record Book (String author, String title) {

String getTitle() {
return this.title;
    }

String getAuthor() {
return this.author;
    }

};

public class ToMapEx {
public static void main(String[] args) {
List<Book> books = Arrays.asList(
new Book("Stephen King", "The Shining"),
new Book("Stephen King", "It"),
new Book("J.K. Rowling", "Harry Potter")
        );

Map<String, String> authorMap = books.stream()
                .collect(Collectors.toMap(
Book::getAuthor,                    // Key Mapper
Book::getTitle,                     // Value Mapper
(v1, v2) -> v1 + " & " + v2         // Merge Function
));

System.out.println(authorMap);
// {J.K. Rowling=Harry Potter, Stephen King=The Shining & It}
}
}

{J.K. Rowling=Harry Potter, Stephen King=The Shining & It}