Add Undo property to JTextPane

Question

Hi
I am getting problem for adding UNDO functionality to jtextpane.My requirement is to undo or restore the data by clicking a button.
According to my application requirement i have to add multiple(as many as required) panels to a scroll pane dynamically.And these panels consist of a text pane and a combo box.when user will click on the undo button the data in the text pane should be restored.
For this i made a panel class(DynamicPanels in the code below which contains the text pane and combo boxes) and creating array of panel classes and adding these panels through loop.
The problem here is , as i am adding the panels dynamically. while trying to add any logic for undo in side the loop it is giving class cast exception ( by applying logic in different way null pointer exception as here the instances are created dynamically i.e runtime).
Please help me to overcome this issue.
Thanks.
//UndoInTextPaneFrame.java
import javax.swing.*;
import javax.swing.event.UndoableEditEvent;
import javax.swing.event.UndoableEditListener;
import javax.swing.text.Document;
import javax.swing.undo.CannotUndoException;
import javax.swing.undo.UndoManager;
public class UndoInTextPaneFrame extends JFrame{
    final static int NO_OFPANELS=10;
    DynamicPanels[] arrayOfPanels=new DynamicPanels[NO_OFPANELS];
    private javax.swing.JButton jButton1;
    private javax.swing.JPanel jPanel1;
    private javax.swing.JScrollPane jScrollPane1;
    public UndoInTextPaneFrame(){
        jScrollPane1 = new javax.swing.JScrollPane();
        jPanel1 = new javax.swing.JPanel();
        jButton1 = new javax.swing.JButton("UNDO");
        jButton1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                System.out.println("UNDO button clicked");
        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setTitle("Frame to add UNDO property");
        setSize(400,300);
        getContentPane().setLayout(new javax.swing.BoxLayout(getContentPane(), javax.swing.BoxLayout.Y_AXIS));
        jScrollPane1.setHorizontalScrollBarPolicy(javax.swing.ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
        jScrollPane1.setVerticalScrollBarPolicy(javax.swing.ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
        jPanel1.setLayout(new javax.swing.BoxLayout(jPanel1, javax.swing.BoxLayout.Y_AXIS));
        for(int i=0;i<arrayOfPanels.length;i++){
            arrayOfPanels= new DynamicPanels(i);
jPanel1.add(arrayOfPanels[i]);
//LOGIC FOR UNDO
// JTextPane txtPane=(JTextPane)arrayOfPanels[i].getComponent(0);
jScrollPane1.setViewportView(jPanel1);
getContentPane().add(jScrollPane1);
getContentPane().add(jButton1);
public static void main(String args[]) {
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
new UndoInTextPaneFrame().setVisible(true);
class DynamicPanels extends JPanel{
final UndoManager undo;
private JTextPane jTextPane1;
private JComboBox jComboBox1;
private JScrollPane jScrollPane1;
public DynamicPanels(int number) {
jTextPane1 = new JTextPane();
jComboBox1=new JComboBox();
jScrollPane1=new JScrollPane();
Document doc = jTextPane1.getDocument();
undo= new UndoManager();
// Listen for undo and redo events
doc.addUndoableEditListener(new UndoableEditListener() {
public void undoableEditHappened(UndoableEditEvent evt) {
undo.addEdit(evt.getEdit());
try {
if (undo.canUndo()) {
undo.undo();
} catch (CannotUndoException e) {
jComboBox1.setModel(new javax.swing.DefaultComboBoxModel(new String[] { "Item 1", "Item 2", "Item 3", "Item 4" }));
jScrollPane1.setViewportView(jTextPane1);
setLayout(new javax.swing.BoxLayout(this, javax.swing.BoxLayout.X_AXIS));
add(jScrollPane1);
add(jComboBox1);

Answer 1

https://forums.oracle.com/forums/thread.jspa?threadID=2469362&tstart=0