根据网络覆盖的范围可分为（）

A、NOVELL网

B、广域网

C、局域网

D、INTERNET网

本题的功能是监听鼠标的操作。鼠标置于窗口中单击时(左键或右键)，在单击的地方会画一个小矩形，如果将鼠标置于小矩形上，则鼠标光标状态改为小十字，按下鼠标左键可拖曳，双击鼠标左键(或右键)时，小矩形消失。
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import java.awt.geom.*;
import javax.swing.*;
public class java3

public static void main(String[]args)

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


class MouseFrame extends JFrame

public MouseFrame()

setTitle("java3");
setSize(DEFAULT_WIDTH,DEFAULT_HEIGHT);
MousePanel panel=new MousePanel();
Container contentPane=geeContentPane();
contentPane.add(panel);

public static final int DEFAULT_WIDTH=300;
public static final int DEFAULT_HEIGHT=200;

class MousePanel extends JPanel

public MousePanel()

squares=new ArrayList();
current=null;
addMouseListener(new MouseHandler());
addMouseMotionListener(new MouseMorionHandler());

public void paintComponent(Graphics g)

super.paintComponent(g);
Graphics2D g2=(Graphics2D)g;
for(int i=0;i＜squares.size();i++)
g2.draw((Rectangle2D)squares.get(i));

public void find(Point2D p)

for(int i=0;i＜squares.size();i++)

Rectangle2D r=(Rectangle2D)squares.get(i);
if(r.contains(p))return r;

return null;

public void add(Point2D p)

double x=p.getX();
double y=p.getY();
current=new Rectangle2D.Double(
x-SIDELENGTH/2,
y-SIDELENGTH/2,
SIDELENGTH,
SIDELENGTH),
squares.add(current);
repaint();

public void remove(Rectangle2D s)

if(s==null)return;
if(s==current)current=null;
squares.remove(s);
repaint();

private static final int SIDELENGTH=10;
private ArrayList squares;
private Rectangle2D current;
private class MouseHandler extends MouseActionListener

public void mousePressed(MouseEvent event)

current=find(event.getPoint());
if(current==null)
add(event.getPoint());

public void mouseClicked(MouseEvent event)

current=find(event.getPoint());
if(current!=null && event.getClickCount()＞=2)
remove(current);


private class MouseMotionHandler implements MouseMotionListener

public void mouseMoved(MouseEvent event)

if(find(event.getPoint)==null)
setCursor(Cursor.getDefaultCursor());
else
setCursor(Cursor.getPredefinedCursor
(Cursor.CROSSHAIR_CURSOR));

public void mouseDragged(MouseEvent event)

if(current!=null)

int x=event.getX();
int y=event.getY();
current.setFrame(
x-SIDELENGTH/2,
y-SIDELENGTH/2,
SIDELENGTH,
SIDELENGTH);
repaint();