求助,一个tableheader的问题

[dlttyy]

为甚么不显示table的标题?有什么问题存在,请各位大大指教,谢谢

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTable;
import javax.swing.table.JTableHeader;

public class tableExercise extends JFrame {

private JPanel jContentPane = null;

private JTable table = null;

private JTableHeader tblheader = null;

Object[][] content = { { "george", "Linux", new Integer(34) },
{ "Bill", "Unix", new Integer(30) },
{ "Sam", "Windows", new Integer(32) }, };

Object[] title = { "姓名", "课程", "年龄" };

/**
* This is the default constructor
*/
public tableExercise() {
super();
initialize();
}

/**
* This method initializes this
*
* @return void
*/
private void initialize() {
this.setSize(300, 200);
this.setContentPane(getJContentPane());
this.setTitle("表");
}

/**
* This method initializes jContentPane
*
* @return javax.swing.JPanel
*/
private JPanel getJContentPane() {
if (jContentPane == null) {
jContentPane = new JPanel();
jContentPane.setLayout(null);
jContentPane.add(getTable(), null);
jContentPane.add(gettblHeader(), null);
}
return jContentPane;
}

/**
* This method initializes jTable
*
* @return javax.swing.JTable
*/
private JTable getTable() {
if (table == null) {
table = new JTable(content, title);
table.setBounds(new java.awt.Rectangle(20, 20, 250, 120));
}
return table;
}

private JTableHeader gettblHeader() {
if (tblheader == null) {
tblheader = table.getTableHeader();
}
return tblheader;
}
}

[harrisecho]

//Overload.java for ChenSinan
public class Overload{

public static void display()
{ System.out.println( "\nThis is the default message;" ); }

public static void display(int num)
{ System.out.println("Integer value is: "+ num); }

public static void display(char ch)
{ System.out.println("Char value is: "+ ch); }

public static void display(String string)
{ System.out.println("Char value is: "+ string); }

public static void main(String argv[]){
try{
display(); //print the default message

int integer = 233;
display(integer); //print ...

char ch = 'c';
display(ch);

String string1 = "string";
display(string1);

String string2 = "0001";
display(string2);


} catch (Exception e) {
System.out.println("error happened in main() when involve display();");};
}

}