关于鼠标事件的问题
我的程序定义了一个panel，每次在鼠标移进这个panel的时候，鼠标上就回显示一个小球，移出的时候小球消失。但是我的程序运行结果什么都不显示，我觉得我的思路没问题，但是在实现的时候可能有问题，请大家看一下程序，多多指教，谢谢
import   java.awt.*;
import   java.awt.event.*;
import   javax.swing.*;
import   java.awt.geom.*;
import   java.util.*;
/*
  *   When   the   mouse   move   to   the   panel,there   will   be   a  
  *   ball   appear     at   the   same   time.when   it   moves   out   of   the   panel
  *   the   ball   disappear.
  */
public   class   MousewithBall   {
      public   static   void   main(String[]   args){
    JFrame   frame=new   DrawBallFrame();
    frame.setSize(300,200);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
   
      }
}

class   DrawBallFrame   extends   JFrame{
public   DrawBallFrame(){
    panel=new   DrawBallPanel();
    add(panel);
}

private   DrawBallPanel   panel;
}

class   DrawBallPanel   extends   JPanel{
private   ArrayList <Point>   points;
public   DrawBallPanel(){
      points=new   ArrayList <Point> ();
      //gets   tje   bounds   of   the   panel
      r=   this.getBounds();
      //add   the   listener
      addMouseMotionListener(new   MouseMotionHandler());
}

public   void   paintComponent(Graphics   g){
super.paintComponents(g);
Graphics2D   g2=(Graphics2D)g;
for(Point   r:points){
    double   x=r.getX();
    double   y=r.getY();
    //draw   the   ball
    g2.fillOval((int)x,(int)y,15,15);
}
}

public   Point   returnPoint(int   x,int   y){
return   new   Point(x,y);
}  
public   void   add(Point   p){
points.add(p);
}
private   Rectangle   r;
private   class   MouseMotionHandler   extends   MouseMotionAdapter{
public   void   mouseMoved(MouseEvent   e){
    int   x=e.getX();
    int   y=e.getY();
    if(r.contains(x,y))
    {returnPoint(x,y);
    repaint();}
}

}
}



------解决方案--------------------
private Rectangle r;
这个没有初始化值