This is my main class:
public class Main {

public static void main(String[] args) throws BookNotFoundException {
    // TODO Auto-generated method stub
    //Library library = new Library();
    LibraryHashMap libhmap = new LibraryHashMap();

     // Adding books
    libhmap.addBook(new Book(1, "The Great Gatsby", "F. Scott Fitzgerald"));
    libhmap.addBook(new Book(2, "1984", "George Orwell"));
    libhmap.addBook(new Book(3, "To Kill a Mockingbird", "Harper Lee"));
    libhmap.addBook(new Book(4, "Oliver Twist", "Charles Dickens"));
    libhmap.addBook(new Book(5, "Harry Potter Series", "J.K. Rowling"));
    libhmap.addBook(new Book(6, "Death on the nile", "Agatha Christy"));
    libhmap.addBook(new Book(7, "Murder on the Orient Express", "Agatha Christy"));
    libhmap.addBook(new Book(8, "A Christmas Carol", "Charles Dickens"));

    // Display all books
    System.out.println("=== Display All Books ===");
    libhmap.displayBooks();

}

Library class:
public class LibraryHashMap {

 private Map<Integer, Book> books; // Collection to store books

    // Constructor
    public LibraryHashMap() {
        this.books = new HashMap<>();
    }

    // Add a book to the library
    public void addBook(Book book) {
        books.put(book.getID(),book);
        System.out.println("Book added: " + book.getTitle());
    }

    // Display all books in the library
    public void displayBooks() {
        if (books.isEmpty()) {
            System.out.println("The library is empty.");
        } else {
            System.out.println("Books in the library:");
            for (Map.Entry<Integer, Book> entry : books.entrySet()) {
                System.out.println("Book ID: "+entry.getKey()+" Book name: "+entry.getValue());
            }
        }
    }

}

Book Class:
public class Book {

private int id;
private String title;
private String author;

public Book(int id, String title, String author) {
    this.id = id;
    this.title = title;
    this.author = author;
}

public int getID() {
    return id;
}

public String getTitle() {
    return title;
}

public String getAuthor() {
    return author;
}

public String toString() {
    return "Book ID:"+id+" Book Title:"+title+" Book Author:"+author;
}

}

Now, I want to write a function to list all unique authors in the library by introducing TreeSet or HashSet since both these collections stores unique elements only . I believe I have to use the getAuthor() function but somehow I am not how to construct it. Please let me know how to do that?