Why JPanel.focusGaind and Lost don't work?

Question

Please take a look at the following code (I've missed the imports purposely)

public class MainFrame extends JFrame {
    private JPanel contentPane;
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    MainFrame frame = new MainFrame();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }
    public MainFrame() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 450, 300);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        setContentPane(contentPane);
        contentPane.setLayout(null);
        JTabbedPane tabbedPane = new JTabbedPane(JTabbedPane.TOP);
        tabbedPane.setBounds(10, 11, 414, 240);
        contentPane.add(tabbedPane);
        JPanel panel = new JPanel();
        panel.addFocusListener(new FocusListener() {

            @Override
            public void focusLost(FocusEvent arg0) {
                System.out.println("lost");
                // I want to do something here, if I reach here!
            }

            @Override
            public void focusGained(FocusEvent arg0) {
                System.out.println("gained");
                // I want to do something here, if I reach here!
            }
        });
        tabbedPane.addTab("New tab", null, panel, null);
        JButton button = new JButton("New button");
        panel.add(button);
        JPanel panel_1 = new JPanel();
        tabbedPane.addTab("New tab", null, panel_1, null);
        JPanel panel_2 = new JPanel();
        tabbedPane.addTab("New tab", null, panel_2, null);
    }
}

I've created this class to test it and then add the onFocusListener in my main code, but it's not working the way I expect. Please tell what's wrong or is this the right EvenetListener at all?

Answer 1

1. JPanels are not focusable by default. If you ever wanted to use a FocusListener on them, you'd first have to change this property via setFocusable(true).
2. But even if you do this, a FocusListener is not what you want.
3. Instead I'd look to listen to the JTabbedPane's model for changes. It uses a SingleSelectionModel, and you can add a ChangeListener to this model, listen for changes, check the component that is currently being displayed and if your component, react.
4. You are using setBounds and null layouts, something that you will want to avoid doing if you are planning on creating and maintaining anything more than a toy Swing program.

---

Edit
For example:

import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.Dimension;
import javax.swing.*;
import javax.swing.event.*;

@SuppressWarnings("serial")
public class MainPanel extends JPanel {
   private static final int PREF_W = 450;
   private static final int PREF_H = 300;
   private static final int GAP = 5;
   private static final int TAB_COUNT = 5;

   private JTabbedPane tabbedPane = new JTabbedPane();

   public MainPanel() {
      for (int i = 0; i < TAB_COUNT; i++) {
         JPanel panel = new JPanel();
         panel.add(new JButton("Button " + (i + 1)));
         panel.setName("Panel " + (i + 1));
         tabbedPane.add(panel.getName(), panel);         
      }

      setBorder(BorderFactory.createEmptyBorder(GAP, GAP, GAP, GAP));
      setLayout(new BorderLayout());
      add(tabbedPane, BorderLayout.CENTER);

      tabbedPane.getModel().addChangeListener(new ChangeListener() {

         @Override
         public void stateChanged(ChangeEvent evt) {
            Component component = tabbedPane.getSelectedComponent();
            System.out.println("Component Selected: " + component.getName());
         }
      });
   }

   @Override
   public Dimension getPreferredSize() {
      return new Dimension(PREF_W, PREF_H);
   }

   private static void createAndShowGui() {
      MainPanel mainPanel = new MainPanel();

      JFrame frame = new JFrame("MainPanel");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

Answer 2

JPanel is a lightweight container and it is not a Actionable component so it does not get focus events. It lets you add focus listener because of swing component hierarchy. In Order to get tab selected events you need to use JTabbedPane#addChangeListener.

Hope this helps.