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( )