186. Streaming custom code to map

Let’s assume that we have the following legacy class:

public class Post {
    
  private final int id;
  private final String title;
  private final String tags;
  public Post(int id, String title, String tags) {
    this.id = id;
    this.title = title;
    this.tags = tags;
  }
  ...
  public static List<String> allTags(Post post) {
        
    return Arrays.asList(post.getTags().split("#"));
  }
}

So we have a class that shapes some blog posts. Each post has several properties, including its tags. The tags of each post are actually represented as a string of tags separated by hashtag (#). Whenever we need the list of tags for a given post, we can call the allTags() helper. For instance, here is a list of posts and their tags:

List<Post> posts = List.of(
  new Post(1, "Running jOOQ", "#database #sql #rdbms"),
  new Post(2, "I/O files in Java", "#io #storage #rdbms&quot...