java - Calendar.set() 无法正常工作

Question

我想获取上个月的结束日期，我正在获取当前月份并将其减一并设置该月份值。但是 Calender.set(Calender.Month, lastMonth) 不仅仅适用于特定日期。

public static String getLastMonthEndDate(Date nowDate, String dateFormat) 
{
    final String METHOD_NAME = "getLastMonthEndDate" ;


    SimpleDateFormat formatedDate = new SimpleDateFormat(dateFormat);


    Calendar calendar = new GregorianCalendar(2014, Calendar.OCTOBER, 31); // Hard coded as 2014, oct 31st for the sake of example.
    //calendar.setTime(nowDate);
    System.out.println(calendar.getTime());
    if(calendar.get(Calendar.MONTH) == Calendar.JANUARY)
    {
        calendar.set(Calendar.YEAR, calendar.get(Calendar.YEAR) - 1);
    }

    //System.out.println("calendar.get(Calendar.MONTH) -1 : " + (calendar.get(Calendar.MONTH) -1));
    int lastMonth = (calendar.get(Calendar.MONTH) -1);
    System.out.println("Last month : "+lastMonth);
    calendar.set(Calendar.MONTH, lastMonth);
    //System.out.println(calendar.getTime());
    //System.out.println("(calendar.get(Calendar.MONTH) : " + calendar.get(Calendar.MONTH));
    System.out.println("calendar.getActualMaximum(Calendar.DAY_OF_MONTH) : " + calendar.getActualMaximum(Calendar.DAY_OF_MONTH));

    calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMaximum(Calendar.DAY_OF_MONTH));
    System.out.println("calendar.getTime() : " + calendar.getTime());

    return formatedDate.format(calendar.getTime());
}



public static void main(String[] args)
{

     Date dCurrentDate = getCurrentDate();
     String strLastMonthEndDateInYYYY = getLastMonthEndDate(dCurrentDate, "MM/dd/yyyy");
     System.out.println("strLastMonthEndDateInYYYY : "+strLastMonthEndDateInYYYY);

}

下面是我得到的输出

output : 


Fri Oct 31 00:00:00 IST 2014

Last month : 8

calendar.getActualMaximum(Calendar.DAY_OF_MONTH) : 31

calendar.getTime() : Wed Oct 01 00:00:00 IST 2014

strLastMonthEndDateInYYYY : 10/01/2014

不知道错在哪里。请帮忙。

Answer 1

它不起作用，因为您在日历上操作后将日期设置为“09/31/2014”，并且 2014 年 9 月没有第 31 天。默认情况下，java Calendar是 lenient ，并将该日期解释为“10/01/2014”(2014 年 10 月 1 日)。

这是relevant part of the Javadoc :

When a Calendar is in lenient mode, it accepts a wider range of calendar field values than it produces. When a Calendar recomputes calendar field values for return by get(), all of the calendar fields are normalized. For example, a lenient GregorianCalendar interprets MONTH == JANUARY, DAY_OF_MONTH == 32 as February 1.

这是代码相关部分的注释版本，用于解释发生的情况:

Calendar calendar = new GregorianCalendar(2014, Calendar.OCTOBER, 31); // 10/31/2014
// [...]
int lastMonth = (calendar.get(Calendar.MONTH) -1); // lastMonth will be Calendar.SEPTEMBER
calendar.set(Calendar.MONTH, lastMonth); // internal value of the calendar is now 09/31/2014
// [..]
System.out.println("calendar.getActualMaximum(Calendar.DAY_OF_MONTH) : " + calendar.getActualMaximum(Calendar.DAY_OF_MONTH)); // returns 31 because this is a get() method, so 09/31/2014 will be converted to 10/01/2014, and there are 31 days in October (but the internal representation of the calendar will still be 09/31/2014)
// [...]
calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMaximum(Calendar.DAY_OF_MONTH)); // internal representation is still 09/31/2014, so this will not change the value
// [...]
return formatedDate.format(calendar.getTime()); // this is a get() method, so the value will be converted to 10/01/2014

更好的解决方案是将您的日历设置为该月的第一天，然后减去一天:

Calendar calendar = new GregorianCalendar(2014, 9, 31);
calendar.set(Calendar.DAY_OF_MONTH, 1);
calendar.add(Calendar.DAY_OF_MONTH, -1);
System.out.println((new SimpleDateFormat("MM/dd/yyyy")).format(calendar.getTime())); // 09/30/2014