我一直收到错误：

Exception in thread "main" java.lang.IllegalArgumentException: adding a window to a container

at java.awt.Container.checkNotAWindow(Container.java:483)

at java.awt.Container.addImpl(Container.java:1084)

at java.awt.Container.add(Container.java:966)

at Lab2.EmployeeGUI.main(EmployeeGUI.java:28)

有人可以帮助我,告诉我我做错了什么吗？

我是初学程序员.

package Lab2;

import java.awt.*;

import java.awt.event.ActionEvent;

import java.awt.event.ActionListener;

import javax.swing.*;

/**

*

* @author Jim Doyle

*/

public class EmployeeGUI extends JFrame implements ActionListener {

JTextField fName, mName, lName, phone, sal, years;

JComboBox boxTitle, boxDept;

DefaultListModel lstdefault;

JList project;

DbWork dbw = new DbWork("Lab2");

DbWork Title = new DbWork("Lab2");

DbWork Dept = new DbWork("Lab2");

DbWork Prjs = new DbWork("Lab2");

DbWork PrjList = new DbWork("Lab2");

public static void main(String[] args) {

EmployeeGUI app = new EmployeeGUI();

JFrame frame = new JFrame("Employee Interface by Jim Doyle");

frame.getContentPane().add(app, BorderLayout.CENTER);

frame.setSize(300, 300);

frame.setVisible(true);

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

}

public EmployeeGUI() {

JPanel labels = new JPanel();

labels.setLayout(new GridLayout(8,1));

labels.add(new JLabel("First Name"));

labels.add(new JLabel("MI"));

labels.add(new JLabel("Last Name"));

labels.add(new JLabel("Title"));

labels.add(new JLabel("Telephone"));

labels.add(new JLabel("Salary"));

labels.add(new JLabel("Department"));

labels.add(new JLabel("Years in Service"));

getContentPane().add(labels, BorderLayout.WEST);

JPanel fields = new JPanel();

fields.setLayout(new GridLayout(8,1));

fName = new JTextField(15);

mName = new JTextField(15);

lName = new JTextField(15);

phone = new JTextField(15);

sal = new JTextField(15);

years = new JTextField(15);

boxTitle = new JComboBox();

boxDept = new JComboBox();

fields.add(fName);

fields.add(mName);

fields.add(lName);

fields.add(boxTitle);

fields.add(phone);

fields.add(sal);

fields.add(years);

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

JPanel prjinfo = new JPanel();

prjinfo.setLayout(new GridLayout(1,2));

prjinfo.add(new JLabel("Project Description"));

project = new JList();

lstdefault = new DefaultListModel();

// add items to title combo box

while(Title.nextRecord()) {

String txtTit = Title.getField(1);

if(txtTit!=null) {

boxTitle.addItem(Title.getField(1));

}

}

// add items to department combo box

while(Dept.nextRecord()) {

String txtDept = Dept.getField(2);

if(txtDept!=null) {

boxDept.addItem(Dept.getField(2));

}

}

while(PrjList.nextRecord()) {

lstdefault.addElement(PrjList.getField(1));

}

project = new JList(lstdefault);

project.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);

prjinfo.add(project);

getContentPane().add(prjinfo, BorderLayout.SOUTH);

}

public void actionPerformed(ActionEvent e) {

String button = e.getActionCommand();

if(button == "First") {

if(dbw.firstRecord()) {

Execute();

}

}

else if(button == "Next") {

if(dbw.nextRecord()) {

Execute();

}

}

else if(button == "Save") {

String sql = "UPDATE FirstName, MiddleName, LastName, WorkPhone, Salary, YearsInService FROM Employee;";

dbw.processQuery(sql);

}

}

private void action() {

boxTitle.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {

JComboBox b = (JComboBox)e.getSource();

String ntitle = (String)b.getSelectedItem();

updateTitle(ntitle);

}

});

}

private void Execute() {

fName.setText(dbw.getField(1));

mName.setText(dbw.getField(2));

lName.setText(dbw.getField(3));

phone.setText(dbw.getField(5));

sal.setText(dbw.getField(6));

years.setText(dbw.getField(8));

String ftext = dbw.getField(4);

int dx = TitleList(ftext);

boxTitle.setSelectedIndex(dx);

String dtext = dbw.getField(7);

int dx2 = DeptList(dtext);

boxDept.setSelectedIndex(dx2);

action();

}

int TitleList(String title) {

int dx = 0;

for(int z=0; z

if(title.equals(boxTitle.getItemAt(z))) {

dx = z;

}

}

return dx;

}

int DeptList(String dept) {

int dx = 0;

for(int z=0; z

if(dept.equals(boxDept.getItemAt(z))) {

dx = z;

}

}

return dx;

}

private void updateTitle(String title) {

}

}