[Tiny] How to Group List Elements in Java

If you need to group list elements by some property of an element, Collectors.groupingBy() methods could help.

Here is an example:

record Comment(Author author, String text, CommentType commentType) { }
record Author(String name) {}
enum CommentType {
    GREETING, QUESTION, OTHER
}

Now, we have a list of comments:

List<Comment> comments = ...;

And we want to group comments by author, i.e., convert this list into Map<Author, List<Comment>>. Here is how we can do this:

public Map<Author, List<Comment>> groupByAuthor(List<Comment> comments) {
    return comments.stream().collect(Collectors.groupingBy(Comment::author));
}

And if we want to go further and group comments by author first, and then by comment type, then we can do it like this:

public Map<Author, Map<CommentType, List<Comment>>> groupByAuthorAndType(List<Comment> comments) {
    return comments.stream()
            .collect(Collectors.groupingBy(
                    Comment::author, Collectors.groupingBy(Comment::commentType)
            ));
}

There are many more use cases where the three Collectors.groupingBy() methods could help. Take a look at Javadocs for more details.

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