How to handle if statement for mockito tests and multiple when methods

I have a test that I am stuck in , it looks like this

 public CartItem addBookToCartItem(Book book, User user, int qty) throws AccessDeniedException {
        List<CartItem> cartItemList = findByShoppingCart(user.getShoppingCart());

        for (CartItem cartItem : cartItemList) {
            if (book.getId() == cartItem.getBook().getId()) {
                cartItem.setQty(cartItem.getQty() + qty);
                cartItem.setSubTotal(new BigDecimal(book.getOurPrice()).multiply(new BigDecimal(qty)));
                cartItemRepository.save(cartItem); //it shows there is a      problem with this line 
                return cartItem;
            }
        }

        CartItem cartItem = new CartItem();
        cartItem.setShoppingCart(user.getShoppingCart());
        cartItem.setBook(book);

        cartItem.setQty(qty);
        cartItem.setSubTotal(new BigDecimal(book.getOurPrice()).multiply(new BigDecimal(qty)));
        cartItem = cartItemRepository.save(cartItem);

        BookToCartItem bookToCartItem = new BookToCartItem();
        bookToCartItem.setBook(book);
        bookToCartItem.setCartItem(cartItem);

        bookToCartItemRepository.save(bookToCartItem);

        return cartItem;
    }

I am posting the whole code cause I don't know where the problem is and how to go about it . My test looks like this

 @Test
    public void addBookToCartItemTest() throws Exception {

      /// entities initialized here 
        List<CartItem> cartItemList = Arrays.asList(cartItem, cartItem1);

        when(cartItemRepository.findByShoppingCart(user.getShoppingCart())).thenReturn(cartItemList);

        for (CartItem item : cartItemList) {

            if (book1.getId() == item.getBook().getId()) {
                item.setQty(item.getQty() + qty);
                item.setSubTotal(new BigDecimal(book1.getOurPrice()).multiply(new BigDecimal(qty)));
                given(cartItemRepository.save(item)).willReturn(cartItem);

            }
        }

            CartItem newCartItem = new CartItem();
            newCartItem.setShoppingCart(user.getShoppingCart());
            newCartItem.setBook(book1);

            newCartItem.setQty(qty);
            newCartItem.setSubTotal(new BigDecimal(book1.getOurPrice()).multiply(new BigDecimal(qty)));

            when(cartItemRepository.save(newCartItem)).thenReturn(newCartItem);

            BookToCartItem bookToCartItem1 = new BookToCartItem();
            bookToCartItem1.setBook(book1);
            bookToCartItem1.setCartItem(newCartItem);

            when(bookToCartItemRepository.save(bookToCartItem1)).thenReturn(bookToCartItem1);

            cartItemService.addBookToCartItem(book1, user, qty);

            // Assert.assertNotNull(cartItem1);
            // Assert.assertEquals(newCartItem, cartItem1);

            Mockito.verify(cartItemRepository).save(newCartItem); //and this line also has a  problem
            Mockito.verify(bookToCartItemRepository).save(bookToCartItem1);
       // Mockito.verify(cartItemRepository).save(item);

error

Argument(s) are different! Wanted:
com.valentine.repository.CartItemRepository#0 bean.save(
    com.valentine.domain.CartItem@2f05be7f
);
-> at com.valentine.service.CartItemServiceTest.addBookToCartItemTest(CartItemServiceTest.java:110)
Actual invocation has different arguments:
com.valentine.repository.CartItemRepository#0 bean.save(
    com.valentine.domain.CartItem@640f11a1
);
-> at com.valentine.service.CartItemServiceImpl.addBookToCartItem(CartItemServiceImpl.java:48)

Comparison Failure:  <Click to see difference>

Argument(s) are different! Wanted:
com.valentine.repository.CartItemRepository#0 bean.save(
    com.valentine.domain.CartItem@2f05be7f
);

I think the problem is how tp handle the multiple when methods or is there another way to go about it , Any suggestion would suffice thank you .