Как создать приложение Swing CRUD на платформе NetBeans 6.8

Создание классов сущностей

В этом разделе вы создаете классы сущностей из выбранной базы данных.

1. В этом примере используйте окно «Службы» для подключения к образцу базы данных, которая входит в состав среды IDE NetBeans:

   

   Примечание. В качестве альтернативы вы можете использовать любую базу данных и адаптировать шаги, которые следуют, к вашему конкретному варианту использования. В случае MySQL см. Подключение к базе данных MySQL .

2. В IDE выберите Файл | Новый проект, за которым следует Java | Java Class Library для создания нового библиотечного проекта с именем CustomerLibrary.
   
3. В окне «Проекты» щелкните правой кнопкой мыши проект библиотеки и выберите «Файл | Новый файл с последующим сохранением | Классы сущностей из базы данных. В мастере выберите свою базу данных и нужные вам таблицы. Здесь мы выбираем «Клиент», а затем «Код скидки» добавляется автоматически, так как между этими двумя таблицами существует связь.

   

4. Specify the persistence strategy, which can
   be any of the available options. Here, since we need
   to choose something, we’ll
   choose EclipseLink:

   

5. Specify «demo» as the name of
   the package where the entity classes will be generated.

   

6. Click Finish. Once you have completed this step, look at the generated code and notice
   that, among other things, you now have a persistence.xml file in a folder
   called META-INF, as well as entity classes for each of your tables:

   

7. Build the Java Library and you will have a JAR file in the library
   project’s «dist» folder, which you can view in the Files window:

   

Wrapping the Entity Class JAR in a Module

In this section, you add your first module to your application!
The new NetBeans module will wrap the JAR file
you created in the previous section.

1. Right-click the DBManager’s Modules node in the Projects window
   and choose Add New Library.
2. Select the JAR you created in the previous subsection
   and complete the wizard, specifying any values you like. Let’s assume
   the application is for dealing with customers at shop.org, in which case
   a unique identifier «org.shop.model» is appropriate for the code name base:

   

You now have your first custom module in your new application,
wrapping the JAR containing the entity classes and
the persistence.xml file:

Creating Other Related Modules

In this section, you create two new modules, wrapping the EclipseLink JARs,
as well as the database connector JAR.

1. Do the same as you did when creating the library wrapper for the
   entity class JAR, but this time for the EclipseLink JARs, which are in
   the «CustomerLibrary» Java library that you created earlier:

   

   Note: In the Library Wrapper Module wizard,
   you can use Ctrl-Click to select multiple JARs.

2. Next, create yet another library wrapper module, this time for the Java DB client JAR,
   which is available in your JDK distribution, at db/lib/derbyclient.jar.

Designing the User Interface

In this section, you create a simple prototype user interface, providing
a window that uses a JTextArea to display data retrieved
from the database.

1. Right-click the DBManager’s Modules node in the Projects window
   and choose Add New. Create a new module named CustomerViewer, with
   the code name base org.shop.ui.
2. In the Projects window, right-click the new module and choose
   New | Window Component. Specify that it should be created in the editor
   position and that it should open when the application starts. Set Customer
   as the window’s class name prefix.
3. Use the Palette (Ctrl-Shift-8) to drag
   and drop a JTextArea on the new window:

   

4. Add this
   to the end of the TopComponent constructor:

   EntityManager entityManager = Persistence.createEntityManagerFactory("CustomerLibraryPU").createEntityManager();
   Query query = entityManager.createQuery("SELECT c FROM Customer c");
   List<Customer> resultList = query.getResultList();
   for (Customer c : resultList) {
     jTextArea1.append(c.getName() + " (" + c.getCity() + ")" + "\n");
   }

   Note: Since you have not set dependencies on the modules that
   provide the Customer object and the persistence JARs, the statements
   above will be marked with red error underlines. These will be fixed
   in the section that follows.

Above, you can see references to a persistence unit named «CustomerLibraryPU»,
which is the name set in the persistence.xml file. In addition,there is a reference
to one of the entity classes, called Customer, which is in the entity classes module.
Adapt these bits to your needs, if they are different to the above.

