//this is an example in java tutorial:

import javax.swing.*;

import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.GridBagLayout;
import java.awt.GridBagConstraints;

import java.awt.event.ContainerEvent;
import java.awt.event.ContainerListener;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import java.util.Vector;

public class ContainerEventDemo extends JPanel
implements ContainerListener,
ActionListener {
JTextArea display;
JPanel buttonPanel;
JButton addButton, removeButton, clearButton;
Vector buttonList;
static final String ADD = "add";
static final String REMOVE = "remove";
static final String CLEAR = "clear";
static final String newline = "\n";

public ContainerEventDemo() {
super(new GridBagLayout());
GridBagLayout gridbag = (GridBagLayout)getLayout();
GridBagConstraints c = new GridBagConstraints();

//Initialize an empty list of buttons.
buttonList = new Vector(10, 10);

//Create all the components.
addButton = new JButton("Add a button");
addButton.setActionCommand(ADD);
addButton.addActionListener(this);

removeButton = new JButton("Remove a button");
removeButton.setActionCommand(REMOVE);
removeButton.addActionListener(this);

buttonPanel = new JPanel(new GridLayout(1,1));
buttonPanel.setPreferredSize(new Dimension(200, 75));
buttonPanel.addContainerListener(this);

display = new JTextArea();
display.setEditable(false);
JScrollPane scrollPane = new JScrollPane(display);
scrollPane.setPreferredSize(new Dimension(200, 75));

clearButton = new JButton("Clear text area");
clearButton.setActionCommand(CLEAR);
clearButton.addActionListener(this);

c.fill = GridBagConstraints.BOTH; //Fill entire cell.
c.weighty = 1.0; //Button area and message area have equal height.
c.gridwidth = GridBagConstraints.REMAINDER; //end of row
gridbag.setConstraints(scrollPane, c);
add(scrollPane);

c.weighty = 0.0;
gridbag.setConstraints(clearButton, c);
add(clearButton);

c.weightx = 1.0; //Add/remove buttons have equal width.
c.gridwidth = 1; //NOT end of row
gridbag.setConstraints(addButton, c);
add(addButton);

c.gridwidth = GridBagConstraints.REMAINDER; //end of row
gridbag.setConstraints(removeButton, c);
add(removeButton);

c.weighty = 1.0; //Button area and message area have equal height.
gridbag.setConstraints(buttonPanel, c);
add(buttonPanel);

setPreferredSize(new Dimension(400, 400));
setBorder(BorderFactory.createEmptyBorder(20,20,20,20));
}

public void componentAdded(ContainerEvent e) {
displayMessage(" added to ", e);
}

public void componentRemoved(ContainerEvent e) {
displayMessage(" removed from ", e);
}

void displayMessage(String action, ContainerEvent e) {
display.append(((JButton)e.getChild()).getText() //public Component getChild()
+ " was" //Returns the component that was affected by the event.
+ action
+ e.getContainer().getClass().getName() //public Container getContainer()
+ newline);
display.setCaretPosition(display.getDocument().getLength());
}

/*
* This could have been implemented as two or three
* classes or objects, for clarity.
*/
public void actionPerformed(ActionEvent e) {
String command = e.getActionCommand();

if (ADD.equals(command)) {
JButton newButton = new JButton("JButton #"
+ (buttonList.size() + 1));
buttonList.addElement(newButton);
buttonPanel.add(newButton);
buttonPanel.revalidate(); //Make the button show up.

} else if (REMOVE.equals(command)) {
int lastIndex = buttonList.size() - 1;
try {
JButton nixedButton = (JButton)buttonList.elementAt(lastIndex);
buttonPanel.remove(nixedButton);
buttonList.removeElementAt(lastIndex);
buttonPanel.revalidate(); //Make the button disappear.
buttonPanel.repaint(); //Make the button disappear. line:127
//java.awt.Component public void repaint()
} catch (ArrayIndexOutOfBoundsException exc) {}
} else if (CLEAR.equals(command)) {
display.setText("");
}
}

/**
* Create the GUI and show it. For thread safety,
* this method should be invoked from the
* event-dispatching thread.
*/
private static void createAndShowGUI() {
//Make sure we have nice window decorations.
JFrame.setDefaultLookAndFeelDecorated(true);

//Create and set up the window.
JFrame frame = new JFrame("ContainerEventDemo");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

//Create and set up the content pane.
JComponent newContentPane = new ContainerEventDemo();
newContentPane.setOpaque(true); //content panes must be opaque
frame.setContentPane(newContentPane);

//Display the window.
frame.pack();
frame.setVisible(true);
}

public static void main(String[] args) {
//Schedule a job for the event-dispatching thread:
//creating and showing this application's GUI.
javax.swing.SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGUI();
}
});
}
}
Masters:
Now this program can remove all the buttons that are added, but if you comment the 127th line, the last button cannot be removed! Why?
Please help me!!