我建 table . 我想从活动窗口添加数据 . 但是a.getTabNum()和其他返回的vatiables的值为 null 这是我的代码：

JButton add = new JButton();

add.setText("+");

add.setHorizontalAlignment(SwingConstants.TRAILING);

panel.add(add);

//When user clicks he can add new employee

add.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent event) {

AddEmp a = new AddEmp();

a.setSize(400,140);

a.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

a.setVisible(true);

String post = a.getPost();

String nameSurname = a.getNameSurname();

String tabNum= a.getTabNum();

Connection connect = null;

PreparedStatement statement1 = null;

try {

connect = DriverManager

.getConnection("jdbc:mysql://localhost/tabel?user=root&password=4499669");

statement1 = connect

.prepareStatement("insert into employee(tab_num, name_surname, position, id_dept) values(?, ?, ?, ?)");

int a1= Integer.parseInt(tabNum);

statement1.setInt(1, a1);

statement1.setString(2,nameSurname);

statement1.setString(3, post);

statement1.setInt(4, num);

statement1.executeUpdate();

// / List of departments

//

} catch (Exception ex) {

ex.printStackTrace();

} finally {

try {

connect.close();

} catch (SQLException e1) {

e1.printStackTrace();

}

}

}

});

这里是ma AddEmp类：`public class AddEmp extends JFrame {private static final long serialVersionUID = 1L; JTextField a = new JTextField(30); JTextField b = new JTextField(27); JTextField c = new JTextField(23); AddEmp self = this; JLabel a1 =新JLabel(“Ф.И.О”); JLabel b1 =新JLabel(“Должность”); JLabel c1 =新JLabel(“Табельныйномер”); JButton o = new JButton(“OK”);

public AddEmp(){

super("Добавление сотрудника");

setResizable(false);

JPanel p = new JPanel();

p.add(a1);

p.add(a);

p.add(b1);

p.add(b);

p.add(c1);

p.add(c);

o.setHorizontalAlignment(SwingConstants.RIGHT);

p.add(o);

getContentPane().add(p, BorderLayout.CENTER);

o.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent arg0) {

try{

JOptionPane.showMessageDialog(null, "Сотрудник добавлен");

self.setVisible(false);

self.dispose();

}catch(Exception e){

e.printStackTrace();

}

}

});

}

public String getPost() {

return b.getText();

}

public String getNameSurname() {

return a.getText();

}

public String getTabNum() {

return c.getText();

}

}`