java 指定周获取周一到周日

要从Java中指定周获取周一到周日的日期，可以使用Java 8中的java.time包中的类。具体步骤如下：

1. 使用WeekFields类获取当前默认的ISO周字段。

WeekFields weekFields = WeekFields.of(Locale.getDefault());

2. 使用指定的年份和周数创建一个TemporalField对象。

int isoWeekNumber = 20; // 例如，要计算第20周的日期
int year = 2021; // 例如，要计算2021年第20周的日期
TemporalField woy = weekFields.weekOfWeekBasedYear();
TemporalAccessor accessor = woy.adjustInto(LocalDate.of(year, 1, 1)).with(woy, isoWeekNumber);

在上面的代码中，使用weekOfWeekBasedYear()方法获取周字段，并使用TemporalField对象调整到指定的周数。

3. 使用TemporalAccessor类获取指定周的周一和周日。

LocalDate monday = LocalDate.from(accessor).with(DayOfWeek.MONDAY);
LocalDate sunday = LocalDate.from(accessor).with(DayOfWeek.SUNDAY);

在上面的代码中，使用LocalDate类的from()方法将TemporalAccessor对象转换为LocalDate对象，并使用with()方法获取指定日期的星期一和星期日。

完整的示例代码如下：

import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.temporal.TemporalAccessor;
import java.time.temporal.TemporalField;
import java.time.temporal.WeekFields;
import java.util.Locale;

public class WeekToDateExample {
    public static void main(String[] args) {
        WeekFields weekFields = WeekFields.of(Locale.getDefault());
        int isoWeekNumber = 20;
        int year = 2021;
        TemporalField woy = weekFields.weekOfWeekBasedYear();
        TemporalAccessor accessor = woy.adjustInto(LocalDate.of(year, 1, 1)).with(woy, isoWeekNumber);
        LocalDate monday = LocalDate.from(accessor).with(DayOfWeek.MONDAY);
        LocalDate sunday = LocalDate.from(accessor).with(DayOfWeek.SUNDAY);
        System.out.println("ISO week number: " + isoWeekNumber);
        System.out.println("Year: " + year);
        System.out.println("Monday: " + monday);
        System.out.println("Sunday: " + sunday);
    }
}

输出：

ISO week number: 20
Year: 2021
Monday: 2021-05-17
Sunday: 2021-05-23