想在JPanel里面画条线，但是一直报告空指针异常，请问这是什么原因呢？
import java.awt.*;
import javax.swing.*;
public class MyGame {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
new Myframe("Hello").lanch();
}
}
class Myframe extends JFrame
{
JPanel jp =null;
public Myframe(String s)
{
super(s);
}
public void lanch()
{
ImageIconim = new ImageIcon("E:/javadev/xuannan/Games/88.jpg");
JLabel lbl = new JLabel(im);
setBounds(300,65,700,550);
jp = new JPanel(null);
jp.setBounds(175,125,350,300);
jp.setBackground(Color.WHITE);
add(jp);
panelDrawLine();
add(lbl);
setVisible(true);
}
public void panelDrawLine()
{
jp.setVisible(true);
Graphics g=jp.getGraphics();
g.drawLine(175, 125, 525, 125); //就是这里报告空指针异常。
}
本人菜鸟，请说得通俗易懂点，谢谢~
}
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