本题使用下拉菜单来控制字体，窗口中有一个标签和一个下拉菜单，当选中下拉菜单中的任一项

问题：

本题使用下拉菜单来控制字体，窗口中有一个标签和一个下拉菜单，当选中下拉菜单中的任一项字体时，标签上字符串的字体就随之改变。
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
class ComboBoxFrame extends JFrame______
public ComboBoxFrame()
setTitle("java2");
setSize(300,200);
addWindowListener(new WindowAdapter()
public void windowClosing(WindowEvent e)
System.exit(0);

);
style=new JComboBox();
style.setEditable(true);
style.addItem("Serif");
style.addItem("SansSerif");
style.addItem("Monospaced");
style.addItem("Dialog");
style.addItem("DialogInput");
style.addActionListener(this);
JPanel p=new JPanel();
p.add(style);
getContentPane().add(p,"South");
panel=new ComboBoxTestPanel();
getContentPane().add(panel,"Center");

public void actionPerformed(ActionEvent evt)
JComboBox source=(JComboBox)______;
String item=(String)source.getSelectedItem();
panel.setStyle(item);

private ComboBoxTestPanel panel;
private JComboBox style;

class ComboBoxTestPanel extends JPanel
public ComboBoxTestPanel()
setStyle("Serif");

public void setStyle(String s)
setFont(new Font(s,Font.PLAIN,12));
repaint();

public void paintComponent(Graphics g)
super.paintComponent(g);
g.drawstring("Welcome to China!",0,50);


public class java2
public static void main(String[]args)
JFrame frame=new ComboBoxFrame();
frame.show();