java根据年份算出总计有多少周，以及对应周的起止时间

Java中可以使用`java.time.Year`和`java.time.temporal.WeekFields`类来计算一年中的周数，以及对应周的起止时间。以下是示例代码：

import java.time.*;
import java.time.temporal.WeekFields;

public class WeekCalculator {
    public static void main(String[] args) {
        int year = 2021;
        LocalDate startOfYear = Year.of(year).atDay(1);
        LocalDate endOfYear = Year.of(year).atDay(365).plusDays(Year.of(year).isLeap() ? 1 : 0); // 判断闰年
        WeekFields weekFields = WeekFields.ISO;
        int numOfWeeks = endOfYear.get(weekFields.weekOfWeekBasedYear()) - startOfYear.get(weekFields.weekOfWeekBasedYear()) + 1;
        System.out.println("Year " + year + " has " + numOfWeeks + " weeks.");
        for (int i = 1; i <= numOfWeeks; i++) {
            LocalDate startOfWeek = startOfYear.with(weekFields.weekOfWeekBasedYear(), i).with(weekFields.dayOfWeek(), 1);
            LocalDate endOfWeek = startOfYear.with(weekFields.weekOfWeekBasedYear(), i).with(weekFields.dayOfWeek(), 7);
            System.out.println("Week " + i + " starts on " + startOfWeek + " and ends on " + endOfWeek);
        }
    }
}

在以上代码中，我们首先获取指定年份的第一天和最后一天。然后使用`WeekFields.ISO`获取一周的定义（即周的起始日为周一）。接着，我们计算出该年的周数，即最后一周减去第一周再加1。最后，我们循环计算每周的起止时间，以便输出。