Setting Dependencies

Running the Prototype

1. In your TopComponent, delete the JTextArea
   in the Design view and comment out its
   related code in the Source view:

   EntityManager entityManager =  Persistence.createEntityManagerFactory("CustomerLibraryPU").createEntityManager();
   Query query = entityManager.createQuery("SELECT c FROM Customer c");
   List<Customer> resultList = query.getResultList();
   //for (Customer c : resultList) {
   //    jTextArea1.append(c.getName() + " (" + c.getCity() + ")" + "\n");
   //}

2. Right-click the CustomerViewer module, choose Properties,
   and use the Libraries tab to set dependencies on the Nodes API and
   the Explorer & Property Sheet API.
3. Next, change the class signature to implement ExplorerManager.Provider:

   final class CustomerTopComponent extends TopComponent implements ExplorerManager.Provider

You will need to override getExplorerManager()

@Override
public ExplorerManager getExplorerManager() {
    return em;
}

At the top of the class, declare and initialize the ExplorerManager:

private static ExplorerManager em = new ExplorerManager();

Note: Watch Top 10 NetBeans APIs
for details on the above code, especially the screencast dealing with the Nodes API
and the Explorer & Property Sheet API.

4. Switch to the TopComponent Design view,
   right-click in the Palette, choose Palette Manager | Add from JAR. Then browse to
   the org-openide-explorer.jar, which is in platform11/modules folder,
   within the NetBeans IDE installation directory. Choose the BeanTreeView and complete
   the wizard. You should now see BeanTreeView in the Palette. Drag it from
   the Palette and drop it on the window.
5. Create a factory class that will create a new BeanNode
   for each customer in your database:

   import demo.Customer;
   import java.beans.IntrospectionException;
   import java.util.List;
   import org.openide.nodes.BeanNode;
   import org.openide.nodes.ChildFactory;
   import org.openide.nodes.Node;
   import org.openide.util.Exceptions;
   
   public class CustomerChildFactory extends ChildFactory<Customer> {
   
       private List<Customer> resultList;
   
       public CustomerChildFactory(List<Customer> resultList) {
           this.resultList = resultList;
       }
   
       @Override
       protected boolean createKeys(List<Customer> list) {
           for (Customer customer : resultList) {
               list.add(customer);
           }
           return true;
       }
   
       @Override
       protected Node createNodeForKey(Customer c) {
           try {
               return new BeanNode(c);
           } catch (IntrospectionException ex) {
               Exceptions.printStackTrace(ex);
               return null;
           }
       }
   
   }

6. Back in the CustomerTopComponent,
   use the ExplorerManager to pass the result list
   from the JPA query in to the Node:

   EntityManager entityManager =  Persistence.createEntityManagerFactory("CustomerLibraryPU").createEntityManager();
   Query query = entityManager.createQuery("SELECT c FROM Customer c");
   List<Customer> resultList = query.getResultList();
   em.setRootContext(new AbstractNode(Children.create(new CustomerChildFactory(resultList), true)));
   //for (Customer c : resultList) {
   //    jTextArea1.append(c.getName() + " (" + c.getCity() + ")" + "\n");
   //}

7. Run the application. Once the application is running,
   open the Properties window. Notice that even though the data
   is available, displayed in a BeanTreeView, the BeanTreeView
   is not synchronized with the Properties window, which is available
   via Window | Properties. In other words, nothing is displayed
   in the Properties window when you move up and down the tree hierarchy.
8. Synchronize the Properties window with the BeanTreeView
   by adding the following to the constructor in the TopComponent:

   associateLookup(ExplorerUtils.createLookup(em, getActionMap()));

Here we add the TopComponent’s ActionMap and ExplorerManager
to the Lookup of the TopComponent.

9. Run the application again and notice that the Properties window
   is now synchronized with the explorer view:

Now you are able to view your data in a tree hierarchy, as you would
be able to do with a JTree. However, you’re also able to swap
in a different explorer view without needing to change the model at all
because the ExplorerManager mediates between the model and the
view. Finally, you are now also able to synchronize the view with the
Properties window.

