How to implement Serializable in jpa using inheritance

Question

When i tried to map class with 2 referenced columns, I got this error:

Caused by: java.lang.ClassCastException: entities.Client cannot be cast to java.io.Serializable

I read some similar question When and why JPA entities should implement Serializable interface?

And i saw this answer:

It has one more parameters named referencedColumnName. This parameter declares the column in the targeted entity that will be used to the join. Note that when using referencedColumnName to a non primary key column, the associated class has to be Serializable.

1.Why? whats happening in the background?

@MappedSuperclass
public abstract class Person {



    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    @Column(name = "full_name")
    private String fullName;
    private String email;
    @Column(name = "phone_number")
    private String phoneNumber;
    private String address;

    public Person()
    {

    }

    public Person(String fullName, String phoneNumber, String email, String address )
    {
        this.fullName = fullName;
        this.email = email;
        this.phoneNumber = phoneNumber;
        this.address = address;

    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getFullName() {
        return fullName;
    }

    public void setFullName(String fullName) {
        this.fullName = fullName;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getPhoneNumber() {
        return phoneNumber;
    }

    public void setPhoneNumber(String phoneNumber) {
        this.phoneNumber = phoneNumber;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }




}

@Entity
@Table(name = "CLIENTS")
public class Client extends Person {

    @OneToMany(mappedBy = "client", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Reservation> reservations = new ArrayList<>();

    public Client() {

    }

    public Client(String fullName, String phoneNumber, String email, String address) {

        super(fullName, phoneNumber, email, address);
    }

    public List<Reservation> getReservations() {
        return reservations;
    }

    public void addReservation(Reservation reservation) {
        this.reservations.add(reservation);
        reservation.setClient(this);
    }

    public void removeReservation(Reservation reservation) {
        this.getReservations().remove(reservation);
        reservation.setClient(null);

    }

}

@Entity
@Table(name = "SUPPLIERS")
public class Supplier extends Person {

    @OneToMany(mappedBy = "supplier", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<SupplierItem> items = new ArrayList<>();

    public Supplier() {

    }

    public Supplier(String fullName, String phoneNumber,
            String email, String address) {

        super(fullName, phoneNumber, email, address);

    }

    public List<SupplierItem> getItems() {
        return items;
    }

    public void addItem(SupplierItem item) {
        this.getItems().add(item);
        item.setSupplier(this);
    }

    public void removeItem(SupplierItem item) {
        this.getItems().remove(item);
        item.setSupplier(null);

    }

}

public class Reservation {


    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "reservation_id")
    private Integer id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumns({
    @JoinColumn(name = "client_id", referencedColumnName="id"),
    @JoinColumn(name = "client_full_name", referencedColumnName="full_name")})

    private Client client;

When i execute the code:

 Client client = new Client("Benny Hagadol", "0523612324","Bennyhagadol@gmail.com","Florentin 34 Tel Aviv");
 Reservation re = new Reservation();
 client.addReservation(re);
 new ClientManager().addClient(client);


 public void addClient(Client client) {
        this.em.getTransaction().begin();
        this.em.persist(client);
        this.em.getTransaction().commit();

    }

The exception:

 14, 2019 10:10:32 PM org.hibernate.engine.transaction.jta.platform.internal.JtaPlatformInitiator initiateService
INFO: HHH000490: Using JtaPlatform implementation: [org.hibernate.engine.transaction.jta.platform.internal.NoJtaPlatform]
Hibernate: 
    insert 
    into
        CLIENTS
        (address, email, full_name, phone_number) 
    values
        (?, ?, ?, ?)
Hibernate: 
    insert 
    into
        RESERVATIONS
        (client_id, client_full_name, created_on, date_to_supply, last_updated, total_sum) 
    values
        (?, ?, ?, ?, ?, ?)
Exception in thread "main" javax.persistence.RollbackException: Error while committing the transaction
    at org.hibernate.internal.ExceptionConverterImpl.convertCommitException(ExceptionConverterImpl.java:81)
    at org.hibernate.engine.transaction.internal.TransactionImpl.commit(TransactionImpl.java:104)
    at managers.ClientManager.addClient(ClientManager.java:31)
    at GUI.Menu.main(Menu.java:148)
Caused by: java.lang.ClassCastException: entities.Client cannot be cast to java.io.Serializable
    at org.hibernate.type.CollectionType.getKeyOfOwner(CollectionType.java:446)
    at org.hibernate.engine.internal.Collections.processReachableCollection(Collections.java:166)
    at org.hibernate.event.internal.FlushVisitor.processCollection(FlushVisitor.java:50)
    at org.hibernate.event.internal.AbstractVisitor.processValue(AbstractVisitor.java:104)
    at org.hibernate.event.internal.AbstractVisitor.processValue(AbstractVisitor.java:65)
    at org.hibernate.event.internal.AbstractVisitor.processEntityPropertyValues(AbstractVisitor.java:59)
    at org.hibernate.event.internal.DefaultFlushEntityEventListener.onFlushEntity(DefaultFlushEntityEventListener.java:182)
    at org.hibernate.event.internal.AbstractFlushingEventListener.flushEntities(AbstractFlushingEventListener.java:233)
    at org.hibernate.event.internal.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:93)
    at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:39)
    at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:108)
    at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1358)
    at org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:436)
    at org.hibernate.internal.SessionImpl.flushBeforeTransactionCompletion(SessionImpl.java:3235)
    at org.hibernate.internal.SessionImpl.beforeTransactionCompletion(SessionImpl.java:2403)
    at org.hibernate.engine.jdbc.internal.JdbcCoordinatorImpl.beforeTransactionCompletion(JdbcCoordinatorImpl.java:447)
    at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.beforeCompletionCallback(JdbcResourceLocalTransactionCoordinatorImpl.java:183)
    at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.access$300(JdbcResourceLocalTransactionCoordinatorImpl.java:40)
    at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl$TransactionDriverControlImpl.commit(JdbcResourceLocalTransactionCoordinatorImpl.java:281)
    at org.hibernate.engine.transaction.internal.TransactionImpl.commit(TransactionImpl.java:101)
    ... 2 more

After i know that i have to implement Serializable (not only for good practice) How do i have to do that?

2. The subclasses has to implement Serializable? or only the superclass?

3. Do i have to declare serialVersionUID?

4. If the subclasses has to implement Serializable, They has to declare serialVersionUID?