Simple REST web service

Question

I am a young computer science student programming mainly in Java.

I wrote a simple RESTful web service using Spring Boot Framework and the DDD architecture pattern. The project is a library management system that allows users to browse, book and borrow books.

I was wondering if I could get feedback from more experienced programmers about my code in general.

Here is the entire project repository: Project repository

@NamedEntityGraph(
        name = "book-entity-graph",
        attributeNodes = {
                @NamedAttributeNode(value = "author"),
                @NamedAttributeNode(value = "genre"),
                @NamedAttributeNode(value = "publisher")
        }
)
@AggregateRoot
@Entity
public class Book extends BaseEntity {

    @Column(nullable = false)
    private String title;

    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    private Author author;

    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    private Genre genre;

    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    private Publisher publisher;

    @Column(nullable = false)
    private String description;

    @Column(nullable = false)
    private LocalDate publicationDate;

    private boolean loaned;

    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "book")
    @OrderColumn(name = "index")
    private List<Reservation> reservations;

    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "book")
    private List<Loan> loans;

    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "book")
    private List<Rating> ratings;

    private double averageRating = 0.; // Additional field to increase performance

    protected Book() {
        reservations = new ArrayList<>();
        loans = new ArrayList<>();
        ratings = new ArrayList<>();
    }

    public Book(String title, Author author, Genre genre, Publisher publisher, String description,
                LocalDate publicationDate) {
        this();
        setTitle(title);
        setAuthor(author);
        setGenre(genre);
        setPublisher(publisher);
        setDescription(description);
        setPublicationDate(publicationDate);
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = Objects.requireNonNull(title);
    }

    public Author getAuthor() {
        return author;
    }

    private void setAuthor(Author author) {
        this.author = Objects.requireNonNull(author);

        if (!author.getBooks().contains(this)) {
            author.addBook(this);
        }
    }

    public void changeAuthor(Author author) {
        Objects.requireNonNull(author);
        this.author.removeBook(this);
        setAuthor(author);
    }

    public Genre getGenre() {
        return genre;
    }

    private void setGenre(Genre genre) {
        this.genre = Objects.requireNonNull(genre);

        if (!genre.getBooks().contains(this)) {
            genre.addBook(this);
        }
    }

    public void changeGenre(Genre genre) {
        Objects.requireNonNull(genre);
        this.genre.removeBook(this);
        setGenre(genre);
    }

    public Publisher getPublisher() {
        return publisher;
    }

    private void setPublisher(Publisher publisher) {
        this.publisher = Objects.requireNonNull(publisher);

        if (!publisher.getBooks().contains(this)) {
            publisher.addBook(this);
        }
    }

    public void changePublisher(Publisher publisher) {
        Objects.requireNonNull(publisher);
        this.publisher.removeBook(this);
        setPublisher(publisher);
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = Objects.requireNonNull(description);
    }

    public LocalDate getPublicationDate() {
        return publicationDate;
    }

    public void setPublicationDate(LocalDate publicationDate) {
        this.publicationDate = Objects.requireNonNull(publicationDate);
    }

    public boolean isLoaned() {
        return loaned;
    }

    public void setLoaned(boolean loaned) {
        this.loaned = loaned;
    }

    public List<Reservation> getReservations() {
        return Collections.unmodifiableList(reservations);
    }

    public List<Reservation> getReservations(Reservation.Status status) {
        return reservations.stream()
                .filter(r -> r.getStatus() == status)
                .collect(Collectors.toUnmodifiableList());
    }

    public void addReservation(Reservation reservation) {
        Objects.requireNonNull(reservation);

        if (!reservation.getBook().equals(this)) {
            throw new IllegalStateException("The reservation has a different book");
        }
        reservations.add(reservation);
    }

    public List<Loan> getLoans() {
        return Collections.unmodifiableList(loans);
    }

    public void addLoan(Loan loan) {
        Objects.requireNonNull(loan);

        if (!loan.getBook().equals(this)) {
            throw new IllegalArgumentException("The loan has a different book");
        }
        loans.add(loan);
    }

    public List<Rating> getRatings() {
        return Collections.unmodifiableList(ratings);
    }

    public void addRating(Rating rating) {
        Objects.requireNonNull(rating);

        if (!rating.getBook().equals(this)) {
            throw new IllegalArgumentException("The rating has a different book");
        }
        averageRating = (averageRating * ratings.size() + rating.getValue()) / (ratings.size() + 1);
        ratings.add(rating);
    }

    public void removeRating(Rating rating) {
        if (!ratings.contains(rating)) {
            throw new IllegalArgumentException("The book doesn't contain this rating");
        }
        if (ratings.size() > 1) {
            averageRating = (averageRating * ratings.size() - rating.getValue()) / (ratings.size() - 1);
        } else {
            averageRating = 0.;
        }
        ratings.remove(rating);
    }

    public double getAverageRating() {
        return averageRating;
    }
}

Answer 1

I checked your code from project repository and one feedback from my end -

Use proper HTTP response codes as mentioned here.

You have used 200 for POST which can be 201 (CREATED)

and 201 for DELETE requests which can be 200 (OK) or 202 (Not Accepted) or 204 (No Content)