import javax.swing.JApplet;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.net.*;
import java.io.*;

public class ApplicantApplet extends JApplet
{
static JPanel panel;
Container content;
                  JButton buttonAccept;
                  static ApplicantApplet applicant;//声明为静态变量
                  
                  GridBagLayout gl;
GridBagConstraints gbc;

                  public void init()
{ //初始化按钮
                   gl=new GridBagLayout();
 gbc=new GridBagConstraints();
 content = getContentPane();
 content.setLayout(gl);
                   buttonAccept = new JButton("Accept");
                   

                  gbc.anchor=GridBagConstraints.NORTHEAST;
gbc.gridx=8;
gbc.gridy=11;
gl.setConstraints(buttonAccept, gbc);

                  content.add(buttonAccept);
validateAction validateButton = new validateAction( );
buttonAccept.addActionListener(validateButton);
                  }

class validateAction implements ActionListener
{
public void actionPerformed(ActionEvent evt)
{
                  Object obj = evt.getSource();
if(obj == buttonAccept)
{  
try
{
URL docurl=new URL("http","127.0.0.1",1002,"D:/MYDOCUMENT/Applet.html");

applicant.getAppletContext().showDocument(docurl,"_self");
                                                     //执行时编译器报错，说是NullPointerException 
                                                     
               catch (MalformedURLException e)
{
                                                      System.out.println("Can't open the URL ");
                                                      }
                                    }
}
}
}
执行时编译器报错，说是NullPointerException ,getAppletContext( ).showDocument()是用来从服务器端下载页面的。是不是不能将applicant声明为static呢？请赐教。

static ApplicantApplet applicant;一个对象的声明，如果没有new，则被默认为null，所以这句执行后applicant is null;在后面你也没有声明，所以在执行applicant.getAppletContext().showDocument(docurl,"_self");时，它还是null,执行时，编译器报错肯定说是NullPointerException
你可以改为getAppletContext().showDocument(docurl,"_self");就OK了。

static ApplicantApplet applicant;//声明为静态变量
申明为静态变量由什么用？？
没有new,根本都用不了。。。