java object数组显示异常

class InsertRecord extends JDialog implements ActionListener{ //负责插入记录的类
      JLabel hintLabel;
      Object[] name={"学号","姓名","性别","年龄"};
      Object[][] a=new Object[1][4];
      JTable table;
      JButton enterInsert; 
      Connection con;
      Statement sql; 
      ResultSet rs;
      String num;
      InsertRecord(String s){
         setTitle(s);
         hintLabel=new JLabel("输入新记录:");
         table=new JTable(a,name);
         enterInsert=new JButton("插入新记录");
         setLayout(null); 
         Box baseBox=Box.createHorizontalBox();
         baseBox.add(hintLabel);
         baseBox.add(new JScrollPane(table));
         baseBox.add(enterInsert);
         add(baseBox);
         baseBox.setBounds(10,40,600,38);
         enterInsert.addActionListener(this);
         setBounds(120,160,700,200);
     }
     public void actionPerformed(ActionEvent e){
      /*  try{  con=DriverManager.getConnection("jdbc:odbc:jec","system","jec");
              sql=con.createStatement();
              int k=sql.executeUpdate
              ("INSERT INTO s VALUES('a[0][0]','a[0][1]','a[0][2]','a[0][3]')");
              if(k==1)
                 JOptionPane.showMessageDialog
                 (this,"插入记录成功","成功",JOptionPane.PLAIN_MESSAGE); 
              table=new JTable(a,name);
              con.close();
        }  
        catch(SQLException ee){ 
               JOptionPane.showMessageDialog
              (this,"插入记录失败"+ee,"失败",JOptionPane.ERROR_MESSAGE);
        }  */
      System.out.println(a[0][0]+""+a[0][1]+""+a[0][2]+""+a[0][3]);
     }      
 }
为什么a[0][3]显示为null 其他的都显示正常 谁能帮我解决一下