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"Doug Rickard" <rickard@primus.com.au> wrote in message
news:39e107db@news.iprimus.com.au...
> variable, or a class that will return a formatted date/time string given a
> simple format definition string.

If all you want to do is convert dates which you *already know* are going to
be in 'DD-MMM-YYYY' format (eg, in a flat text file), then what you need is
a java.text.SimpleDateFormat().

  DateFormat df = new SimpleDateFormat("dd-MMM-yyyy");
  Date end_date = df.parse("12-JAN-1987");

If you have something more complicated in mind:

Java has three related classes that do date/time stuff.

java.util.Date. This class encapsulates the idea of a moment in time. A date
is like a number. The only operations you can do on a date is compare one to
another for greaterthan/lessthan/equalto. It is an "opaque" class - it's
actual contents are NOYB, however, most jvm implementations will represent
dates as a number of miliseconds since 1-Jan-1970 GMT.

java.util.Calendar. This class encapsulates the idea of dividing time up
into named intervals (days, months, eras, lunar cycles etc). A calendar is a
thing that says "that point of time is during a Tuesday in March". One
could, in theory, make up a calendar for mars or for the sidreal year
(rather than the solar) and plug it into the existing date/time
infrastructure. Try that with windows!

java.text.DateFormat. This class encapulates the idea of using a text string
to specify a Date.

Oh - and other classes of interest are java.util.Locale, which represents a
geographical, political, or cultural region; and java.util.TimeZone, which
is a time zone (duh).

The locale in which a jvm is running is a system property. Java does not use
environment variables - it uses properties. These properties should have
been set up when the jvm was installed. You get the default Locale with
    Locale.getDefault();

and the default TimeZone with
    TimeZone.getDefault();

which are a static methods. There may be more than one Locale in a timezone
(eg: nth and sth america), and more than one Timezone in a Locale (eg, the
USA). For what you want you probably don't need these classes directly, but
if you need the user to indicate where they are, you can construct a
picklists from Locale.getAvailableLocales(), and TimeZone.getAvailableIDs().
This also handy for internationalising your app with resource bundles (a
Locale also includes the concept of a language, CA_EN vs. CA_FR).

The next thing you need is a Calendar. If you use
  Calendar.getInstance(),

you'll get a calendar which is of the default type for your system locale,
initialised to the current time. In most places, the calendar you get back
will be an instance of GregorianCalendar. I suppose there is an
implementation of the Chinese calendar out there somewhere if you need it.
If you are giving your users a picklist of locales, you can use
   Calendar.getInstance(Locale)
   Calendar.getInstance(TimeZone)
   Calendar.getInstance(TimeZone, Locale)

to get a calendar for the locale they chose. Of course, you can specifically
use a GregorianCalendar
   Calendar cal = new GregorianCalendar();
   Calendar cal = new GregorianCalendar(user_locale);

if you don't want to deal with the Year of the Rat, roman Ides and Calends,
or Jovian sidreal years (the day on Jupiter is 10 earth-hours long).
Calendars allow you to do things like adding one day to a date, or finding
the first weekday of next month. Some things are specific to certain types
of calendar - eg, finding the next year of the Rat. If you are dealing with
concepts like "month" and "day of the week", then you may need to admit that
you are assuming a Gregorian calendar, and explicitly construct one. It's
nothing to be ashamed of, everyone does it.

So. You have a localised calendar. Each calendar may have several ways of
specifying dates as text. To convert text strings to non-text values, you
use a java.Text.Format. There is a subclass of this, java.Text.DateFormat,
for handling dates.

To get a default date formatter for the current locale, use
    java.text.DateFormat.getInstance();
    java.text.DateFormat.getDateInstance();
    java.text.DateFormat.getTimeInstance();
    java.text.DateFormat.getDateTimeInstance();
as applicable
*** MOST OF THE TIME, THIS IS ALL YOU NEED TO DO ***

just call
  formatter.parse(userInputString)
to convert a string to a date, or
   formatter.format(employeeTerminationDate)
to convert a date to a string

'Xcuse the shouting, but that was for people just flicking through this
post. getDateInstance should correctly return dd/mm/yyyy, mm/dd/yyyy,
dd.mm.yyyy, yyyy.mm.dd etc for wherever the user happens to be when it is
run.

You can ask for a long, medium or short date/time by calling
    DateFormat.getInstance(DateFormat.LONG)
    DateFormat.getInstance(DateFormat.MEDIUM)
    DateFormat.getInstance(DateFormat.SHORT)
    DateFormat.getInstance(DateFormat.DEFAULT)
    DateFormat.getInstance(DateFormat.FULL)

The meanings of which depend on the calendar and the locale.

And, of course, you can
    DateFormat.getDateInstance(DateFormat.FULL, Locale.FRENCH)

For a long format french date format, and I presume
    DateFormat df = new DateFormat();
    df.setCalendar(new WiccanLunarCalendar());

to plan your next sabbat.

And this is as much as I know about the topic. It took me a while to figure
it out, but it's very very neat, really. Cheers!

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