a result that is not quite right. DateFormat instances stubbornly insist on using Pacific Standard Time, so you have to tell them what time zone you're in:

DateFormat plain = DateFormat.getInstance();
String now = plain.format(new Date());	// 4/9/97 9:06 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 code below 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 says how to format the time:

DateFormat df = DateFormat.getDateInstance(DateFormat.DEFAULT);	// 09-Apr-97
DateFormat tf = DateFormat.getTimeInstance(DateFormat.DEFAULT);	// 9:18:27 AM
DateFormat dtf = // Wednesday, April 09, 1997 9:18:27 o'clock AM EDT
    DateFormat.getDateTimeInstance(DateFormat.FULL, DateFormat.FULL);

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

DateFormat df =     // 9 avr. 97 
    DateFormat.getDateInstance(DateFormat.DEFAULT, Locale.FRANCE);
DateFormat tf =     // 9:27:49 
    DateFormat.getTimeInstance(DateFormat.DEFAULT, Locale.GERMANY);
DateFormat dtf =    // mercoledi 9 aprile 1997 9.27.49 GMT-04:00
    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 the calls to parse() below 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 09, 1997 2:22:22 o'clock PM EST");	
  df = DateFormat.getDateTimeInstance(DateFormat.MEDIUM, DateFormat.MEDIUM);
  d = df.parse("09-Apr-97 2:22:22 PM");	
  df = DateFormat.getDateTimeInstance(DateFormat.LONG, DateFormat.LONG);
  d = df.parse("April 09, 1997 2:22:22 PM EST");	
  d = df.parse("09-Apr-97 2:22:22 PM");//ParseException; detail level mismatch
catch (Exception e) {}

9.3.1 The Year 2000 Problem

There's been a lot of talk lately about the millenium bug. This refers to the expected failure of some software in the year 2000, when programs that use two digits to represent years "roll over" and interpret "00" as 1900 instead of 2000. Java is mostly safe from this error. The Date class has no specific field for year and is thus immune to this problem. Internally, Java keeps track of time in milliseconds since some base date, which is fundamentally safe; the field used to hold the current count won't overflow for a very long time. The only time you could run into this error in Java is when you use a DateFormat to parse a date string with a two-digit year. Two-digit years are automatically prefixed with 19 for the current century. My advice is to use a four-digit year when you expect to parse a date string.

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