@Entity()
@Table(name="Clients")
public class Client extends BasePersistentObject {
   
    private static Logger log = Logger.getLogger(Client.class);
   
   
    @Id
    @Column(name="Id")
    @GeneratedValue(strategy=GenerationType.AUTO)
    private long id;
   
    @Column(name="companyName")
    private String companyName;
   
    @OneToMany(mappedBy="client", cascade=CascadeType.ALL)
    private List<Address> addresses;

    // What annotations? Should reference to an object of the addresses list.
    private Address mainAddress;

    // What annotations? Should reference to an object of the addresses list.
    private Address billingAddress;

    // What annotations? Should reference to an object of the addresses list.
    private Address shippingAddress;

    // The rest of methods, contructors, getters and setters.

}

@Entity()
@Table(name="Addresses")
public class Address extends BasePersistentObject {
   
    @Id
    @Column(name="Id")
    @GeneratedValue(strategy=GenerationType.AUTO)
    private long id;
   
    @ManyToOne()
    @JoinColumn(name="Client_ID", nullable=false)
    private Client client;
   
    @Column(name="Address")
    private String address;

    // ...

}

public Address getBillingAddress() {
   return getAddresses.get(BILLING_ADDRESS_KEY);
}