哪位高手能帮小弟我看上这个异常：在掉数据库数据时，才出错

package bookstore;import java.awt.BorderLayout;import java.awt.FlowLayout;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import java.sql.Connection;import java.sql.DriverManager;import java.sql.SQLException;import java.sql.Statement;import java.sql.ResultSet;import javax.swing.Box;import javax.swing.JButton;import javax.swing.JDialog;import javax.swing.JLabel;import javax.swing.JOptionPane;import javax.swing.JPanel;import javax.swing.JScrollPane;import javax.swing.JTable;import javax.swing.JTextField;public class bookDialog extends JDialog implements ActionListener{    JLabel jLabel1;    JButton jButton1;    JTextField jTextField1;        Box box1;    JPanel jPanel1;        JTable table;    Object a[][];    Object name[] = {"图书编号","书名","类别","是否借出","借阅人编号"};        Connection con;    Statement sql;    ResultSet rs;        bookDialog() {        jLabel1 = new JLabel("图书编号(回车确认)：");        jTextField1 = new JTextField(10);        jButton1 = new JButton("查询全部");        a = new Object[1][5];        table = new JTable(a,name);                box1 = Box.createHorizontalBox();                jPanel1 = new JPanel(new FlowLayout(FlowLayout.CENTER));                box1.add(jLabel1);        box1.add(Box.createHorizontalStrut(40));        box1.add(jTextField1);        box1.add(Box.createHorizontalStrut(40));        box1.add(jButton1);                jPanel1.add(box1);                add(jPanel1,BorderLayout.SOUTH);        add(new JScrollPane(table),BorderLayout.CENTER);                jTextField1.addActionListener(this);        jButton1.addActionListener(this);                setBounds(200, 250, 435, 300);            }    @Override    public void actionPerformed(ActionEvent e) {        if (e.getSource() == jTextField1) {            try {                String s = jTextField1.getText().trim();                int num = Integer.parseInt(s);                con = DriverManager.getConnection("jdbc:odbc:bookstore","","");                sql = con.createStatement();                rs  = sql.executeQuery("SELECT * FROM book WHERE bno = '"+num+"' ");                if (!rs.next()) {                    JOptionPane.showMessageDialog(this, "学号卜存在","提示",JOptionPane.WARNING_MESSAGE);                }                    else {                   try {                    a[0][0] = rs.getInt(1);                    a[0][1] = rs.getString(2);                    a[0][2] = rs.getString(3);                    a[0][3] = rs.getString(4);                    a[0][4] = rs.getInt(5);                    table.repaint();                   }                    catch (SQLException e2) {e2.printStackTrace();}                   con.close();                 }            }            catch (SQLException ee) {System.out.println(ee);}                        table = new JTable(a,name);            getContentPane().removeAll();            add(jPanel1,BorderLayout.SOUTH);            add(new JScrollPane(table),BorderLayout.CENTER);        }                if (e.getSource() == jButton1) {            try {                con = DriverManager.getConnection("jdbc:odbc:bookstore","","");                sql = con.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,ResultSet.CONCUR_READ_ONLY);                rs  = sql.executeQuery("SELECT * FROM book");                int lastnum = rs.getRow();                a= new Object[lastnum][5];                int k = 0;                rs.beforeFirst();                while (rs.next()) {                    try {                        a[k][0] = rs.getInt(1);                        a[k][1] = rs.getString(2);                        a[k][2] = rs.getString(3);                        a[k][3] = rs.getString(4);                        a[k][4] = rs.getInt(5);                                k++;                       }                        catch (SQLException e2) {e2.printStackTrace();}                }                con.close();            } catch (SQLException ee) {                System.out.println(ee);            }                        table = new JTable(a,name);            getContentPane().removeAll();            add(jPanel1,BorderLayout.SOUTH);            add(new JScrollPane(table),BorderLayout.CENTER);        }    }     }