Question

I’m using a JTabbedPane and want to do some field validation at the moment the user selects another Tab. Potentially, depending on their reply to a messagebox, keep them on the same tab to correct bad data.
I’m implementing the ChangeListener interface and handling stateChanged events however getSelectedIndex() returns the newly selected tab.
Is there easy way to
1) Determine which was the previously selected Tab and
2) Prevent the user from going to the newly selected Tab in the case of data validation failure? A stateChanged event cannot be consumed.
Thanks in advance,

Answer 1

Hmm you could try something like this which is basically adding a ChangeListener to monitor current tab and to keep previous tab selection:

public class JTabbedPaneExample extends JFrame {

    private JTabbedPane tabbedPane;
    private JPanel panel1, panel2, panel3;
    private int currentTabIndex = 0, previousTabIndex = 0;

    public JTabbedPaneExample() {
        createAndShowUI();
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                JTabbedPaneExample jTabbedPaneExample = new JTabbedPaneExample();
            }
        });
    }

    private void createAndShowUI() {
        setTitle("JTabbedPaneExample");
        setSize(300, 300);
        setResizable(false);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);
        addComponentsToContentPane(getContentPane());
        addListeners();
        //pack();
        setVisible(true);
    }

    private void addComponentsToContentPane(Container contentPane) {
        tabbedPane = new JTabbedPane();
        panel1 = new JPanel();
        panel2 = new JPanel();
        panel3 = new JPanel();
        tabbedPane.insertTab("tab1", null, panel1, null, 0);
        tabbedPane.insertTab("tab2", null, panel2, null, 1);
        tabbedPane.insertTab("tab3", null, panel3, null, 2);
        contentPane.add(tabbedPane);
    }

    private void addListeners() {
        tabbedPane.addChangeListener(new ChangeListener() {

            @Override
            public void stateChanged(ChangeEvent ce) {
                previousTabIndex = currentTabIndex;
                currentTabIndex = tabbedPane.getSelectedIndex();
                System.out.println("Current tab is:" + currentTabIndex);
                System.out.println("Previous tab is:" + previousTabIndex);
            }
        });
    }
}

EDIT: here is a newer version which will only allow a tab change if the validation variable is set to ‘true’ this is done via a button click just to show the logic:

public class JTabbedPaneExample extends JFrame {

    private JTabbedPane tabbedPane;
    private JPanel panel1, panel2, panel3;
    private int currentTabIndex = 0, previousTabIndex = 0;
    private boolean valid = false;
    private JButton changeVariableBtn;

    public JTabbedPaneExample() {
        createAndShowUI();
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                JTabbedPaneExample jTabbedPaneExample = new JTabbedPaneExample();
            }
        });
    }

    private void createAndShowUI() {
        setTitle("JTabbedPaneExample");
        setSize(300, 300);
        setResizable(false);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);
        addComponentsToContentPane(getContentPane());
        addListeners();
        //pack();
        setVisible(true);
    }

    private void addComponentsToContentPane(Container contentPane) {
        tabbedPane = new JTabbedPane();
        panel1 = new JPanel();
        panel2 = new JPanel();
        panel3 = new JPanel();
        changeVariableBtn = new JButton("Set validation to true");
        tabbedPane.insertTab("tab1", null, panel1, null, 0);
        tabbedPane.insertTab("tab2", null, panel2, null, 1);
        tabbedPane.insertTab("tab3", null, panel3, null, 2);
        contentPane.add(tabbedPane, BorderLayout.NORTH);
        contentPane.add(changeVariableBtn, BorderLayout.CENTER);
    }

    private void addListeners() {
        tabbedPane.addChangeListener(new ChangeListener() {

            private boolean automatedStateChange = false;

            @Override
            public void stateChanged(ChangeEvent ce) {
                //this is used so when we allow the user not to go to the new tab by setting the tabb index to the previous one we dont want our changelistener to fire again as if the user were changing the tabs
                if (automatedStateChange) {
                    automatedStateChange = false;
                } else {
                    previousTabIndex = currentTabIndex;
                    currentTabIndex = tabbedPane.getSelectedIndex();
                }

                if (valid) {
                    System.out.println("Current tab is:" + currentTabIndex);
                    System.out.println("Previous tab is:" + previousTabIndex);
                    System.out.println("Validation succeeded: " + valid);
                    changeVariableBtn.doClick();
                } else {
                    System.out.println("You need to enter all valid data first!");
                    automatedStateChange = true;
                    tabbedPane.setSelectedIndex(previousTabIndex);
                }
            }
        });
        changeVariableBtn.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent ae) {
                if (valid) {
                    valid = false;
                    changeVariableBtn.setText("Set validation to true");
                } else {
                    valid = true;
                    changeVariableBtn.setText("Set validation to false");
                }
            }
        });
    }
}