来吧.指点下小鸟

import java.awt.*;import javax.swing.*;import java.awt.event.*;public class ball extends JPanel implements KeyListener{                private int x = 20;//小球的大小                private int y = 20;//小球的大小                private int a = 50;//小球的位置                private int b = 50;//小球的位置                private int x1 = 200;//窗体的大小                private int y1 = 200;//窗体的大小    ball(){        JFrame jf = new JFrame();        jf.setLocation(200,200);        jf.setSize(x1,y1);        JPanel p1 = new p();//覆盖组建的paint方法才能用        jf.getContentPane().add(p1);        jf.setVisible(true);        jf.addKeyListener(this);        try{             while(true){                Thread.sleep(50);                p1.repaint();                }            }catch(InterruptedException e){                e.printStackTrace();            }        }            public void keyTyped(KeyEvent e){    }    public void keyPressed(KeyEvent e){        //System.out.println(e.getKeyCode());        //System.out.println(e.VK_KP_DOWN);        if(e.getKeyCode() == 40){//下                y += 4;                    }        if(e.getKeyCode() == 39){//右                if(x>=x1-(x-b/2)){//请问怎么计算小球撞到窗体边缘.我感觉老是不行帮我鲁下思路吧高人                                        }else{                x += 4;                            }            }        if(e.getKeyCode() == 37){//左                x -= 4;                    }        if(e.getKeyCode() == 38){//上                y -= 4;                    }    }    public void keyReleased(KeyEvent e){        //System.out.println("+++++++++++++++");    }    public static void main(String[] args){        ball b = new ball();                //b.paint();        }                private class p extends JPanel{                    public void paint(Graphics g){                        super.paint(g);                           g.setColor(Color.blue);                        //System.out.println(a +"a"+ b+"b" + x +"x"+ y+"y");                        g.fillOval(x,y,a,b);                        g.drawOval(x,y,a,b);                                            }            }}