Hibernate does not allow to save data with similar id

I'm learning Spring and few days ago i started learning Hibernate. I have my studying project where i need to create a shop with products. Here is entity class

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import javax.persistence.*;

@Entity
@Table(name = "cart")
@Data
@NoArgsConstructor
@AllArgsConstructor
public class Cart {
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")

    private int id;
    @Column(name = "quantity")
    private int quantity;
    @Column(name = "mask")
    private String mask;
    @Column(name = "price")
    private int price;

So, i create interface

import org.springframework.data.jpa.repository.JpaRepository;

public interface CartRepository extends JpaRepository<Cart, Integer> {
}

and create controllers

@Autowired
    CartRepository cartList;

    @RequestMapping("/add-black-mask")
    public String addBlackMask() {
        cartList.save(new Cart(1, 1, "black", 3));

        return "masks/add-black-mask";
    }

    @RequestMapping("/add-build-mask")
    public String addBuildMask() {
        cartList.save(new Cart(2, 1, "build", 5));

        return "masks/add-build-mask";
    }


  @RequestMapping(value = "/save_product_to_cart")
    public ModelAndView saveProduct(@ModelAttribute(value = "cart")
                                            Cart cart, BindingResult result) {
        int index = Integer.parseInt(String.valueOf(cartList.count()));
        if (cart.getId() == 0) {
            cart.setId(index + 1);
            cartList.save(cart);
        } else {
            Cart cart1 = cartList.getOne(cart.getId());
            cart1.setMask(cart.getMask());
            cart1.setQuantity(cart.getQuantity());
            cart1.setPrice(cart.getPrice());
            cartList.save(cart1);
        }
        return new ModelAndView("redirect:/");
    }

Also, there are some other controllers for view, Thymeleaf etc, but its ok. My problem is - when i save my product 1 time - its ok, but when i save second - it didnt work( i think because i can't save 2 rows with similar ID) So it seems i have UNIQUE ID in my table and it can not be repeated. Question is - how can i delete unique id or change my code in any way? Thanks in advance!

p.s. i read some other topics here but it didnt help me.

when you use GENERATIONTYPE.IDENTITY you are asking hibernate to let the database handle Ids for you, you should not set it yourself. you are changing the value of the id, just create a new product, set all the fields and inside a transactional context, save your product. Also always use wrapped versions of primitives for serialization purposes. (Long is an object but long is a primitive.) you can google boxing and unboxing and learn more about this.

Let me answer this question:
First of all, using annotations as our configure method is just a convenient method instead of coping the endless XML configuration file.

The @Idannotation is inherited from javax.persistence.Id， indicating the member field below is the primary key of current entity. Hence your Hibernate and spring framework as well as you can do some reflect works based on this annotation. for details please check javadoc for Id

The @GeneratedValue annotation is to configure the way of increment of the specified column(field). For example when using Mysql, you may specify auto_increment in the definition of table to make it self-incremental, and then use

@GeneratedValue(strategy = GenerationType.IDENTITY)

in the Java code to denote that you also acknowledged to use this database server side strategy. Also, you may change the value in this annotation to fit different requirements.

1. Define Sequence in database

For instance, Oracle has to use sequence as increment method, say we create a sequence in Oracle:

create sequence oracle_seq;

2. Refer the database sequence

Now that we have the sequence in database, but we need to establish the relation between Java and DB, by using @SequenceGenerator:

@SequenceGenerator(name="seq",sequenceName="oracle_seq")

sequenceName is the real name of a sequence in Oracle, name is what you want to call it in Java. You need to specify sequenceName if it is different from name, otherwise just use name. I usually ignore sequenceName to save my time.

3. Use sequence in Java

Finally, it is time to make use this sequence in Java. Just add @GeneratedValue:

@GeneratedValue(strategy=GenerationType.SEQUENCE, generator="seq")

The generator field refers to which sequence generator you want to use. Notice it is not the real sequence name in DB, but the name you specified in name field of SequenceGenerator.

4. Complete

So the complete version should be like this:

public class Cart
{
    @Id
    @SequenceGenerator(name="seq",sequenceName="oracle_seq")        
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="seq")               
    private Integer id;
}

Now start using these annotations to make your JavaWeb development easier.

On top of that I would like you to understand all 4 ways of ID generation in hibernate. You can think of reading in your free time

1. GenerationType.AUTO
2. GenerationType.IDENTITY (your case)
3. GenerationType.SEQUENCE
4. GenerationType.TABLE {Rarely used nowdays}