如何获取当前周和月的第一天？

Question

我有几个事件的日期以 毫秒[1] 表示，我想知道当前周和当前月份内有哪些事件，但我不知道如何获取第一天（日/月/年) 并将其转换为毫秒，与该月的第一天相同。

 [1]Since January 1, 1970, 00:00:00 GMT

原文由 cesarlinux 发布，翻译遵循 CC BY-SA 4.0 许可协议

Answer 1

本周以毫秒为单位：

 // get today and clear time of day
Calendar cal = Calendar.getInstance();
cal.set(Calendar.HOUR_OF_DAY, 0); // ! clear would not reset the hour of day !
cal.clear(Calendar.MINUTE);
cal.clear(Calendar.SECOND);
cal.clear(Calendar.MILLISECOND);

// get start of this week in milliseconds
cal.set(Calendar.DAY_OF_WEEK, cal.getFirstDayOfWeek());
System.out.println("Start of this week:       " + cal.getTime());
System.out.println("... in milliseconds:      " + cal.getTimeInMillis());

// start of the next week
cal.add(Calendar.WEEK_OF_YEAR, 1);
System.out.println("Start of the next week:   " + cal.getTime());
System.out.println("... in milliseconds:      " + cal.getTimeInMillis());

本月以毫秒为单位：

 // get today and clear time of day
Calendar cal = Calendar.getInstance();
cal.set(Calendar.HOUR_OF_DAY, 0); // ! clear would not reset the hour of day !
cal.clear(Calendar.MINUTE);
cal.clear(Calendar.SECOND);
cal.clear(Calendar.MILLISECOND);

// get start of the month
cal.set(Calendar.DAY_OF_MONTH, 1);
System.out.println("Start of the month:       " + cal.getTime());
System.out.println("... in milliseconds:      " + cal.getTimeInMillis());

// get start of the next month
cal.add(Calendar.MONTH, 1);
System.out.println("Start of the next month:  " + cal.getTime());
System.out.println("... in milliseconds:      " + cal.getTimeInMillis());

原文由 COME FROM 发布，翻译遵循 CC BY-SA 3.0 许可协议

Answer 2

可以借助 java.util.Calendar 确定一周的第一天，如下所示：

 Calendar calendar = Calendar.getInstance();
calendar.clear();
calendar.setTimeInMillis(timestamp);
while (calendar.get(Calendar.DAY_OF_WEEK) > calendar.getFirstDayOfWeek()) {
    calendar.add(Calendar.DATE, -1); // Substract 1 day until first day of week.
}
long firstDayOfWeekTimestamp = calendar.getTimeInMillis();

一个月的第一天可以确定如下：

 Calendar calendar = Calendar.getInstance();
calendar.clear();
calendar.setTimeInMillis(timestamp);
while (calendar.get(Calendar.DATE) > 1) {
    calendar.add(Calendar.DATE, -1); // Substract 1 day until first day of month.
}
long firstDayOfMonthTimestamp = calendar.getTimeInMillis();

非常冗长，是的。

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Java 7 将带有大大改进的日期和时间 API ( JSR-310 )。如果你还不能切换，那么你可以尽可能使用 JodaTime ，这使得它变得不那么复杂：

 DateTime dateTime = new DateTime(timestamp);
long firstDayOfWeekTimestamp = dateTime.withDayOfWeek(1).getMillis();

和

DateTime dateTime = new DateTime(timestamp);
long firstDayOfMonthTimestamp = dateTime.withDayOfMonth(1).getMillis();

原文由 BalusC 发布，翻译遵循 CC BY-SA 2.5 许可协议