Update

In this section, you first create an editor. The editor will be provided
by a new NetBeans module. So, you will first create a new module. Then, within
that new module, you will create a new TopComponent, containing two JTextFields,
for each of the columns you want to let the user edit. You will need to
let the viewer module communicate with the editor module. Whenever a new
Node is selected in the viewer module, you will add the current Customer
object to the Lookup. In the editor module, you will listen to the
Lookup for the introduction of Customer objects. Whenever a
new Customer object is introduced into the Lookup, you will
update the JTextFields in the editor.

Next, you will synchronize your JTextFields
with the NetBeans Platform’s Undo, Redo, and Save functionality. In other words,
when the user makes changes to a JTextField, you want the
NetBeans Platform’s existing functionality to become available so that,
instead of needing to create new functionality, you’ll simply be able to
hook into the NetBeans Platform’s support. To
this end, you will need to use the UndoRedoManager, together with the
SaveCookie.

1. Create a new module, named CustomerEditor, with org.shop.editor as
   its code name base.
2. Right-click the CustomerEditor module and choose New | Window Component.
   Make sure to specify that the window should appear in the editor position and
   that it should open when the application starts. In the final panel of the wizard,
   set «Editor» as the class name prefix.
3. Use the Palette (Ctrl-Shift-8) to add two JLabels and two JTextFields
   to the new window. Set the texts of the labels to «Name» and «City» and
   set the variable names of the two JTextFields to jTextField1
   and jTextField2.

   In the GUI Builder, the window should now look something like this:

   

4. Go back to the CustomerViewer module and change its layer.xml
   file to specify that the CustomerTopComponent window will appear in the explorer mode.

   Note: Right-click the application project and choose «Clean», after
   changing the layer.xml file. Why? Because whenever you run the
   application and close it down, the window positions are stored in the
   user directory. Therefore, if the CustomerViewer was initially
   displayed in the editor mode, it will remain in the editor mode,
   until you do a «Clean», thus resetting the user directory (i.e., thus
   deleting the user directory) and enabling
   the CustomerViewer to be displayed in the position currently set in the
   layer.xml file.

Also check
that the BeanTreeView in the CustomerViewer will stretch horizontally
and vertically when the user resizes the application. Check this by opening the window,
selecting the BeanTreeView, and then clicking the arrow buttons in the
toolbar of the GUI Builder.

5. Run the application and make sure that you see the following when the
   application starts up:

   

6. Now we can start adding some code.
   Firstly, we need to show the currently selected Customer object in the editor:
   • Start by tweaking the CustomerViewer module so that the current Customer object
     is added to the viewer window’s Lookup whenever a new Node
     is selected. Do this by creating an AbstractNode, instead of a BeanNode,
     in the CustomerChildFactory class. That enables you to add the
     current Customer object to the Lookup of the Node,
     as follows (note the «Lookups.singleton(c)» below):

     @Override
     protected Node createNodeForKey(Customer c) {
         Node node = new AbstractNode(Children.LEAF, Lookups.singleton(c));
         node.setDisplayName(c.getName());
         node.setShortDescription(c.getCity());
         return node;
     //        try {
     //            return new BeanNode(c);
     //        } catch (IntrospectionException ex) {
     //            Exceptions.printStackTrace(ex);
     //            return null;
     //        }
     }

   Now, whenever a new Node is created, which
   happens when the user selects a new customer in the viewer, a new
   Customer object is added to the Lookup of the Node.

