Getting the Date from a Week Number in Java

When I started to publish week notes to my site through my Micropub server, I wanted to programmatically set up the metadata for a post, which looks like:

title: "Week Notes 21#49"
description: "What happened in the week of 2021-12-06?"

In this case, we need the short year name, the week number, and the ISO8601 date for the Monday of that week.

Looking around for a way to do this in Java, I found that we can use IsoFields in conjunction with LocalDate to create the date, for instance:

int year = 2020;
int week = 53;
LocalDate desiredDate =
  LocalDate.now()
  .withYear(year)
  // this doesn't work
  .with(IsoFields.WEEK_OF_WEEK_BASED_YEAR, week):

System.out.println(desiredDate);

Fortunately this StackOverflow answer highlights that we can use TemporalAdjusters.previousOrSame to pick up the Monday of the week, like so:

int year = 2020;
int week = 53;
LocalDate desiredDate =
  LocalDate.now()
  .withYear(year)
  .with(IsoFields.WEEK_OF_WEEK_BASED_YEAR, week)
  .with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY));

System.out.println(desiredDate);