面板

Question

package other;import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JFrame;
import javax.swing.JPanel;public  class paint2 extends JFrame{
static JPanel p;
public static JPanel f()
{
p=new JPanel();
p.setLayout(new BorderLayout());
Graphics g=p.getGraphics();
g.setColor(p.getForeground());
g.drawLine(1, 1, 50, 50);
return p;
}
public static void main(String[] main){
paint2  a = new paint2();
a.setSize(600,400);
a.add(f());
a.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
a.setVisible(true);
}
}
为什么这个会运行错误？该怎么改？

Answer 1

g为null，解释原因见http://zhidao.baidu.com/question/180810909.html?push=related

Answer 2

那为什么在这可以？
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
public class TestApplet extends Applet
{
     String strFont;
     int xOrigin,yOrigin;
      public void init()
      {    strFont=getParameter("font");
          addMouseListener(new MouseAdapter()
           {
                public void mousePressed(MouseEvent e)
                {
                       xOrigin=e.getX();
                       yOrigin=e.getY();
                }
                public void mouseReleased(MouseEvent e)
                {
                       Graphics g=getGraphics();
                       g.setColor(Color.blue);
                       g.drawLine(xOrigin,yOrigin,e.getX(),e.getY());
                }
           });
      }
      public void start()
      {
           System.out.println("start");
      }
      public void stop()
      {
          System.out.println("stop");
      }
      public void destroy()
      {
          System.out.println("destroy");
      }
      public void paint(Graphics g)
      {                                                 
          Font f=new Font(strFont,Font.BOLD,30); 
          g.setFont(f);
          g.setColor(Color.blue);
           g.drawString("维新科学",0,30);
      }
}

Answer 3

一个面板在被show出来
就是setVisible(true)出来之前
graphics是默认为null的

Answer 4

那在setVisible(true)之后graphics就不为null了吗？为什么？

Answer 5

是吗？？？ 应该是空的，把画图的语句放在setVisible(true)之后，框架出来后，还是抱空指针异常，
说明之后graphics是空的