本题的功能是监听鼠标左右键的单击，以及面板中滚动条的添加。在窗口的画板中单击鼠标左键，在单击的位置绘制一个圆，当绘制的圆大于画板的大小时，画板就添加滚动条，在画板中单击鼠标右键，则清除画板中的所有图形。
import javax.swing.*;
import javax.swing.event.MouseInputAdapter;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
public class java3 extends JPanel
private Dimension size;
private Vector objects;
private final Color colors[]=
Color.red,Color.blue,Color.green,Color.orange,
Color.cyan,Color.magenta,Color.darkGray,Color.yellow;
private final int color_n=colors.length;
JPanel drawingArea;
public java3()
setOpaque(true);
size=new Dimension(0,0);
objects=new Vector();
JLabel instructionsLeft=new JLabel("单击鼠标左键画圆.");
JLabel instructionsRight=new JLabel("单击鼠标右键清空画板.");
JPanel instructionPanel=new JPanel(new GridLayout(0,1));
instructionPanel.add(instructionsLeft);
instructionPanel.add(instructionsRight);
drawingArea=new JPanel()
protected void paintComponent(Graphics g)
super.paintComponent(g);
Rectangle rect;
for(int i=0;i＜objects.size();i++)
rect=(Rectangle)objects.elementAt(i);
g.setColor(colors[(i%color_n)]);
g.fillOval(rect.x,rect.y,rect.width,rect.height);
;
drawingArea.setBackground(Color.white);
drawingArea.addMouseListener(new MouseListener());
JScrollPane scroller=new JScrollPane(drawingArea);
scroller.setPreferredSize(new Dimension(200,200));
setLayout(new BorderLayout());
add(instructionPanel,BorderLayout.NORTH);
add(scroller,BorderLayout.CENTER);
class MyMouseListener extends mouseInputAdapter
final int W=100;
final int H=100;
public void mouseReleased(MouseEvent e)
boolean changed=false;
if(SwingUtilities.isRightMouseButton(e))
objects.removeAllElements();
size.width=0;
size.height=0;
changed=true;
else
int x-e.getX()-W/2;
int y=e.getY()-H/2;
if(x＜0)x=0;
if(Y＜0)y=0;
Rectangle rect=new Rectangle(x,y,W,H);
objects.addElement(rect);
drawingArea.scrollRectToVisible(rect);
int this_width=(x+W+2);
if(this width＞size.width)
size.width=this_width;changed=true;
int this_height=(y+H+2);
if(this_height＞size.height)
size.height=this_height;changed=true;
if(changed)
drawingArea.setPreferredSize(size);
drawingArea.revalidate();
drawingArea.paint();
public static void main(String args[])
JFrame frame=new JFrame("java3");
frame.addWindowListener(new WindowAdapter()
public void windowClosing(WindowEvent e)System.exit(0);
);
frame.setContentPane(new java3());
frame.pack();
frame.setVisible(true);