Calendar. A Calendar maps out time. With it, we divide time into days, into months and years. In Java, a Calendar is used to manage time values.
Dates, DateFormat. Other classes too are helpful. A Date instance is used to store a point in time. A DateFormat helps us convert time Strings.
A Calendar example. This example combines several time-oriented classes in Java. It uses the DateFormat class to parse a String into a Date. And it sets up a Calendar from that Date.
Detail This class provides the parse method. With parse, we can convert a String into its Date representation.
Detail This represents a point in time. The DateFormat parse method returns a Date instance.
Detail With Calendar we access values from the Date. We can set a Calendar and get values, like hours, from it.
import java.text.DateFormat;
import java.text.ParseException;
import java.util.Calendar;
import java.util.Date;
public class Program {
public static void main(String[] args) throws ParseException {
// Get DateFormat and parse a String into a Date.DateFormat format = DateFormat.getInstance();
Date date = format.parse("05/14/14 1:06 PM");
// Set date in a Calendar.Calendar cal = Calendar.getInstance();
cal.setTime(date);
// Get hours from the Calendar.
int hours = cal.get(Calendar.HOUR_OF_DAY);
if (hours == 13) {
System.out.println(true);
}
}
}true
After, before. Two Calendars can be compared. After initializing them with set(), we call after and before to see which is earlier in time.
Detail This returns negative one, 0 or one. The int indicates the relative order of the two Calendars.
import java.util.Calendar;
public class Program {
public static void main(String[] args) {
// Set date to 5/13.Calendar cal = Calendar.getInstance();
cal.set(2014, 5, 13);
// Set date to 5/14.Calendar cal2 = Calendar.getInstance();
cal2.set(2014, 5, 14);
// See if first date is after, before second date.
boolean isAfter = cal.after(cal2);
boolean isBefore = cal.before(cal2);
System.out.println(isAfter);
System.out.println(isBefore);
// Compare first to second date.
int compare = cal.compareTo(cal2);
System.out.println(compare);
}
}false
true
-1
GetInstance. The DateFormat and Calendar classes are abstract ones. We cannot instantiate them with new. But with getInstance we can use their helpful methods.
Calendar, get. We access values from a Calendar with the get method. To get a certain field, we pass a constant int like Calendar.HOUR_OF_DAY. This returns that value.
Format. We can display Calendars with format strings and the String.format method. After a lowercase "t" we specify our time-formatting characters.
Often to manipulate, test and format dates in Java, many classes must be used together. This is at first challenging. But with practice, these classes are easier to use.
Custom methods. Writing custom methods to deal with dates is a burden. Built-in code, as we find in the Date, Calendar and DateFormat classes, is already tested and correct.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.