Presents your 
JAVA E-NEWSLETTER for January 6, 2003

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EXPLORE THE BENEFITS OF JAVA.UTIL.CALENDAR

The Calendar class, introduced in JDK 1.1, is a different style of date
handling class. Imagine it as a typical calendar on the wall, with lots
of dates and pages that you can turn.

The basis of the Calendar class is the concept of fields. Each time
element is a field, and these fields are represented by static fields 
in the Calendar class. Using these fields, values may be obtained or set 
using the get/set methods.

// a default Calendar, set to this instance in time
Calendar cal = Calendar.getInstance();
int year = cal.get(Calendar.YEAR);
cal.set(Calendar.MONTH, Calendar.NOVEMBER);

The Calendar class provides the ability to move between dates with the
add method and the roll method. Both methods take a field to modify and a
value to modify it by, where that value may be positive or negative. 
The only difference is that add will overflow values into higher fields. 
For example, if you take away three days from September 3, you get:

    Calendar cal = Calendar.getInstance();
    cal.add(Calendar.DATE, -3);
    // value is: Sat Aug 31 23:43:19 EDT 2002

However rolling back three days will give:

    Calendar cal = Calendar.getInstance();
    cal.roll(Calendar.DATE, -3);
    // value is: Mon Sep 30 23:43:47 EDT 2002

This is why you'll primarily use the add method.

There is one more piece of functionality hidden in the most common
subclass of the Calendar class--the isLeapYear method.

    Calendar cal = Calendar.getInstance();
    boolean leapYear = ( (GregorianCalendar)cal ).isLeapYear(2002);
    // this is false

Despite being an instance method, the isLeapYear method behaves like a
static method, requiring the year in question to be passed into the
calendar.

By taking over the date-modification functionality, the
java.util.Calendar class seems like a more complex version of Date. But 
the extra punch
it carries, to say nothing of the internationalization abilities it 
has,
makes it well worth the learning curve.

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