      /**
       * The constants used in this Content Widget.
       */
      public static interface CwConstants extends Constants,
          ContentWidget.CwConstants {
        String[] cwStackPanelContacts();

        String[] cwStackPanelContactsEmails();

        String cwStackPanelContactsHeader();

        String cwStackPanelDescription();

        String[] cwStackPanelFilters();

        String cwStackPanelFiltersHeader();

        String[] cwStackPanelMailFolders();

        String cwStackPanelMailHeader();

        String cwStackPanelName();
      }

      /**
       * Specifies the images that will be bundled for this example.
       *
       * We will override the leaf image used in the tree. Instead of using a blank
       * 16x16 image, we will use a blank 1x1 image so it does not take up any
       * space. Each TreeItem will use its own custom image.
       */
      public interface Images extends Tree.Resources {
        ImageResource contactsgroup();

        ImageResource defaultContact();

        ImageResource drafts();

        ImageResource filtersgroup();

        ImageResource inbox();

        ImageResource mailgroup();

        ImageResource sent();

        ImageResource templates();

        ImageResource trash();

        /**
         * Use noimage.png, which is a blank 1x1 image.
         */
        @Source("noimage.png")
        ImageResource treeLeaf();
      }

      /**
       * An instance of the constants.
       */
      private CwConstants constants;

      /**
       * Initialize this example.
       */
      @Override
      public Widget onInitialize() {
        // Get the images
        Images images = (Images) GWT.create(Images.class);

        // Create a new stack panel
        DecoratedStackPanel stackPanel = new DecoratedStackPanel();
        stackPanel.setWidth("200px");

        // Add the Mail folders
        String mailHeader = getHeaderString(constants.cwStackPanelMailHeader(),
            images.mailgroup());
        stackPanel.add(createMailItem(images), mailHeader, true);

        // Add a list of filters
        String filtersHeader = getHeaderString(
            constants.cwStackPanelFiltersHeader(), images.filtersgroup());
        stackPanel.add(createFiltersItem(), filtersHeader, true);

        // Add a list of contacts
        String contactsHeader = getHeaderString(
            constants.cwStackPanelContactsHeader(), images.contactsgroup());
        stackPanel.add(createContactsItem(images), contactsHeader, true);

        // Return the stack panel
        stackPanel.ensureDebugId("cwStackPanel");
        return stackPanel;
      }

      /**
       * Create the list of Contacts.
       *
       * @param images the {@link Images} used in the Contacts
       * @return the list of contacts
       */
      private VerticalPanel createContactsItem(Images images) {
        // Create a popup to show the contact info when a contact is clicked
        HorizontalPanel contactPopupContainer = new HorizontalPanel();
        contactPopupContainer.setSpacing(5);
        contactPopupContainer.add(new Image(images.defaultContact()));
        final HTML contactInfo = new HTML();
        contactPopupContainer.add(contactInfo);
        final PopupPanel contactPopup = new PopupPanel(true, false);
        contactPopup.setWidget(contactPopupContainer);

        // Create the list of contacts
        VerticalPanel contactsPanel = new VerticalPanel();
        contactsPanel.setSpacing(4);
        String[] contactNames = constants.cwStackPanelContacts();
        String[] contactEmails = constants.cwStackPanelContactsEmails();
        for (int i = 0; i < contactNames.length; i++) {
          final String contactName = contactNames[i];
          final String contactEmail = contactEmails[i];
          final HTML contactLink = new HTML("<a href=\"javascript:undefined;\">"
              + contactName + "</a>");
          contactsPanel.add(contactLink);

          // Open the contact info popup when the user clicks a contact
          contactLink.addClickHandler(new ClickHandler() {
            public void onClick(ClickEvent event) {
              // Set the info about the contact
              contactInfo.setHTML(contactName + "<br><i>" + contactEmail + "</i>");

              // Show the popup of contact info
              int left = contactLink.getAbsoluteLeft() + 14;
              int top = contactLink.getAbsoluteTop() + 14;
              contactPopup.setPopupPosition(left, top);
              contactPopup.show();
            }
          });
        }
        return contactsPanel;
      }

      /**
       * Create the list of filters for the Filters item.
       *
       * @return the list of filters
       */
      private VerticalPanel createFiltersItem() {
        VerticalPanel filtersPanel = new VerticalPanel();
        filtersPanel.setSpacing(4);
        for (String filter : constants.cwStackPanelFilters()) {
          filtersPanel.add(new CheckBox(filter));
        }
        return filtersPanel;
      }

      /**
       * Create the {@link Tree} of Mail options.
       *
       * @param images the {@link Images} used in the Mail options
       * @return the {@link Tree} of mail options
       */
      private Tree createMailItem(Images images) {
        Tree mailPanel = new Tree(images);
        TreeItem mailPanelRoot = mailPanel.addItem("foo@example.com");
        String[] mailFolders = constants.cwStackPanelMailFolders();
        addItem(mailPanelRoot, images.inbox(), mailFolders[0]);
        addItem(mailPanelRoot, images.drafts(), mailFolders[1]);
        addItem(mailPanelRoot, images.templates(), mailFolders[2]);
        addItem(mailPanelRoot, images.sent(), mailFolders[3]);
        addItem(mailPanelRoot, images.trash(), mailFolders[4]);
        mailPanelRoot.setState(true);
        return mailPanel;
      }

      /**
       * Get a string representation of the header that includes an image and some
       * text.
       *
       * @param text the header text
       * @param image the {@link ImageResource} to add next to the header
       * @return the header as a string
       */
      private String getHeaderString(String text, ImageResource image) {
        // Add the image and text to a horizontal panel
        HorizontalPanel hPanel = new HorizontalPanel();
        hPanel.setSpacing(0);
        hPanel.setVerticalAlignment(HasVerticalAlignment.ALIGN_MIDDLE);
        hPanel.add(new Image(image));
        HTML headerText = new HTML(text);
        headerText.setStyleName("cw-StackPanelHeader");
        hPanel.add(headerText);

        // Return the HTML string for the panel
        return hPanel.getElement().getString();
      }