   • Let’s
     now change the editor module in such a way that its window
     will end up listening for Customer objects being added to the Lookup. First,
     set a dependency in the editor module on the module that provides
     the entity class, as well as the module that provides the persistence
     JARs.
   • Next, change the EditorTopComponent class signature
     to implement LookupListener:

     public final class EditorTopComponent extends TopComponent implements LookupListener

   • Override the
     resultChanged so that the JTextFields are updated whenever
     a new Customer object is introduced into the Lookup:

     @Override
     public void resultChanged(LookupEvent lookupEvent) {
         Lookup.Result r = (Lookup.Result) lookupEvent.getSource();
         Collection<Customer> coll = r.allInstances();
         if (!coll.isEmpty()) {
             for (Customer cust : coll) {
                 jTextField1.setText(cust.getName());
                 jTextField2.setText(cust.getCity());
             }
         } else {
             jTextField1.setText("[no name]");
             jTextField2.setText("[no city]");
         }
     }

   • Now that the LookupListener is defined,
     we need to add it to something. Here, we add it to
     the Lookup.Result obtained from the global context.
     The global context proxies the context of the selected Node.
     For example, if «Ford Motor Co» is selected in the tree hierarchy,
     the Customer object for «Ford Motor Co» is added to the Lookup
     of the Node which, because it is the currently selected Node,
     means that the Customer object for «Ford Motor Co» is now available in
     the global context. That is what is then passed to the resultChanged,
     causing the text fields to be populated.

     All of the above starts happening, i.e., the LookupListener
     becomes active, whenever the editor window is opened, as you can see below:

     @Override
     public void componentOpened() {
         result = Utilities.actionsGlobalContext().lookupResult(Customer.class);
         result.addLookupListener(this);
         resultChanged(new LookupEvent(result));
     }
     
     @Override
     public void componentClosed() {
         result.removeLookupListener(this);
         result = null;
     }

   Since the editor window is opened when the application starts, the
   LookupListener is available at the time that the application starts up.

   • Finally, declare the result variable at the top of the class, like this:

     private Lookup.Result result = null;

   • Run the application again and notice that the editor window
     is updated whenever you select a new Node:

     

     However, notice what happens when you switch the focus to the editor window:

     

     Because the Node is no longer current, the Customer object
     is no longer in the global context. This is the case because, as pointed out
     above, the global context proxies the Lookup of the current Node.
     Therefore, in this case, we cannot use the global context. Instead, we will
     use the local Lookup provided by the Customer window.

   Rewrite this line:

   result = Utilities.actionsGlobalContext().lookupResult(Customer.class);

   To this:

   result = WindowManager.getDefault().findTopComponent("CustomerTopComponent").getLookup().lookupResult(Customer.class);

   The string «CustomerTopComponent» is the ID of the CustomerTopComponent, which
   is a string constant that you can find in the source code of the CustomerTopComponent. One
   drawback of the approach above is that now our EditorTopComponent only works if it
   can find a TopComponent with the ID «CustomerTopComponent». Either this needs to
   be explicitly documented, so that developers of alternative editors can know that they
   need to identify the viewer TopComponent this way, or you need to rewrite the
   selection model, as described here by Tim Boudreau.

   If you take one of the above approaches, you will find that the context is not lost
   when you switch the focus to the EditorTopComponent, as shown below:

   

   Note: Since you are now using AbstractNode, instead of BeanNode,
   no properties are shown in the Properties window. You need to provide these yourself,
   as described in the Nodes API Tutorial.

7. Secondly, let’s work on the Undo/Redo functionality. What we’d
   like to have happen is that whenever the user makes a change to one
   of the JTextFields, the «Undo» button and the «Redo» button,
   as well as the related menu items in the Edit menu, become enabled. To
   that end, the NetBeans Platform makes the UndoRedo.Manager available.
   • Declare and instantiate a new UndoRedoManager at the top of the
     EditorTopComponent:

     private UndoRedo.Manager manager = new UndoRedo.Manager();

   • Next, override the getUndoRedo() method in the EditorTopComponent:

     @Override
     public UndoRedo getUndoRedo() {
         return manager;
     }

   • In the constructor of the EditorTopComponent, add
     a KeyListener to the JTextFields and, within
     the related methods that you need to implement, add the UndoRedoListeners:

     jTextField1.getDocument().addUndoableEditListener(manager);
     jTextField2.getDocument().addUndoableEditListener(manager);
     

