how to persist an object of a subclass from an existing super class entity object?

hello, i have a user entity which inherits from collaborator entity, and i use @Inheritance(strategy = InheritanceType.JOINED) in the Collaborator entity.
I do not find the way to persist a user entity that belongs to an existing collaborator,
i can find a collaborator by em.findById(2);
but how to associate a new user object to this collaborator object ?

If you User "has a" Collaborator, then you simply set it.

i.e.
user.setCollaborator(em.find(Collaborator.class, 2));
em.persist(user);

But you seem to be confused about inheritance wanting to make a new User "become" a Collaborator? If so, then you should not be using inheritance, but have User have a OneToOne relationship to Collaborator.

hello, thak you for your help,
but i am not confused in inheritance,
User "is a" Collaborator, so i can not simply set it.
so, how i will set it in this situation?
I do not find the way to persist a user entity that belongs to an existing collaborator, I can add a user object and it results to 2 new created rows in the data base but what i want to do is to add only a user object and not a new collaborator.
I can find a collaborator by Collaborator clbr=em.findById(2);
and i want to add a user that its collaborator is clbr but how to associate the new user object to this collaborator object ?
Below my entities

@Inheritance(strategy = InheritanceType.JOINED)
@Entity
@Table(name = "COLLABORATEUR")
public class Collaborator implements Serializable {

private static final long serialVersionUID = 2421761198342915033L;

/**
    * Unique ID used as a primary key
    */
   @Id
   @Column(name = "MATRICULE")
   protected Integer id;
   /**
    * The name of the collaborator
    */
   @NotNull
   @Column(name = "NOM")
   protected String name;
   /**
    * The surname of the collaborator
    */
   @NotNull
   @Column(name = "PRENOM")
   protected String surname;
   /**
    * The mail of the collaborator
    */
   @Column(name = "MAIL")
   protected String mail;

/**
    * @return the mail
    */
   public String getMail() {
      return mail;
   }

/**
    * @param mail
    *            the mail to assign for the collaborator
    */
   public void setMail(String mail) {
      this.mail = mail;
   }

/**
    * @return the reference number
    */
   public Integer getId() {
      return id;
   }

/**
    * @param id
    *            the the reference number to set
    */
   public void setId(Integer id) {
      this.id = id;
   }

/**
    * @return the name
    */
   public String getName() {
      return name;
   }

/**
    * @param name
    *            the name to set
    */
   public void setName(String name) {
      this.name = name;
   }

/**
    * @return the surname
    */
   public String getSurname() {
      return surname;
   }

/**
    * @param surname
    *            the surname to set
    */
   public void setSurname(String surname) {
      this.surname = surname;
   }

/**
    * @return the String representation of user
    */
   public String toString() {
      return String.format("[id=%d , name=%s, surName=%s, mail=%s, userName=%s]", getId(), getName(), getSurname(), getMail());
   }
}

Code:
@Entity
@Table(name="UTILISATEUR")
public class User extends Collaborator implements Serializable {

private static final long serialVersionUID = 6568398464819949491L;
   
   /**
    * The username used for authenticating
    */
   @NotNull
   @Column(name="UTI_USERNAME")
   private String username;
   /**
    * The password MD5 hash
    */
   @Column(name="UTI_PASSWORD_HASH")
   private String passwordHash;   
   /**
    * The list of privileges of the user
   */
   @ManyToMany(targetEntity = Privilege.class)
   @JoinTable(name = "UTILISATEUR_PRIVILEGE", joinColumns = @JoinColumn(name = "UTI_ID"), inverseJoinColumns = @JoinColumn(name = "PRIVILEGE_ID"))
   private Set<Privilege> privileges;
   
      
   /**
    * @return the username
    */
   public String getUsername() {
      return username;
   }
   /**
    * @param username the username to set
    */
   public void setUsername(String username) {
      this.username = username;
   }
   /**
    * @return the passwordHash
    */
   public String getPasswordHash() {
      return passwordHash;
   }
   /**
    * @param passwordHash the passwordHash to set
    */
   public void setPasswordHash(String passwordHash) {
      this.passwordHash = passwordHash;
   }
   /**
    * @return the privileges
    */
   public Set<Privilege> getPrivileges() {
      return privileges;
   }
   /**
    * @param privileges the privileges to set
    */
   public void setPrivileges(Set<Privilege> privileges) {
      this.privileges = privileges;
   }
   
   /**
    * @return the String representation of user
    */
   public String toString() {
      return String.format("[id=%d , userName=%s, password=%s, collaborator=%s]", getId(), getUsername(), getPasswordHash()/*, getCollaborator().getName()*/);
   }
}

Must I change my strategy to table per class?