如何从 Java 中的日历 object 构建天、月、年的列表？

Question

I want to build a date widget for a form, which has a select list of months, days, years. since the list is different based on the month and year, i cant hard code it to 31 days. (eg february has 28 days not 30 or 31 and some years even 29 days) How can I use the calendar or joda object to build me these lists.

Answer 1

I strongly recommend that you avoid the built-in date and time APIs in Java.

Instead, use Joda Time . This library is similar to the one which will (hopefully,) make it into Java 7. and is much more pleasant to use than the built-in API.

Now, is the basic problem that you want to know the number of days in a particular month?

EDIT: Here's the code (with a sample):

import org.joda.time.*;
import org.joda.time.chrono.*;

public class Test   
{
    public static void main(String[] args)
    {        
        System.out.println(getDaysInMonth(2009, 2));
    }

    public static int getDaysInMonth(int year, int month)
    {
        // If you want to use a different calendar system (e.g. Coptic)
        // this is the code to change.
        Chronology chrono = ISOChronology.getInstance();
        DateTimeField dayField = chrono.dayOfMonth();        
        LocalDate monthDate = new LocalDate(year, month, 1);
        return dayField.getMaximumValue(monthDate);
    }
}

Answer 2

Here is an example of creating a list of months:

String[] months = new DateFormatSymbols().getMonths();
    List<String> allMonths = new ArrayList<String>();
    for(String singleMonth: months){
        allMonths.add(singleMonth);
    }
    System.out.println(allMonths);

Answer 3

tl;dr

int lengthOfMonth = 
    YearMonth.from( 
                      LocalDate.now( ZoneId.of( "America/Montreal" ) ) 
                  )
             .lengthOfMonth() ;

java.time

The Answer by Jon Skeet is correct but now outdated. The Joda-Time project is now in maintenance mode, with the team advising migration to the java.time classes.

LocalDate

The code in java.time is similar to that of Joda-Time , with a LocalDate class. The LocalDate class represents a date-only value without time-of-day and without time zone.

A time zone is crucial in determining a date. For any given moment, the date varies around the globe by zone. For example, a few minutes after midnight in Paris France is a new day while still “yesterday” in Montréal Québec .

ZoneId z = ZoneId.of( “America/Montreal” );
LocalDate today = LocalDate.now( z );

You may interrogate for each part, year number, month, etc. Note that months are number sanely, 1-12 for January-December (unlike in the legacy classes).

int year = today.getYear();
int monthNumber = today.getMonthValue(); // 1-12 for January-December.
int dayOfMonth = today.getDayOfMonth();

You can assemble a LocalDate object from those parts.

LocalDate ld = LocalDate.of( year , monthNumber , dayOfMonth );

YearMonth

To ask for the length of the month , use the YearMonth class.

YearMonth ym = YearMonth.from( ld );
int lengthOfMonth = ym.lengthOfMonth();

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old date-time classes such as java.util.Date , .Calendar , & java.text.SimpleDateFormat .

The Joda-Time project, now in maintenance mode , advises migration to java.time.

To learn more, see the Oracle Tutorial . And search Stack Overflow for many examples and explanations.

Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport and further adapted to Android in ThreeTenABP (see How to use… ).

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval , YearWeek , YearQuarter , and more .

Answer 4

The Calendar object will tell you the number of days in the current month using getActualMaximum(Calendar.DAY_OF_MONTH) . See an example here .

From that you can update your lists on each change.

Answer 5

There are a number of date picker implementations out there for java, which can be used via swing or a web ui. I would attempt to reuse one of these and avoid writing your own.