Using calculated properties of an entity

Question

Hi all,
I am having some serious trouble in finding out how this works. Maybe I'm not Googling correctly, but I have spent many hours already on this problem and I'm not getting anywhere. I am also reasonably sure I have checked out all the Netbeans tutorials on this matter, but alas. So I guess, I could do with some help.
This is really a basic (beginner?) question about persistence. As a side note: I am using Netbeans 6.7
I am running a database and all my tables have corresponding entity classes. I have a GUI in which I want to display a table from the database, together with some calculated values.
I'll make the setup as simple as possible. Take the simple database created by
CREATE DATABASE administration;
USE administration;
CREATE TABLE customers (
  id INT NOT NULL AUTO_INCREMENT,
  firstname VARCHAR(50) NOT NULL,
  surname VARCHAR(50 NOT NULL,
  PRIMARY KEY(id)
) ENGINE=INNODB;
INSERT INTO customers (firstname, surname)
  VALUES ('John', 'Adams'), ('Caroline', 'Nixon')Based on this database, Netbeans creates an entity class
@Entity
@Table(name = "customers")
@NamedQueries({
  @NamedQuery(name = "Customers.findAll", query = "SELECT c FROM Customers c"),
  @NamedQuery(name = "Customers.findById", query = "SELECT c FROM Customers c WHERE c.id = :id"),
  @NamedQuery(name = "Customers.findByFirstname", query = "SELECT c FROM Customers c WHERE c.firstname= :firstname"),
  @NamedQuery(name = "Customers.findBySurname", query = "SELECT c FROM Customers c WHERE c.surname= :surname")})
public class Customers implements Serializable {
  @Transient
  private PropertyChangeSupport changeSupport = new PropertyChangeSupport(this);
  private static final long serialVersionUID = 1L;
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  @Basic(optional = false)
  @Column(name = "id")
  private Integer id;
  @Basic(optional = false)
  @Column(name = "firstname")
  private String firstname;
  @Basic(optional = false)
  @Column(name = "surname")
  private String surname;
  // CONSTRUCTORS
  // GET AND SET METHODS
  // A FEW SIMPLE @override METHODS
  public void addPropertyChangeListener(PropertyChangeListener listener) {
    changeSupport.addPropertyChangeListener(listener);
  } public void removePropertyChangeListener(PropertyChangeListener listener) {
    changeSupport.removePropertyChangeListener(listener);
}Now, using the Netbeans GUI tools I create a JTable with values bound to Customers. This works fine: the apllication reads the database and displays correct values in the table (however, externally making a change in the database is not updated in the table, nor are changes in the table sent to the database, but that is a different matter - gonna work on that after this).
The next step is creating a calculated property, say "private String adressee" which is calculated by Java code the be for example "C. Nixon" or "J. Adams". (*This is my question: what is the correct way of implementing something like this, so that it is updated in the table when firstname or surname are altered*?) Now the problems start. Simply defining this property gives errors, because it is automatically interpreted as a persistent value. So the apllication does an SQL query interpreting 'adressee' as a column, which of course results in an error. Alternatively, I can define 'adressee' as @Transient. Now, the application compiles and runs, but 'adressee' is never updated in the table. Note that I update it manually: that is, I have the program listen to PropertyChange for surname and firstname, and the generate the correct value for adressee. This works, but is not reflected in the table. I have also tried to refresh() the JTableBinding when a PropertyChange occurs, but this returns a NullPointerException. Moreover, even if that should work, I am convinced there must be a way more elegant method.
I anyone would be so kind as to enlighten me, please do!
Greets

Answer 1

Somehow, retrying my first technique worked. Maybe it is because I am now using Hobernate, set to Eager (not Lazy, which gave me compiling errors!). For future reference, here is how I did it.
The MySQL table is called 'relaties'. 'contactpersoon' is a value which I want to calculate from several other values from this table and its related tables.
I use Netbeans to create entity classes from database. One of which is Relaties:
@Entity
@Table(name = "relaties", catalog = "TK", schema = "")
public class Relaties implements Serializable {
  @Transient
  private PropertyChangeSupport changeSupport = new PropertyChangeSupport(this);
  private static final long serialVersionUID = 1L;
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  @Basic(optional = false)
  @Column(name = "id")
  private Integer id;
  // AND MANY OTHER COLUMNS, THE USUAL ENTITY CODE
  @Transient
  private String contactpersoon;
  public String getContactpersoon() {
    return contactpersoon;
  public void setContactpersoon(String contactpersoon) {
    String oldContactpersoon = this.contactpersoon;
    this.contactpersoon = contactpersoon;
    changeSupport.firePropertyChange("contactpersoon", oldContactpersoon, contactpersoon);
  public void genContactpersoon() {
    // calculation code - note that I had to catch NullPointerExceptions for Columns without @Basic(optional = false)
    this.setContactpersoon(s);
  public void addPropertyChangeListener(PropertyChangeListener listener) {
    changeSupport.addPropertyChangeListener(listener);
  public void removePropertyChangeListener(PropertyChangeListener listener) {
    changeSupport.removePropertyChangeListener(listener);
}I now make another class, in which I put everything I think Relaties should do itself, but I can't put in there because of compilation errors (my bad?):
public class RelatiesHelper implements PropertyChangeListener {
  public void init(Relaties r) {
    r.addPropertyChangeListener(this);
    r.genContactpersoon();
    r.genAdressering();
  public void propertyChange(PropertyChangeEvent evt) {
    if (evt.getPropertyName().equals("achternaam") || evt.getPropertyName().equals("tussenvoegsels") || evt.getPropertyName().equals("voornaam") || evt.getPropertyName().equals("titel")) { // these are column titles of the properties on which 'contactpersoon' depends
      ((Relaties) evt.getSource()).genContactpersoon();
}I now made a class ViewRelaties, which includes a JTable. The values of this JTable can now be bound in the usual way, including 'contactpersoon':
public class ViewRelaties
        extends javax.swing.JPanel {
  /** Creates new form ViewRelaties */
  public ViewRelaties() {
    initComponents(); // code to create table and initialize a Relaties-list, bind values etc.
    ListIterator<Relaties> l = this.relatiesList.listIterator();
    RelatiesHelper rh = new RelatiesHelper();
    while (l.hasNext()) {
      rh.init(l.next());
  // THE initComponents() CODE
  private javax.persistence.EntityManager AdminPUEntityManager;
  private javax.persistence.Query relatiesQuery;
  private java.util.List<admin.entities.Relaties> relatiesList;
}Hope this helps someone :)
Edited by: Quinten1 on Aug 1, 2009 5:21 AM