填空题 本题的功能是通过按钮来选择窗口显示的风格。窗口中有三个按钮：“Metal”、“Motif和“WindoWS“，单击任何一个按钮，就能将窗口的风格改变为按钮名称所对应的风格。
import java.awt*;
import java.awt.event.*;
import javax.swing.*;
class PlafPanel extends JPanel implements Actionlis-tener
public______()
metalButton=new JButton("Metal");
motifButton=new JButton("Motif");
windowsButton=new JButton("Windows");
add(metalButton);
add(motifButton);
add(windowsButton);
metalButlon.addActionListener(this);
motifButton.addActionListener(this);
windowsButton.addActionListener(this);

public void actionPerformed(ActionEvent evt)
Object source=evt.getSource();
string plaf="";
if(source==metaIButton)
pIaF="javax.swing.plaf.metal.MetalLookAncl-Feel";
else if(source==motifButton)
plaf="com.sun.java.swing.plaf.motif.Moti-fLookAndFeel";
else if(source==windowsButton)
plaf="com.sun.java.swing.plaf.windows.windowsLookAndFeel";
try
UIManager.setLookAndFeel(______);
SwingUtilities.updateComponentTreeUI(ttlis);

catch(Exception e)

private JButton metalButton;
private JButton motifButton;
private JButton windowsButton;

class PlafFrame extends JFrame
public PlafFrame()
setTitle("simple");
setSize(300,200)；
addWindowListener(new WindowAdapter()
public void windowClosing(WindowEvent e)
System.exit(0);

);
Container contentPane=getContentPane();
contentPane.add(new PlafPanel());


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