从周数中查找一周的开始日期

Question

我使用 servlet 和 JSP 构建了一个 Web 应用程序，并在我的 Servlet 中计算周数：

 private int findWeekNumber(String myDate) 
    throws ParseException{
        SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd");
        Date date;
        date = df.parse(myDate);
        Calendar cal = Calendar.getInstance();
        cal.setTime(date);
        int week = cal.get(Calendar.WEEK_OF_YEAR);
        System.out.println("week number is:" + week);

        return week;
    }

然后我将week 保存到我的数据库中。然后我从我的数据库中检索week。如何从week 获取一周的开始 - 结束日期（周开始是星期一）？假设weekNumber 是 30，我想计算 20/07/2015 - 26/07/2015。同样的功能是here

Answer 1

从周数开始：

Calendar cal = Calendar.getInstance();
cal.setTime(date);
int week = cal.get(Calendar.WEEK_OF_YEAR);
int year = cal.get(Calendar.YEAR);

Calendar calendar = Calendar.getInstance();
calendar.clear();
calendar.set(Calendar.YEAR, year);


calendar.set(Calendar.WEEK_OF_YEAR, week);
calendar.set(Calendar.DAY_OF_WEEK, 1);
   // Now get the first day of week.
Date sDate = calendar.getTime();
System.out.println(sDate);

calendar.set(Calendar.WEEK_OF_YEAR, (week));
calendar.set(Calendar.DAY_OF_WEEK, 7);
Date eDate = calendar.getTime();
System.out.println(eDate);

另一种解决方案

IFF - 没有周数

Date date = new Date();
Calendar c = Calendar.getInstance();
c.setTime(date);
int dayOfWeek = c.get(Calendar.DAY_OF_WEEK) - c.getFirstDayOfWeek();
c.add(Calendar.DAY_OF_MONTH, -dayOfWeek);

Date weekStart = c.getTime();
// we do not need the same day a week after, that's why use 6, not 7
c.add(Calendar.DAY_OF_MONTH, 6); 
Date weekEnd = c.getTime();

System.out.println(weekStart);
System.out.println(weekEnd);

Answer 2

您可以为此使用Calendar，

 Calendar cal = Calendar.getInstance();
 cal.set(Calendar.WEEK_OF_YEAR, week);
 Date yourDate = cal.getTime();

 cal.setTime(yourDate);//Set specific Date of which start and end you want

 Date start,end;

 cal.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
 start = cal.getTime();//Date of Monday of current week

 cal.add(Calendar.DATE, 6);//Add 6 days to get Sunday of next week
 cal.set(Calendar.DAY_OF_WEEK, Calendar.SUNDAY);
 end = cal.getTime();//Date of Sunday of current week
 System.out.println(start +" - "+ end);

Answer 3

Java 8 版本

LocalDate week = LocalDate.now().with(ChronoField.ALIGNED_WEEK_OF_YEAR, yourWeekNumber);

LocalDate start = week.with(DayOfWeek.MONDAY);
LocalDate end = start.plusDays(6);
System.out.println(start +" - "+ end);

Answer 4

java.time

java.util 日期时间 API 及其格式化 API SimpleDateFormat 已过时且容易出错。建议完全停止使用，改用modern Date-Time API^*。

另外，下面引用的是来自home page of Joda-Time的通知：

请注意，从 Java SE 8 开始，用户被要求迁移到 java.time (JSR-310) - JDK 的核心部分，它取代了这个项目。

使用现代日期时间 API java.time 的解决方案：

import java.time.LocalDate;
import java.time.Year;
import java.time.temporal.WeekFields;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        // Test
        int weekNumber = 34;
        System.out.printf("%s - %s%n", getFirstDayOfWeek(weekNumber, Locale.UK),
                getLastDayOfWeek(weekNumber, Locale.UK));
        System.out.printf("%s - %s%n", getFirstDayOfWeek(weekNumber, Locale.US),
                getLastDayOfWeek(weekNumber, Locale.US));
    }

    static LocalDate getFirstDayOfWeek(int weekNumber, Locale locale) {
        return LocalDate
                .of(Year.now().getValue(), 2, 1)
                .with(WeekFields.of(locale).getFirstDayOfWeek())
                .with(WeekFields.of(locale).weekOfWeekBasedYear(), weekNumber);
    }

    static LocalDate getLastDayOfWeek(int weekNumber, Locale locale) {
        return getFirstDayOfWeek(weekNumber, locale).plusDays(6);
    }
}

输出：

2021-08-23 - 2021-08-29
2021-08-15 - 2021-08-21

ONLINE DEMO

请注意，一周的第一天是 Locale-dependent 例如英国是星期一，美国是星期日。根据ISO 8601 标准，现在是星期一。为了比较，请检查US calendar 和UK calendar。

从 Trail: Date Time 了解有关现代日期时间 API 的更多信息。

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^{* 出于任何原因，如果您必须坚持使用 Java 6 或 Java 7，您可以使用 ThreeTen-Backport，它将大部分 java.time 功能向后移植到 Java 6 和 7 . 如果您正在为一个 Android 项目工作并且您的 Android API 级别仍然不符合 Java-8，请检查 Java 8+ APIs available through desugaring 和 How to use ThreeTenABP in Android Project。}