su una label ho messo questo evento qua:
codice:
    private void jLabel12MouseClicked(java.awt.event.MouseEvent evt) {                                      
        try {
            panelAuthor.setVisible(true);
            panelAdd.setVisible(false);
            panelTable.setVisible(false);
            panelEditor.setVisible(false);
            fillCBAuthor();
        } catch (SQLException ex) {
            JOptionPane.showMessageDialog(btnConnect, ex.getMessage());
        } catch (ClassNotFoundException ex) {
            JOptionPane.showMessageDialog(btnConnect, ex.getMessage());
        }
    }
dove fillCBAuthor è questo:
codice:
    private void fillCBAuthor() throws SQLException, ClassNotFoundException {
        comboUpAuthor.removeAllItems();
        comboUpAuthor.addItem("*");
        DoFillCBUPAuthor.fillAuthor();
    }
la combobox ha questo evento:
codice:
    private void comboUpAuthorItemStateChanged(java.awt.event.ItemEvent evt) {
        Object obj = comboUpAuthor.getSelectedItem();
        if (obj == "*") {
            textUpAuthor.setText("");
        } else {
            textUpAuthor.setText(obj.toString());
        }
    }
in pratica succede che se clicco una volta sulla label funziona tutto.
compreso l'itemstatechange e l'altro evento sul pulsante (che nn ho postato perchè nn serve).
se però clicco due volte sulla label la combo si svuota e in netbeans appare questo:
codice:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at libreria.FramePrincipale.comboUpAuthorItemStateChanged(FramePrincipale.java:686)
        at libreria.FramePrincipale.access$900(FramePrincipale.java:9)
        at libreria.FramePrincipale$9.itemStateChanged(FramePrincipale.java:313)
        at javax.swing.JComboBox.fireItemStateChanged(JComboBox.java:1205)
        at javax.swing.JComboBox.selectedItemChanged(JComboBox.java:1253)
        at javax.swing.JComboBox.contentsChanged(JComboBox.java:1309)
        at javax.swing.JComboBox.intervalRemoved(JComboBox.java:1331)
        at javax.swing.AbstractListModel.fireIntervalRemoved(AbstractListModel.java:161)
        at javax.swing.DefaultComboBoxModel.removeAllElements(DefaultComboBoxModel.java:169)
        at javax.swing.JComboBox.removeAllItems(JComboBox.java:751)
        at libreria.FramePrincipale.fillCBAuthor(FramePrincipale.java:700)
        at libreria.FramePrincipale.jLabel12MouseClicked(FramePrincipale.java:589)
        at libreria.FramePrincipale.access$400(FramePrincipale.java:9)
        at libreria.FramePrincipale$4.mouseClicked(FramePrincipale.java:107)
        at java.awt.Component.processMouseEvent(Component.java:6270)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
        at java.awt.Component.processEvent(Component.java:6032)
        at java.awt.Container.processEvent(Container.java:2041)
        at java.awt.Component.dispatchEventImpl(Component.java:4630)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4247)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
        at java.awt.Container.dispatchEventImpl(Container.java:2085)
        at java.awt.Window.dispatchEventImpl(Window.java:2478)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)