How to implement a "one to zero" mapping?

Question

The scenario is as follows: A user may have many or no comments in the newsletters posted on a system - obviously, a comment belongs to a single user.

The problem is that Hibernate does not have any annotations of the type @OneToZero or anything like that, so how can I do this mapping?

On the user’s side (class User), to list the comments a user has made the following:

@OneToMany(mappedBy = "userID", cascade = CascadeType.ALL)
private List<Comment> comments;

I think the above mapping is correct. But on the comments side (class Comment) i don’t know what to do to define that a comment can exist in the same way that it can never be realized.

// como mapear aqui? 
@JoinColumn(name = "fk_user")
private User userID;

I even tried to do with @ManyToOne, but a NullPointerException whenever I try to perform any CRUD operation with the user. A simple Insert is already enough to "crash" the application precisely because the comments are null. But there it is, a user has no comments when this is registered in the system.

My classes are like this:

@Entity
@Table(name = "users")
public class User implements Serializable {

   @Id
   @GeneratedValue
   private Long id;

   @OneToMany(mappedBy = "userID", cascade = CascadeType.ALL)
   private List<Comment> comments;

   /* ...outros atributos... */
}

@Entity
@Table(name = "comments")
public class Comment implements Serializable {

    @Id
    @GeneratedValue
    private Long id;

    @ManyToOne //ManyToOne gera uma NullPointerException, mas creio que esteja próximo do certo
    @JoinColumn(name = "fk_user")
    private User userID; 

    /* ...outros atributos... */
}

Answer 1

Try adding the annotation @NotFound with the attribute (action=NotFoundAction.IGNORE), getting:

@Entity
@Table(name = "comments")
public class Comment implements Serializable {

@Id
@GeneratedValue
private Long id;

@NotFound(action=NotFoundAction.IGNORE) //Linha inserida
@JoinColumn(name = "fk_user")
private User userID; 

/* ...outros atributos... */
}

Sources:

1. https://stackoverflow.com/questions/841354/
2. https://stackoverflow.com/questions/7146064/
3. http://www.concretepage.com/hibernate/example-notfound-hibernate