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.

Note The DateFormat class provides the parse method. With parse, we can convert a String into its Date representation.

Note 2 Date represents a point in time. The DateFormat parse method returns a Date instance.

Note 3 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.

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.