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JAC444: Dates

JAC444: Dates. Tim McKenna Seneca@York. Dates, Calendars, and what year is this?. Java tries to take an OOD approach to “when is now?” import java.util.*; // get util package Date class – represents a point in time Calendar class – a general way to organize time

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JAC444: Dates

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  1. JAC444: Dates Tim McKenna Seneca@York

  2. Dates, Calendars, and what year is this? • Java tries to take an OOD approach to“when is now?” • import java.util.*; // get util package • Date class – represents a point in time • Calendar class – a general way to organize time • java.GregorianCalendar –a specific way to organize time.

  3. Date and Time • current date and time (in milliseconds)after January 1, 1970 00:00:00 GMT • long ms = System.currentTimeMillis( ) • milliseconds as a date & time object • java.util.Date date = new Date(); • Date is mostly deprecated because… • is a Date created at this moment here the same as one created in Australia? same ms but different date/time to us • Date assumes local time zone!

  4. GregorianCalendar • GregorianCalendar extends Calendar • captures ms timestamp and time zone • constructors can create current or other date • getTime( ) returns a Date object • get(Calendar.YEAR / MONTH / DATE / etc ) • set or add(Calendar.YEAR/etc , int ) • boolean gc1.after(gc2) or gc1.before(gc2) • Example: DateTime.java

  5. GregorianCalendar caution • always follow one or more .set(Calendar.XXX) method calls with any .get(Calendar.XXX) call. • GregorianCalendar today = new GregorianCalendar(); // current datetoday.set(Calendar.HOUR_OF_DAY, 0);today.set(Calendar.MINUTE, 0);today.set(Calendar.SECOND, 0);today.get(Calendar.DATE); // update fields

  6. GregorianCalendar alternative • see Joda-Time at http://joda-time.sourceforge.net/index.html • Stephen Colebourne is the Lead Technical Architect … see his presentation in a PDF • Joda-Time may be needed in applications that do a lot of date and time processing

  7. GregorianCalendar utilities • Databrew in the bobjects package offers: • daysInterval()number of days difference between two local dates ignoring time of day. • timeInterval() – "days hh:mm:ss" difference • calendarInterval() – "yy-mm-dd hh:mm:ss" difference

  8. Formatting Date and Time • java.text.SimpleDateFormat - constructors - date formatting symbols - format( ) • only works if set() was followed by get() • Example: FormatDateTime.java

  9. Preset Formatters • java.text.DateFormat - the parent class of SimpleDateFormat • DateFormat constants: SHORT, MEDIUM, LONG, FULL • DateFormat static methods : getDateInstance( ), getTimeInstance( ), getDateTimeInstance( ) retrieve normal format for that system • Example: DefaultFormats.java

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