This is my Cart Entity class:

-----------------------------------------------------------------

@Entity

@Table(name = "carts")

public class Cart implements Serializable {

private static final long serialVersionUID = 3L;

          public Cart() {

        super();

    }

@Id

    private String username;

@ElementCollection

    @CollectionTable(name="cartsbooks", joinColumns=@JoinColumn(name="username"))

    @Column(name="quantity")

    @MapKeyJoinColumn(name="isbn", referencedColumnName="isbn")

    private Map<Book,Integer> articles;

private float bill;

          public String getUsername() {

  return username;

          }

          public void setUsername(String username) {

                    this.username = username;

          }

          public Map<Book,Integer> getArticles() {

  return articles;

          }

          public void setArticles(Map<Book,Integer> articles) {

                    this.articles = articles;

          }

          public float getBill() {

  return bill;

          }

          public void setBill(float bill) {

                    this.bill = bill;

          }

}

-----------------------------------------------------------------

As you can see the getArticles() method returns a HashMap<Book, Integer>, so the subsequent call get(book) should return an Integer value.

Thank you!

Yes, now in my application I can persist books and retrieve them. The problem is when the addArticle() is executed.

In fact since

Integer currentQuantity = cart.getArticles().get(book);

return always null then the book is added on the database but as a different record.

Thank you!

The cart variable is inside the CartBean:

-----------------------------------------------------------------------

@Stateful

@Remote(ICart.class)

public class CartBean implements ICart {

          @PersistenceContext(unitName = "JPABOOK", type=PersistenceContextType.EXTENDED)

    EntityManager entityManager;

          private Cart cart = null;

    public CartBean() {}

    @Override

    public void create(String username) {

              cart = entityManager.find(Cart.class, username);

        if (cart == null) {

                  cart = new Cart();

                  cart.setUsername(username);

                  cart.setBill(0);

                  entityManager.persist(cart);

        }

    }

    @Override

    public void addArticle(Book book, Integer quantity) {

              Integer currentQuantity = cart.getArticles().get(book);

              System.out.println(currentQuantity);

        if (currentQuantity == null) {

                  currentQuantity = 0;

        }

        currentQuantity += quantity;

        cart.getArticles().put(book, currentQuantity);

    }

-----------------------------------------------------------------------

The cart it could be empty or containg some entries.

If I invoke one time addArticle() method then the cart is not empty but cart.getArticles().get(book) always return null even if the cart contains some entries. When I got this situation i figured out that the problem was the lack of hashCode and equals. Now that the methods are implemented nothing is changed. Moreover i've tried using a hashCode() that return always 1 and equals() that return always true but currentQuantity is still null.

I tried to use equals() returning true and hashCode returning 1 only for testing. Because in this way if all books has the same hashcode it would like "all the books are the same". But since even in this case the call returns null i've tried with normal equals() and hashCode() to print the hashCode of the book inserted in the cart and the hashCode of the book inserted. They were equals.

However i can retry to investigate deeply.

Thank u Piotr!

I've just fix the problem! I've changed the code in this way:

-------------------------------------------------------

@Override

          public boolean equals(Object o) {

             if (o==this)

                 return true;

             if (o instanceof Book) {

                       if (this.isbn.compareTo(((Book) o).getIsbn()) == 0)

                                 return true;

                       else return false;

             }

             return false;

          }

-------------------------------------------------------

The problem was the comparison between string! Now I use the compareTo() method and everything correctly works.

Thank you for your time Piotr!