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;
}