无法创建表示 2019 年 12 月 29 日的 Date 对象

Question

我希望这不是 JDK 错误，或者我很想兑现我所有的银行账户，然后把它扔到床垫里...... 这是代码。请注意，将日期设置为 12 月 29 日会立即将年份滚动到 2020 年。此外，toString() 输出显示 Calendar 似乎认为周数为“1”。

import java.util.*; // headers MUST be above the first class
import java.text.*;

public class JDKCalendarBug {

    public static void main(String[] args) {
        SimpleDateFormat sdf = new SimpleDateFormat("MM-dd-YYYY kk:mm");
        Calendar cal = java.util.Calendar.getInstance(TimeZone
                .getTimeZone("EST"));
        cal.clear();
        cal.set(Calendar.YEAR, 2019);
        cal.set(Calendar.MONTH, Calendar.DECEMBER);

        cal.set(Calendar.DAY_OF_MONTH, 26);
        Date t = cal.getTime();
        System.out.print("\n"+sdf.format(t));
        System.out.print("\n"+cal.toString());

        cal.set(Calendar.DAY_OF_MONTH, 27);
        t = cal.getTime();
        System.out.print("\n"+sdf.format(t));
        System.out.print("\n"+cal.toString());

        cal.set(Calendar.DAY_OF_MONTH, 28);
        t = cal.getTime();
        System.out.print("\n"+sdf.format(t));
        System.out.print("\n"+cal.toString());

        cal.set(Calendar.DAY_OF_MONTH, 29);
        t = cal.getTime();
        System.out.print("\n"+sdf.format(t));
        System.out.print("\n"+cal.toString());

    }

}

输出：

12-26-2019 24:00 java.util.GregorianCalendar[time=1577336400000,areFieldsSet=true,areAllFieldsSet=false,lenient=true,zone=sun.util.calendar.ZoneInfo[id="EST",offset=-18000000,dstSavings=0,useDaylight=false ,transitions=0,lastRule=null],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=?,YEAR=2019,MONTH=11,WEEK_OF_YEAR=?,WEEK_OF_MONTH=?,DAY_OF_MONTH=26,DAY_OF_YEAR=?,DAY_OF_WEEK=?, DAY_OF_WEEK_IN_MONTH=?,AM_PM=?,HOUR=?,HOUR_OF_DAY=?,MINUTE=?,SECOND=?,MILLISECOND=?,ZONE_OFFSET=?,DST_OFFSET=?]

2019 年 12 月 27 日 24:00 java.util.GregorianCalendar[time=1577422800000,areFieldsSet=true,areAllFieldsSet=false,lenient=true,zone=sun.util.calendar.ZoneInfo[id="EST",offset=-18000000,dstSavings=0,useDaylight=false ,transitions=0,lastRule=null],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2019,MONTH=11,WEEK_OF_YEAR=52,WEEK_OF_MONTH=4,DAY_OF_MONTH=27,DAY_OF_YEAR=361,DAY_OF_WEEK=6, DAY_OF_WEEK_IN_MONTH=4,AM_PM=0,HOUR=0,HOUR_OF_DAY=0,MINUTE=0,SECOND=0,MILLISECOND=0,ZONE_OFFSET=-18000000,DST_OFFSET=0] 2019 年 12 月 28 日 24:00 java.util.GregorianCalendar[time=1577509200000,areFieldsSet=true,areAllFieldsSet=false,lenient=true,zone=sun.util.calendar.ZoneInfo[id="EST",offset=-18000000,dstSavings=0,useDaylight=false ,transitions=0,lastRule=null],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2019,MONTH=11,WEEK_OF_YEAR=52,WEEK_OF_MONTH=4,DAY_OF_MONTH=28,DAY_OF_YEAR=362,DAY_OF_WEEK=7, DAY_OF_WEEK_IN_MONTH=4,AM_PM=0,HOUR=0,HOUR_OF_DAY=0,MINUTE=0,SECOND=0,MILLISECOND=0,ZONE_OFFSET=-18000000,DST_OFFSET=0]

12-29-2020 24:00 java.util.GregorianCalendar[time=1577595600000,areFieldsSet=true,areAllFieldsSet=false,lenient=true,zone=sun.util.calendar.ZoneInfo[id="EST",offset=-18000000,dstSavings=0,useDaylight=false ,transitions=0,lastRule=null],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2019,MONTH=11,WEEK_OF_YEAR=1,WEEK_OF_MONTH=5,DAY_OF_MONTH=29,DAY_OF_YEAR=363,DAY_OF_WEEK=1, DAY_OF_WEEK_IN_MONTH=5,AM_PM=0,HOUR=0,HOUR_OF_DAY=0,MINUTE=0,SECOND=0,MILLISECOND=0,ZONE_OFFSET=-18000000,DST_OFFSET=0]

Answer 1

问题在于SimpleDateFormat。如果您将格式字符串更改为小写ys，则日期格式正确为12-29-2019 24:00

new SimpleDateFormat("MM-dd-yyyy kk:mm")

Y 对应于“周年”，与“年”不同，并非所有日历都始终支持。根据JavaDocs：

如果指定了周年“Y”并且日历不支持任何 周年，则使用日历年 ('y')。的支持 周年可以通过调用来测试 getCalendar().isWeekDateSupported()。

在这种情况下，支持“周年”，因为底层 Calendar 实现是 GregorianCalendar。据其JavaDocs：

一周年与 WEEK_OF_YEAR 周期同步。之间的所有星期 第一周和最后一周（含）具有相同的周年值。 因此，一周年的第一天和最后一天可能有不同的 日历年值。