Remote interface and associations

[infodavid]

I want to use this interface named 'ICity' :

@Remote
public interface ICity extends Serializable {
 public String getName() throws RemoteException;

 public void setName(String s) throws RemoteException;

 public String getCode() throws RemoteException;

 public void setCode(String s) throws RemoteException;

 public String getCountry() throws RemoteException;

 public void setCountry(String o) throws RemoteException;
}

for association in :

@javax.persistence.Entity(access = AccessType.PROPERTY)
@Table(name = "ENTITIES")
@Inheritance(strategy = InheritanceType.JOINED)
public class Entity extends AbstractSimpleEntity implements IEntity {
 private ICity city = null;
...

 public Entity() { }

 public Entity(Entity o) { super(o); }

 @Id(generate = GeneratorType.TABLE)
 @Column(name = "entity_id", nullable = false, unique = true)
 public long getPrimaryKey() { return id; }

...

 @ManyToOne(fetch = FetchType.LAZY)
 @JoinColumn(name = "city_id")
 public ICity getCity() throws RemoteException { return city; }

 public void setCity(ICity o) throws RemoteException { city = (City)o; }
...
}

but the entity manager can't find the associated and concrete city object (implemented in class 'City').

How can I do with this, which object do I used in the methods of 'Entity' ?