ActionListener in un JTabbedPane

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import javax.swing.JList;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTabbedPane;
import javax.swing.JTree;
import javax.swing.SwingUtilities;
import javax.swing.UIManager;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.DefaultTreeModel;

public class Test extends JFrame implements ActionListener
{
public Test()
{
buildGUI();
}

private void buildGUI()
{
setDefaultCloseOperation(EXIT_ON_CLOSE);
UIManager.put("TabbedPane.contentOpaque", Boolean.FALSE);
JTabbedPane mainTab = new JTabbedPane();
mainTab.setOpaque(false);
mainTab.addTab("Visualizzazione ad albero", buildTreeView());
mainTab.addTab("Visualizzazione a lista", buildListView());
getContentPane().add(mainTab, BorderLayout.CENTER);
// pack(); // Il risultato e' brutto. Metto le dimensioni preferite
setSize(1280, 720);
setLocationRelativeTo(null);

}

private JPanel buildListView()
{
String[] countries = {
"Italia",
"San Marino",
"Vaticano",
"Andorra",
"Principato di Monaco",
"Repubblica Ceca"
};
JPanel panel = new JPanel();
panel.setLayout(new BorderLayout());
JList list = new JList(countries);
panel.add(new JScrollPane(list));
return panel;
}

private JPanel buildTreeView()
{
JPanel panel = new JPanel();
panel.setLayout(new BorderLayout());
DefaultTreeModel treeModel = new DefaultTreeModel(buildTreeNode());
JTree tree= new JTree(treeModel);
panel.add(new JScrollPane(tree), BorderLayout.CENTER);
return panel;
}

private DefaultMutableTreeNode buildTreeNode()
{
DefaultMutableTreeNode root = new DefaultMutableTreeNode("Europa");
DefaultMutableTreeNode rootMediterranean = new DefaultMutableTreeNode("Mediterraneo");
DefaultMutableTreeNode rootNorthern = new DefaultMutableTreeNode("Paesi nordici");
DefaultMutableTreeNode rootMittle = new DefaultMutableTreeNode("MittleEuropa");
DefaultMutableTreeNode rootEast = new DefaultMutableTreeNode("Europa dell'Est");

root.add(rootMediterranean);
root.add(rootNorthern);
root.add(rootMittle);
root.add(rootEast);

DefaultMutableTreeNode nodeItaly = new DefaultMutableTreeNode("Italia");
DefaultMutableTreeNode nodeSpain = new DefaultMutableTreeNode("Spagna");
DefaultMutableTreeNode rootPortugal = new DefaultMutableTreeNode("Europa");
rootMediterranean.add(nodeItaly);
rootMediterranean.add(nodeSpain);
rootMediterranean.add(rootPortugal);

DefaultMutableTreeNode nodeDenmark = new DefaultMutableTreeNode ("Danimarca");
DefaultMutableTreeNode nodeNorway = new DefaultMutableTreeNode ("Norvegia");
DefaultMutableTreeNode nodeSweden = new DefaultMutableTreeNode ("Svezia");
rootNorthern.add(nodeDenmark);
rootNorthern.add(nodeNorway);
rootNorthern.add(nodeSweden);

DefaultMutableTreeNode nodeFrance = new DefaultMutableTreeNode ("France");
DefaultMutableTreeNode nodeGermany = new DefaultMutableTreeNode ("Germany");
DefaultMutableTreeNode nodeSwitzerland = new DefaultMutableTreeNode ("Switzerland");

rootMittle.add(nodeFrance);
rootMittle.add(nodeGermany);
rootMittle.add(nodeSwitzerland);

DefaultMutableTreeNode nodePoland = new DefaultMutableTreeNode ("Polonia");
DefaultMutableTreeNode nodeEstonia = new DefaultMutableTreeNode ("Estonia");
DefaultMutableTreeNode nodeLettonia = new DefaultMutableTreeNode ("Latvia");
rootEast.add(nodePoland);
rootEast.add(nodeEstonia);
rootEast.add(nodeLettonia);
return root;
}

/**
* @param args the command line arguments
*/
public static void main(String[] args)
{
SwingUtilities.invokeLater(() -> {
new Test().setVisible(true);
});
}

@Override
public void actionPerformed(ActionEvent ae) {



}

}