   • Run the application and show the Undo and Redo functionality in action,
     the buttons as well as the menu items. The functionality works exactly as you would expect. You might want to
     change the KeyListener so that not ALL keys cause the undo/redo
     functionality to be enabled. For example, when Enter is pressed, you probably
     do not want the undo/redo functionality to become available. Therefore, tweak
     the code above to suit your business requirements.
8. Thirdly, we need to
   integrate with the NetBeans Platform’s Save functionality:
   • By default, the «Save All» button is available in the
     NetBeans Platform toolbar. In our current scenario, we do not
     want to save «all», because «all» refers to a number of different
     documents. In our case, we only have one «document», which is the
     editor that we are reusing for all the nodes in the tree hirerarchy.
     Remove the «Save All» button and add the «Save» button instead, by adding
     the following to the layer file of the CustomerEditor module:

     <folder name="Toolbars">
         <folder name="File">
             <file name="org-openide-actions-SaveAction.shadow">
                 <attr name="originalFile" stringvalue="Actions/System/org-openide-actions-SaveAction.instance"/>
                 <attr name="position" intvalue="444"/>
             </file>
             <file name="org-openide-actions-SaveAllAction.shadow_hidden"/>
         </folder>
     </folder>

   When you now run the application, you will see a different icon
   in the toolbar. Instead of the «Save All» button, you now have
   the «Save» button available.

   • Set dependencies on the Dialogs API and the Nodes API.
   • In the EditorTopCompontn constructor, add a
     call to fire a method (which will be defined in the next step)
     whenever a change is detected:

     public EditorTopComponent() {
     
             ...
             ...
             ...
     
             jTextField1.getDocument().addDocumentListener(new DocumentListener() {
                 public void insertUpdate(DocumentEvent arg0) {
                     fire(true);
                 }
                 public void removeUpdate(DocumentEvent arg0) {
                     fire(true);
                 }
                 public void changedUpdate(DocumentEvent arg0) {
                     fire(true);
                 }
             });
     
             jTextField2.getDocument().addDocumentListener(new DocumentListener() {
                 public void insertUpdate(DocumentEvent arg0) {
                     fire(true);
                 }
                 public void removeUpdate(DocumentEvent arg0) {
                     fire(true);
                 }
                 public void changedUpdate(DocumentEvent arg0) {
                     fire(true);
                 }
             });
     
             //Create a new instance of our SaveCookie implementation:
             impl = new SaveCookieImpl();
     
             //Create a new instance of our dynamic object:
             content = new InstanceContent();
     
             //Add the dynamic object to the TopComponent Lookup:
             associateLookup(new AbstractLookup(content));
     
         }
     
         ...
         ...
         ...
     

   • Here are the two methods referred to above. First, the method
     that is fired whenever a change is detected. An implementation of
     the SaveCookie from the Nodes API is added to the InstanceContent
     whenever a change is detected:

         public void fire(boolean modified) {
             if (modified) {
                 //If the text is modified,
                 //we add SaveCookie impl to Lookup:
                 content.add(impl);
             } else {
                 //Otherwise, we remove the SaveCookie impl from the lookup:
                 content.remove(impl);
             }
         }
     
         private class SaveCookieImpl implements SaveCookie {
     
             @Override
             public void save() throws IOException {
     
                Confirmation message = new NotifyDescriptor.Confirmation("Do you want to save \""
                         + jTextField1.getText() + " (" + jTextField2.getText() + ")\"?",
                         NotifyDescriptor.OK_CANCEL_OPTION,
                         NotifyDescriptor.QUESTION_MESSAGE);
     
                 Object result = DialogDisplayer.getDefault().notify(message);
                 //When user clicks "Yes", indicating they really want to save,
                 //we need to disable the Save action,
                 //so that it will only be usable when the next change is made
                 //to the JTextArea:
                 if (NotifyDescriptor.YES_OPTION.equals(result)) {
                     fire(false);
                     //Implement your save functionality here.
                 }
             }
         }
     

   • Run the application and notice the enablement/disablement of the
     Save button:

     

   Note: Right now, nothing happens when you click OK in the dialog
   above. In the next step, we add some JPA code for handling persistence of
   our changes.

