Why is @Id needed in Entity?

Question

Java

Why is the @Id field needed in Entity? What happens if I have a field with @Id, but there is no column with the field name?

Answer 1

1) Maybe in JPA?
2) Consider this example:

@Entity
@Table(name="users")
public class UsersDataSet {
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    long user_id;
    //...
}

This means that the id field corresponding to the database will be associated with the user_id field in your class.