val date = "2015-40"
val formatter = DateTimeFormatter.ofPattern("yyyy-ww") 
LocalDate.parse(date, formatter)

import java.time.LocalDate
import java.time.format.DateTimeFormatterBuilder
import java.time.temporal.{ChronoField, DayOfWeek}
import java.util.Locale
object Main extends App {
  val date = "2015-40"
  val formatter = new DateTimeFormatterBuilder()
    .appendPattern("YYYY-ww")
    .parseDefaulting(ChronoField.DAY_OF_WEEK, DayOfWeek.MONDAY.getValue())
    .toFormatter(Locale.FRANCE)
  val ld = LocalDate.parse(date, formatter)
  
  println(ld)
}

2015-09-28

<details>
<summary>英文:</summary>
		String date = &quot;2015-40&quot;;
		DateTimeFormatter formatter = new DateTimeFormatterBuilder()
				.appendPattern(&quot;YYYY-ww&quot;)
				.parseDefaulting(ChronoField.DAY_OF_WEEK, DayOfWeek.MONDAY.getValue())
				.toFormatter(Locale.FRANCE);
		LocalDate ld = LocalDate.parse(date, formatter);
		
		System.out.println(ld);
Sorry that I can write only Java, I trust you to translate to Scala. Output is:
&gt; 2015-09-28
Why were you getting an exception? There were a couple of things missing for us to parse `2015-40` into a `LocalDate`:
 - A `LocalDate` is a calendar date, and week 40 consists of 7 days. Java doesn’t know which of those 7 days you want and refuses to make the choice for you. In my code above I have specified Monday. Any other day of the week should work.
 - A bit subtler. While to a human year 2015 and week 40 is unambiguous, the same is not always the case around New Year where week 1 may begin before New Year or week 52 or 53 extend after New Year. So a calendar year and a week number don’t always define one specific week. Instead we need the concept of a *week year* or *week-based year*. A week year lasts from week 1 inclusive no matter if that means it begins a few days before or after New Year. And it lasts until the last day of the last week, again most often a few days before or after New Year. To tell a `DateTimeFormatter` that we want to parse (or print) the week year we need to use upper case `YYYY` instead of lower case `yyyy` (or `uuuu`).
As an aside, if you can influence the format, consider `2015-W40` with a `W`. This is ISO 8601 format for year and week. In ISO `2015-12` denotes year and month, and many will read it this way. So to disambiguate and avoid misreading.
Edit:
In my explanation I have assumed ISO week scheme (Monday is the first day of week and week 1 is defined as the first week having at least 4 days in the new year). You may pass a different locale to the formatter builder to obtain a different week scheme.
If you are sure that your weeks follow ISO, Andreas’ suggestion in the comment is good enough that we want as part of the answer:
&gt; Alternatively, add the [ThreeTen Extra](https://www.threeten.org/threeten-extra/) library, so you can use the
&gt; [`YearWeek`](https://www.threeten.org/threeten-extra/apidocs/org.threeten.extra/org/threeten/extra/YearWeek.html)
&gt; class, that has a nice [`atDay​(DayOfWeek
&gt; dayOfWeek)`](https://www.threeten.org/threeten-extra/apidocs/org.threeten.extra/org/threeten/extra/YearWeek.html#atDay(java.time.DayOfWeek))
&gt; method for getting a `LocalDate`.
**Link:** [Wikipedia article: ISO 8601](https://en.wikipedia.org/wiki/ISO_8601)
</details>
# 答案2
**得分**: 0
`LocalDate`有三个部分：年、月和月份中的日期。因此，在`year-month`字符串的情况下，根据您的要求，您将需要获取特定日期的`LocalDate`。解析年月字符串就像演示代码中所示的那样直接。
在`year-week`字符串的情况下，您将需要获取特定星期几的`LocalDate`，例如星期一或今天的日期等。此外，与其直接解析字符串，我发现更容易获取年份和周数，然后使用`LocalDate`的方法来获取所需的`LocalDate`实例。
```java
import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.YearMonth;
import java.time.format.DateTimeFormatter;
import java.time.temporal.TemporalAdjusters;
import java.time.temporal.WeekFields;
public class Main {
    public static void main(String[] args) {
        //#################### Year-Month #######################
        // 给定 year-month 字符串
        var yearMonthStr = "2015-08";
        // 从 yearMonthStr 解析出的 LocalDate，位于该月的第一天
        LocalDate date2 = YearMonth.parse(yearMonthStr, DateTimeFormatter.ofPattern("u-M")).atDay(1);
        System.out.println(date2);
        // 从 yearMonthStr 解析出的 LocalDate，位于该月的最后一天
        date2 = YearMonth.parse(yearMonthStr, DateTimeFormatter.ofPattern("u-M")).atEndOfMonth();
        System.out.println(date2);
        // 从 yearMonthStr 解析出的 LocalDate，位于该月的特定日期
        date2 = YearMonth.parse(yearMonthStr, DateTimeFormatter.ofPattern("u-M")).atDay(1).withDayOfMonth(10);
        System.out.println(date2);
        //#################### Year-Week #######################
        // 给定 year-week 字符串
        var yearWeekStr = "2015-40";
        // 在 '-' 上拆分字符串，并获取年份和周数值
        String[] parts = yearWeekStr.split("-");
        int year = Integer.parseInt(parts[0]);
        int week = Integer.parseInt(parts[1]);
        // 拥有年份、周数和今天的日期，例如星期五的 LocalDate
        LocalDate date1 = LocalDate.now()
                            .withYear(year)
                            .with(WeekFields.ISO.weekOfYear(), week);
        System.out.println(date1);
        // 拥有年份、周数和下一个星期一（如果今天是星期一，则为同一天）
        date1 = LocalDate.now()
                .withYear(year)
                .with(WeekFields.ISO.weekOfYear(), week)
                .with(TemporalAdjusters.nextOrSame(DayOfWeek.MONDAY));
        System.out.println(date1);
        // 拥有年份、周数和今天之前的星期一（如果今天是星期一，则为同一天）
        date1 = LocalDate.now()
                .withYear(year)
                .with(WeekFields.ISO.weekOfYear(), week)
                .with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY));
        System.out.println(date1);
    }
}

2015-08-01
2015-08-31
2015-08-10
2015-10-02
2015-10-05
2015-09-28