To use a static variable in another class,but report NullPointerException

Question

when TableMain is running,I run testRecord so that let TableMain add a occur informatin and
happened time in a row in TableMain,but when I run testRecord,java report a NullPointerException and I dont know how to solve this problem,thanks for helping me to check my code;(error report info is in end)
import java.awt.event.*;
import javax.swing.table.*;
import java.io.*;
public class TableMain extends JFrame{
JTable table;
static OwnModel model;
String[] columnHeader={"occur","time"};
class OwnModel extends DefaultTableModel{
public OwnModel(Object[] columnNames,int numRows){
super(columnNames,numRows);
public boolean isCellEditable(int row,int column){
return false;
public TableMain(){
model=new OwnModel(columnHeader,0);
table=new JTable(model);
JScrollPane scroll=new JScrollPane(table);
JButton save=new JButton("save record");
JButton delete=new JButton("delete record");
JPanel buttons=new JPanel();
buttons.add(save);
buttons.add(delete);
JLabel sign=new JLabel("occur record");
Container cp=getContentPane();
cp.add(BorderLayout.NORTH,sign);
cp.add(BorderLayout.CENTER,scroll);
cp.add(BorderLayout.SOUTH,buttons);
this.setSize(300,500);
this.setVisible(true);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
public static void main(String[] args){
new TableMain();
import java.util.Vector;
import java.util.Calendar;
import java.text.SimpleDateFormat;
public class testRecord{
public static void main(String[] args){
SimpleDateFormat simpledf=new SimpleDateFormat("yyyy/MM/dd hh:mm:ss");
String tableName="friends";
Calendar occurtime=Calendar.getInstance();
Vector record=new Vector();
record.add("send 'desc' sql, "+"operated table is :"+tableName);
record.add(simpledf.format(occurtime.getTime()));
System.out.println("model is "+Guide.model);
System.out.println("record is "+record);
Guide.model.addRow(record);
error report:
model is null
record is [send 'desc' sql,operated table is :friends, 2004/05/26 11:39:03]
Exception in thread "main" java.lang.NullPointerException
at testRecord.main(testRecord.java:14)

Answer 1

but I just use this constructor once,never use twice
with same jvm;I thinks my idea is not too very badIt's a public constructor (if I'm looking in the right
place) - it could be called from anywhere. To
intialize a static variable outside the declaration,
use a static initializer:private static String whatever;static {
whatever = "whatever";
}Or simply (in this case):private static String whatever = "whatever";