   • Next, we add JPA code for persisting our change.
     Do so by replacing the comment «//Implement your save functionality here.»
     The comment should be replaced by all of the following:

     EntityManager entityManager = Persistence.createEntityManagerFactory("CustomerLibraryPU").createEntityManager();
     entityManager.getTransaction().begin();
     Customer c = entityManager.find(Customer.class, customer.getCustomerId());
     c.setName(jTextField1.getText());
     c.setCity(jTextField2.getText());
     entityManager.getTransaction().commit();

   Note: The «customer» in customer.getCustomerId() is currently undefined. Add the line
   «customer = cust;» in the resultChanged (as shown below), after declaring Customer customer; at the top
   of the class, so that the current Customer object sets the customer,
   which is then used in the persistence code above to obtain the ID of
   the current Customer object.

   @Override
   public void resultChanged(LookupEvent lookupEvent) {
       Lookup.Result r = (Lookup.Result) lookupEvent.getSource();
       Collection<Customer> c = r.allInstances();
       if (!c.isEmpty()) {
           for (Customer customer : c) {
               customer = cust;
               jTextField1.setText(customer.getName());
               jTextField2.setText(customer.getCity());
           }
       } else {
           jTextField1.setText("[no name]");
           jTextField2.setText("[no city]");
       }
   }

9. Run the application and change some data. Currently, we have no «Refresh»
   functionality (that will be added in the next step) so, to see the changed data, restart the application. Here, for
   example, the tree hierarchy shows the persisted customer name for «Toyota Motor Co»:

   

10. Fourthly, we need to
    add functionality for refreshing the Customer viewer. You might want to
    add a Timer which periodically refreshes the viewer.
    However, in this example, we will add
    a «Refresh» menu item to the Root node so that the user will be able
    to manually refresh the viewer.
    • In the main package of the CustomerViewer module,
      create a new Node, which will replace the AbstractNode
      that we are currently using as the root of the children in the
      viewer. Note that we also bind a «Refresh» action to our
      new root node.

      public class CustomerRootNode extends AbstractNode {
      
          public CustomerRootNode(Children kids) {
              super(kids);
              setDisplayName("Root");
          }
      
          @Override
          public Action[] getActions(boolean context) {
              Action[] result = new Action[]{
                  new RefreshAction()};
              return result;
          }
      
          private final class RefreshAction extends AbstractAction {
      
              public RefreshAction() {
                  putValue(Action.NAME, "Refresh");
              }
      
              public void actionPerformed(ActionEvent e) {
                  CustomerTopComponent.refreshNode();
              }
          }
      
      }

    • Add this method to the CustomerTopComponent, for refreshing
      the view:

      public static void refreshNode() {
          EntityManager entityManager = Persistence.createEntityManagerFactory("CustomerLibraryPU").createEntityManager();
          Query query = entityManager.createQuery("SELECT c FROM Customer c");
          List<Customer> resultList = query.getResultList();
          em.setRootContext(new CustomerRootNode(Children.create(new CustomerChildFactory(resultList), true)));
      } 

    Now replace the code above in the constructor
    of the CustomerTopComponent with a call to the above. As
    you can see, we are now using our CustomerRootNode instead
    of the AbstractNode. The CustomerRootNode includes
    the «Refresh» action, which calls the code above.

    • In your save functionality, add the call to the method above so that,
      whenever data is saved, an automatic refresh takes place. You can
      take different approaches when implementing this extension to
      the save functionality. For example, you might want to create a
      new module that contains the refresh action. That module would
      then be shared between the viewer module and the editor module,
      providing functionality that is common to both.
    • Run the application again and notice that you have a new root node,
      with a «Refresh» action:

      

    • Make a change to some data, save it, invoke the Refresh action, and notice
      that the viewer is updated.

