Thread: Hello, why have an error? someone help me

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import javax.swing.border.*;

public class Exercise16_10 extends JFrame implements ActionListener {
// Declare a panel for displaying message

private MessagePanel messagePanel;
// Declare two buttons to move the message left and right
private JButton jbtLeft, jbtRight;
private JTextField jtfNewMessage = new JTextField(8);
private JComboBox jcboInterval = new JComboBox();
private JRadioButton jrbRed = new JRadioButton("Red");
private JRadioButton jrbGreen = new JRadioButton("Green");
private JRadioButton jrbBlue = new JRadioButton("Blue");
private JCheckBox jchkCentered = new JCheckBox("Center");
private JCheckBox jchkBold = new JCheckBox("Bold");
private JCheckBox jchkItalic = new JCheckBox("Italic");
// Font name
private String fontName = "SansSerif";
// Font style
private int fontStyle = Font.PLAIN;
// Font Size
private int fontSize = 12;

/** Main method */
public static void main(String[] args) {
Exercise16_10 frame = new Exercise16_10();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOS E);
frame.setSize(500, 200);
frame.setVisible(true);
}

/** Default constructor */
public Exercise16_10() {
setTitle("Exercise16_10");

// Create a MessagePanel instance and set colors
/*messagePanel = new MessagePanel("Welcome to Java");
messagePanel.setBackground(Color.white);*/

// Create Panel jpButtons to hold two Buttons "<=" and "right =>"
JPanel jpButtons = new JPanel();
jpButtons.setLayout(new FlowLayout());
jpButtons.add(jbtLeft = new JButton());
jpButtons.add(jbtRight = new JButton());

// Set button text
jbtLeft.setText("<=");
jbtRight.setText("=>");

// Set keyboard mnemonics
jbtLeft.setMnemonic('L'); // When a key is pressed on the keyboard,
//only the menu item that has the mnemonic matching the pressed key will fire an action event.
jbtRight.setMnemonic('R');

// Set icons
//jbtLeft.setIcon(new ImageIcon("image/left.gif"));
//jbtRight.setIcon(new ImageIcon("image/right.gif"));

// Set toolTipText on the "<=" and "=>" buttons
jbtLeft.setToolTipText("Move message to left");
jbtRight.setToolTipText("Move message to right");

// Place panels in the frame
getContentPane().setLayout(new BorderLayout());
getContentPane().add(messagePanel, BorderLayout.CENTER);
getContentPane().add(jpButtons, BorderLayout.SOUTH);

// Register listeners with the buttons
jbtLeft.addActionListener(this);
jbtRight.addActionListener(this);

/** 1.Add a text field labeled “New Message.\uFFFD
* Upon typing a new message in the text field and pressing the Enter
* key, the new message is displayed in the message panel.
*/
jpButtons.add(new JLabel("Enter a new message"));
jpButtons.add(jtfNewMessage);

jtfNewMessage.addActionListener(this);

/** 2. Add a combo box label “Interval\uFFFD that enables the user to select a
* new interval for moving the message. The selection values range from
* 10 to 100 with interval 5. The user can also type a new
* interval in the combo box.
*/
jpButtons.add(new JLabel("Select an interval"));
jpButtons.add(jcboInterval);
for (int interval = 5; interval <= 100; interval += 5) {
jcboInterval.addItem(interval + "");
}

jcboInterval.addActionListener(this);

/**
* 3. Add three radio buttons that enable the user to select the foreground
* color for the message as Red, Green, and Blue.
*/
JPanel panel = new JPanel();
getContentPane().add(panel, BorderLayout.NORTH);

panel.add(jrbRed);
panel.add(jrbGreen);
panel.add(jrbBlue);
ButtonGroup btg = new ButtonGroup();
btg.add(jrbRed);
btg.add(jrbGreen);
btg.add(jrbBlue);
jrbRed.addActionListener(this);
jrbGreen.addActionListener(this);
jrbBlue.addActionListener(this);

/**
* 4. Add three check boxes that enable the user to center the message
* and display it in italic or bold.
*/
panel.add(jchkCentered);
panel.add(jchkBold);
panel.add(jchkItalic);
jchkCentered.addActionListener(this);
jchkBold.addActionListener(this);
jchkItalic.addActionListener(this);

/**
* 5. Add a border titled “Message Panel\uFFFD on the message panel.
*/
//messagePanel.setBorder(new TitledBorder("Message Panel"));
jpButtons.setBorder(new TitledBorder("South Panel"));
panel.setBorder(new TitledBorder("North Panel"));

this.setCursor(new Cursor(Cursor.CROSSHAIR_CURSOR));
}

/** Handle button events */
public void actionPerformed(ActionEvent e) {
if (e.getSource() == jbtLeft) {
messagePanel.moveLeft();
messagePanel.repaint();
} else if (e.getSource() == jbtRight) {
messagePanel.moveRight();
messagePanel.repaint();
} else if (e.getSource() == jtfNewMessage) {
messagePanel.setMessage(jtfNewMessage.getText());
messagePanel.repaint();
} else if (e.getSource() == jcboInterval) {
messagePanel.setInterval(
Integer.parseInt((String) (jcboInterval.getSelectedItem())));
messagePanel.repaint();
} else if (e.getSource() == jrbRed) {
messagePanel.setForeground(Color.red);
} else if (e.getSource() == jrbGreen) {
messagePanel.setForeground(Color.green);
} else if (e.getSource() == jrbBlue) {
messagePanel.setForeground(Color.blue);
} else if (e.getSource() == jchkCentered) {
if (jchkCentered.isSelected()) {
messagePanel.setCentered(true);
} else {
messagePanel.setCentered(false);
}

messagePanel.repaint();
} else if ((e.getSource() == jchkBold) ||
(e.getSource() == jchkItalic)) {
fontStyle = Font.PLAIN;

// Determine a font style
if (jchkBold.isSelected()) {
fontStyle = fontStyle + Font.BOLD;
}
if (jchkItalic.isSelected()) {
fontStyle = fontStyle + Font.ITALIC;
}

// Set font for the message
messagePanel.setFont(new Font(fontName, fontStyle, fontSize));
}
}
}