问答题 本题程序的功能是：主窗口中有两个按钮“Start”和“Close”，单击按钮“Start”后会在窗口左上角生成一个黑色实心小球，小球做斜线运动，碰到面板边缘后反弹几次后停止运动，再次单击按钮“Start”后又会生成一个小球做同样的运动。程序中存在若干错误，请找出并改正(注意：不得改动程序的结构，不得增行或删行)。
import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.util.*;
import javax.swing.*;
public class advance

public static void main (String[] args)

JFrame frame = new BounceFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)
frame.show();

class BounceFrame extends JFrame

public BounceFrame()

setSize(WIDTH,HEIGHT);
setTitle("advance");
Container contentPane = getContentPane();
Canvas = new BallCanvas();
contentPane.add(canvas,BorderLayout.CENTER);
JPanel buttonPanel = new JPanel();
addButton(buttonPanel,"Start",new ActionListener()
public void actionPerformed(ActionEvent evt)

addBall();

);
addButton(buttonPanel,"Close",new ActionListener()
public void actionPerformed(ActionEvent evt)

System.exit (0);

);
contentPane.add(buttonPanel,BorderLayout.SOUTH);

public void addButton(Container c,String title,Listener listener)

JButton button = new JButton(title);
c.add(button);
button.addActionListener(listener);

public void addBall()

try

Ball b = new Ball(canvas);
canvas.add(b);
for (int i = 1;i ＜= 1000; i++)

b.move ();
Runnable.sleep (5);


catch (InterruptedException exception)



private BallCanvas canvas;
public static final int WIDTH = 450;
public static final int HEIGHT = 350;

class BallCanvas extend JPanel
public void add(Ball b)

balls.add(b);

public void paintComponent(Graphics g)
super.paintComponent(g);
Graphics2D g2 = (Graphics2D)g;
for (int i = 0; i ＜ balls.size(); i++)

Ball b = (Ball)balls.get(i);
b.draw(g2);

private ArrayList balls = new ArrayList();

class Ball

public Ball(Component c)

canvas = c;

public void draw(Graphics2D g2)

g2.fill(new Ellipse2D.Double(x,y,XSIZE,YSIZE));

public void move()

x += dx;
y += dy;
if(x ＜ 0)
x = 0;
dx = -dx;

if(x+XSIZE ＞= canvas.getWidth())
x = canvas.getWidth() - XSIZE;
dx = -dx;

if(y ＜ 0)

y = 0;
dy = -dy;

if (y+YSIZE ＞= canvas.getHeight())

y = canvas.getHeight() - YSIZE;
dy = -dy;

canvas.paint(canvas.getGraphics());

private Component canvas;
private static final int XSIZE = 15;
private static final int YSIZE = 15;
private int x = 0;
private int y = 0;
private int dx = 2;
private int dy = 2;