Learning Java, 4th Edition

Dates and Times

Working with dates and times without the proper tools can be a chore. Fortunately, Java has three classes that handle most of the work for you. The java.util.Date class encapsulates a point in time. The java.util.GregorianCalendar class, which extends 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 many languages.^[32]

The separation of the Date and Calendar classes is analogous to having a class representing temperature and a class that translates that temperature to Celsius units. A Date represents an absolute point in time as defined by a number of milliseconds from the reference point: midnight, Jan 1, 1970, GMT. This is the same frame of reference used by the System.currentTimeMillis() call. A Calendar encapsulates a point in time and maps it to higher-level (and messier) notions like years, months, weeks, and days, and deals with discontinuities like leap years. Conceivably, we could define subclasses of Calendar other than the default GregorianCalendar, say JulianCalendar or LunarCalendar, that map time using other sociological or cultural conventions.^[33]

Working with Calendars

The default GregorianCalendar constructor creates a calendar initialized to the current time, in the current time zone:

    GregorianCalendar now = new GregorianCalendar();

However, more generally we can just ...