高分解答,为什么javac编译成功而appletviewer看结果时会出错啊???

[lyhang801]

源代码如下:
import java.applet.*;
import java.awt.*;

import java.awt.event.*;

public class exam extends Applet implements ActionListener
{
Button button1,button2;
TextField text1;
Label a;
Panel p1,p2;
public void init()
{
setLayout(new GridLayout(2,1));
add(p1);add(p2);
p1=new Panel();
p2=new Panel();
button1=new Button("display");
button2=new Button("suaxin");
a=new Label("display");
p1.setLayout(new GridLayout(1,2));
p1.add(button1);
p1.add(button2);
p2.setLayout(new GridLayout(1,2));
p2.add(a);
p2.add(text1);

button1.addActionListener(this);
button2.addActionListener(this);
}
public void actionPerformed(ActionEvent e)
{
if(e.getSource()==button1)
{text1.setText("ok");
}
else if(e.getSource()==button2)
{
p2.removeAll();
}
}
}
编译时用javac exam.java
然后用appletviewer exam.html
exam.html代码为:
<applet code=exam.class height=300 width=300>
</applet>
在窗口中会 start: 出现未初始化applet小程序
Dos中弹出信息:
java.lang.NULLPointerException
at java.awt.Container.addImpl(Container.java:336)
at java.awt.Container.add(Container.java:223)
at exam.init(exam.java:15)
at sun.applet.AppletPanel.run(AppletPanel.java:340)
at java.lang.Thread.run(Thread.java:479)

请哪位大哥解答一下谢谢了

[soloxiao]

代码问题:
import java.applet.*;
import java.awt.*;
import java.awt.event.*;

public class exam extends Applet implements ActionListener
{
Button button1,button2;
TextField text1;
Label a;
Panel p1,p2;
public void init()
{
setLayout(new GridLayout(2,1));
//先new:
p1=new Panel();
p2=new Panel();
add(p1);add(p2);
button1=new Button("display");
button2=new Button("suaxin");
a=new Label("display");
p1.setLayout(new GridLayout(1,2));
p1.add(button1);
p1.add(button2);
p2.setLayout(new GridLayout(1,2));
p2.add(a);
//先new:
text1 = new TextField("test");
p2.add(text1);

button1.addActionListener(this);
button2.addActionListener(this);

}
public void actionPerformed(ActionEvent e)
{
if(e.getSource()==button1)
{text1.setText("ok");
}
else if(e.getSource()==button2)
{
p2.removeAll();
}
}
}

[lyhang801]

谢谢了