填空题

本题的功能是用按钮来控制文字的颜色。窗口中有三个按钮“Yellow”、“Blue”和“Red”，它们分别对应文字标签中文本的颜色为黄色、蓝色和红色，单击任意一个按钮，文字标签中的文本就变成按钮对应的颜色。
import java. awt. * ;
import java. awt. event. * ;
import javax. swing. * ;
class ButtonPanel extends JPanel implements ActionListener
public ButtonPanel()
yellowButton=new JButton("Yellow");
blueButton=new JButton("Blue");
redButton=new JButton("Red");
jl=new JLabel("I am from China!");
add(yellowButton);
add(blueButton);
add(redButton);
add(jl);
yellowButton. addActionListener(this);
blueButton. addActionListener(this);
redButton. addActionListener(this);

public void actionPerformed(ActionEvent evt)
Object source=evt. getSource();
Color color=getForeground();
if(source==yellowButton) color=Color. yellow;
else if(source==blueButton) color=Color. blue;
else if(source==redButton) color=Color. red;
______;
______;

private JButton yellowButton;
private JButton blueButton;
private JButton redButton;
private JLabel jl;

class ButtonFrame extends JFrame
public ButtonFrame()
setTitle("exam_16");
setSize(300,200);
addWindowListener(new WindowAdapter()
public void windowClosing(WindowEvent e)
System. exit(0);

);
Container contentPane=getContentPane();
contentPane, add(new ButtonPanel());


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