...
@Entity
@Table(name="Users")
@SequenceGenerator(name="user_gen", sequenceName="user_id_seq")
public class User implements Serializable {

        @Id
   @GeneratedValue(strategy=GenerationType.IDENTITY, generator="user_gen")
   private int userID;
        //bi-directional many-to-one association to Useraddress
   @OneToMany(mappedBy="user", cascade = { CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REMOVE})
   private Set<Useraddress> useraddresses = new HashSet<Useraddress>();
       
        //Konstruktor

   public Set<Useraddress> getUseraddresses() {
      return this.useraddresses;
   }

   public void setUseraddresses(Set<Useraddress> useraddresses) {
      this.useraddresses = useraddresses;
   }
...
}

...
@Entity
@Table(name="UserAddresses")
@SequenceGenerator(name="useraddresses_gen", sequenceName="useraddresses_id_seq")
public class Useraddress implements Serializable {

        @Id
   @GeneratedValue(strategy=GenerationType.IDENTITY, generator="useraddresses_gen")
   private int addressID;

         private String city;
         private String postcode;
         private String street;
         
        // Getter und Setter

        //bi-directional many-to-one association to User
        @ManyToOne(cascade = CascadeType.ALL)
   @JoinColumn(name="userID")
   private User user;
...
}

...
/**
     * Saves address to person.
     */
   public void saveUseraddress(User user, Useraddress useraddress) {
      
      // UserId zuweisen
      int currentuserId = user.getUserID();
      
      User _user = new User(currentuserId);
      if (_user.getUseraddresses().contains(useraddress)) {
         _user.getUseraddresses().remove(useraddress);
        }

      _user.getUseraddresses().add(useraddress);
      em.persist(useraddress);
      
               return;
   }