[code="java"]
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import java.awt.geom.*;
import javax.swing.*;
public class MouseTest{
public static void main(String args[]){
MouseFrame mf = new MouseFrame();
mf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
mf.setVisible(true);
}
}
class MouseFrame extends JFrame{
public MouseFrame(){
setTitle("MouseTest");
setLocation(300,300);
setSize(DEFAULT_WIDTH,DEFAULT_HEIGHT);
MousePanel mp = new MousePanel();
getContentPane().add(mp);
}
public static final int DEFAULT_WIDTH = 300;
public static final int DEFAULT_HEIGHT = 200;
}
class MousePanel extends JPanel{
private ArrayList squares;
private Rectangle2D current;
private static final int SIDELENGTH = 10;
public MousePanel(){
squares = new ArrayList();
current = null;
addMouseListener(new MouseHandler());
// addMouseMotionListener(new MouseMotionHandler());
}
public void paintComponent(Graphics g){
super.paintComponent(g);
Graphics2D g2 =(Graphics2D)g;
for(Rectangle2D r : squares){
g2.draw(r);
}
}
public Rectangle2D find(Point2D p){
for(Rectangle2D r : squares){
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;
//这个if判断什么意思，起到什么作用？
squares.remove(s);
repaint();
}
private class MouseHandler extends MouseAdapter{
public void mousePressed(MouseEvent e){
//add a new square if the cursor isn't inside a square
current = find(e.getPoint());
if(current == null){
add(e.getPoint());
}
}
public void mouseClicked(MouseEvent e){
//remove the current square if double clicked
current = find(e.getPoint());
if(current!=null&&e.getClickCount()>=2){
remove(current);
}
}
}
// private class MouseMotionHandler implements MouseMotionListener{
// public void mouseMoved(MouseEvent e){
//
// }
// public void mouseDragged(MouseEvent e){
//
// }
// }
}
[/code]
这是书上一个例子的部分程序，请问注释处的if啥意思？不就是把curent传给s嘛？s和此时的current本来就是相等的啊，为什么还要判断？