You have now learned how to let the NetBeans Platform handle changes
to the JTextFields. Whenever the text changes, the NetBeans
Platform Undo and Redo buttons are enabled or disabled. Also, the
Save button is enabled and disabled correctly, letting the user
save changed data back to the database.

Create

In this section, you allow the user to create a new entry in the database.

1. Right-click the CustomerEditor
   module and choose «New Action».
   Use the New Action wizard to create a new «Always Enabled» action. The new
   action should be displayed anywhere in the toolbar and/or anywhere in
   the menu bar. In the next step of the wizard, call the action NewAction.

   Note: Make sure that
   you have a 16×16 icon available, which the wizard forces
   you to select if you indicate that you want the
   action to be invoked from the toolbar.

2. In the New action, let the TopComponent be opened,
   together with emptied JTextFields:

   import java.awt.event.ActionEvent;
   import java.awt.event.ActionListener;
   
   public final class NewAction implements ActionListener {
   
       public void actionPerformed(ActionEvent e) {
           EditorTopComponent tc = EditorTopComponent.getDefault();
           tc.resetFields();
           tc.open();
           tc.requestActive();
       }
   
   }

   Note: The action implements the ActionListener class, which
   is bound to the application via entries in the layer file, put there
   by the New Action wizard. Imagine how easy it will be when you
   port your existing Swing application to the NetBeans Platform, since
   you’ll simply be able to use the same Action classes that you
   used in your original application, without needing to rewrite them
   to conform to Action classes provided by the NetBeans Platform!

In the EditorTopComponent, add the following method for resetting
the JTextFields and creating a new Customer object:

public void resetFields() {
    customer = new Customer();
    jTextField1.setText("");
    jTextField2.setText("");
}

3. In the SaveCookie, ensure that a return of null
   indicates that a new entry is saved, instead of an existing entry
   being updated:

   public void save() throws IOException {
   
       Confirmation message = new NotifyDescriptor.Confirmation("Do you want to save \""
                       + jTextField1.getText() + " (" + jTextField2.getText() + ")\"?",
                       NotifyDescriptor.OK_CANCEL_OPTION,
                       NotifyDescriptor.QUESTION_MESSAGE);
   
       Object result = DialogDisplayer.getDefault().notify(msg);
   
       //When user clicks "Yes", indicating they really want to save,
       //we need to disable the Save button and Save menu item,
       //so that it will only be usable when the next change is made
       //to the text field:
       if (NotifyDescriptor.YES_OPTION.equals(result)) {
           fire(false);
           EntityManager entityManager = Persistence.createEntityManagerFactory("CustomerLibraryPU").createEntityManager();
           entityManager.getTransaction().begin();
           if (customer.getCustomerId() != null) {
               Customer c = entityManager.find(Customer.class, cude.getCustomerId());
               c.setName(jTextField1.getText());
               c.setCity(jTextField2.getText());
               entityManager.getTransaction().commit();
           } else {
               Query query = entityManager.createQuery("SELECT c FROM Customer c");
               List<Customer> resultList = query.getResultList();
               customer.setCustomerId(resultList.size()+1);
               customer.setName(jTextField1.getText());
               customer.setCity(jTextField2.getText());
               //add more fields that will populate all the other columns in the table!
               entityManager.persist(customer);
               entityManager.getTransaction().commit();
           }
       }
   
   }

4. Run the application again and add a new customer to the database.

Delete

In this section, let the user delete a selected entry in the database. Using
the concepts and code outlined above, implement the Delete action yourself.

1. Create a new action, DeleteAction. Decide whether you
   want to bind it to a Customer node or whether you’d rather bind it
   to the toolbar, the menu bar, keyboard shortcut, or combinations of
   these. Depending on where you want to bind
   it, you will need to use a different approach in your code. Read the
   article again for help, especially by looking at how the «New» action
   was created, while comparing it to the «Refresh» action on the root
   node.
2. Get the current Customer object, return an ‘Are you sure?’ dialog,
   and then delete the entry. For help on this point, read the article
   again, focusing on the part where the «Save» functionality is implemented.
   Instead of saving, you now want to delete an entry from the database.