当试图在 java 中获取下个月的最后一个日期时

Question

Trying to run this code it was working perfectly fine till OCT but in NOV it is like firstdate 2019-12-01 & lastdate 2020-12-31

public class Test1 {
    public static void main(String[] args) {
        Calendar calendar = Calendar.getInstance();         
        calendar.add(Calendar.MONTH, 1);
        String date;    
        calendar.set(Calendar.DATE,calendar.getActualMinimum(Calendar.DAY_OF_MONTH));
        Date nextMonthFirstDay = calendar.getTime();
        date=new SimpleDateFormat("YYYY-MM-dd").format(nextMonthFirstDay).toLowerCase();
        System.out.println("firstdate "+ date);

        calendar.set(Calendar.DAY_OF_MONTH,calendar.getActualMaximum(Calendar.DAY_OF_MONTH));
        Date nextMonthLastDay = calendar.getTime();
        date=new SimpleDateFormat("YYYY-MM-dd").format(nextMonthLastDay).toLowerCase();
        System.out.println("lastdate  "+date);

    }
}

I don't know why it is showing like this.. Is it a fault or bug in java?

Answer 1

Change your date format to yyyy-MM-dd (notice lowercase for year)

They both represent a year but yyyy represents the calendar year while YYYY represents the year of the week.

So something like...

date=new SimpleDateFormat("yyyy-MM-dd").format(nextMonthLastDay).toLowerCase();

Hope it helps!

Answer 2

You seem to already have got an answer that works, however, here is one that uses the modern datetime API java.time and is s little more readable than the way you are calculating the first and last day of the next month based on today:

public static void main(String[] args) {
    // base is today
    LocalDate today = LocalDate.now();
    /*
     * create a LocalDate from 
     * - the year of next month (may be different)
     * - the current month plus 1 and 
     * - the first day
     * ——> first day of next month
     */
    LocalDate firstDayOfNextMonth = LocalDate.of(
            today.plusMonths(1).getYear(),
            today.getMonth().plus(1),
            1);
    /*
     * create a LocalDate from 
     * - the first day of next month (just created above)
     * - add a month and
     * - subtract one day
     * ——> last day of next month
     */
    LocalDate lastDayOfNextMonth = firstDayOfNextMonth.plusMonths(1).minusDays(1);

    // print the results
    System.out.println("first date of upcoming month:\t"
                        + firstDayOfNextMonth.format(DateTimeFormatter.ISO_DATE));
    System.out.println("last date of upcoming month:\t"
                        + lastDayOfNextMonth.format(DateTimeFormatter.ISO_DATE));
}

Don't be misled by the formatting, there are significantly fewer lines of code and their output is

first date of upcoming month:   2019-12-01
last date of upcoming month:    2019-12-31