我正在创建通讯簿，所有代码都已完成，但我不断收到错误消息！这是错误：
Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at javax.swing.JFrame.addImpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at MyAddressBook.<init>(MyAddressBook.java:40)
at MyAddressBook.main(MyAddressBook.java:31)
当我运行地址簿时，出现错误并且有一个空框架，框架没有读取内容，请运行代码，你会看到我在说什么。请建议我如何解决这个问题？
我的代码：
import javax.swing.*; import javax.swing.event.DocumentEvent; import javax.swing.event.DocumentListener; import javax.swing.event.ListSelectionEvent; import java.awt.event.*; import java.awt.*; import java.io.*; public class AddressBook {
private JLabel lblFirstname,lblSurname, lblMiddlename, lblPhone,
lblEmail,lblAddressOne, lblAddressTwo, lblCity, lblPostCode, lblPicture;
private JTextField txtFirstName, txtSurname, txtAddressOne, txtPhone,
txtMiddlename, txtAddressTwo, txtEmail, txtCity, txtPostCode;
private JButton btSave, btExit, btDelete;
private JList<String> list;
private DefaultListModel<String> listModel;
private JPanel panel;
PrintWriter out;
public static void main(String[] args) throws Exception{
new AddressBook();
}
public AddressBook() throws Exception{
JFrame frame = new JFrame("My Address Book");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(900,400);
frame.setVisible(true);
out = new PrintWriter (new BufferedWriter (new FileWriter("outfile.txt",true)));
frame.add(panel);
frame.setVisible(true);
}
public void MyPanel() {
panel = new JPanel();
panel.setLayout(null);
panel.setBackground(Color.cyan);
}
public void Fields () {
lblFirstname = new JLabel("First name");
lblFirstname.setBounds(135, 50, 150, 20);
Font styleOne = new Font("Arial", Font.BOLD, 13);
lblFirstname.setFont(styleOne);
panel.add(lblFirstname);
txtFirstName = new JTextField();
txtFirstName.setBounds(210, 50, 150, 20);
panel.add(txtFirstName);
lblSurname = new JLabel ("Surname");
lblSurname.setBounds(385,50,150,20);
Font styleTwo = new Font ("Arial",Font.BOLD,13);
lblSurname.setFont(styleTwo);
panel.add(lblSurname);
txtSurname = new JTextField();
txtSurname.setBounds(450,50,150,20);
panel.add(txtSurname);
lblMiddlename = new JLabel ("Middle Name");
lblMiddlename.setBounds(620,50,150,20);
Font styleThree = new Font ("Arial", Font.BOLD,13);
lblMiddlename.setFont(styleThree);
panel.add(lblMiddlename);
txtMiddlename = new JTextField();
txtMiddlename.setBounds(710,50,150,20);
panel.add(txtMiddlename);
lblPhone = new JLabel("Phone");
lblPhone.setBounds(160,100,100,20);
Font styleFour = new Font ("Arial", Font.BOLD,13);
lblPhone.setFont(styleFour);
panel.add(lblPhone);
txtPhone = new JTextField();
txtPhone.setBounds(210,100,150,20);
panel.add(txtPhone);
lblEmail = new JLabel("Email");
lblEmail.setBounds(410,100,100,20);
Font styleFive = new Font ("Arial", Font.BOLD,13);
lblEmail.setFont(styleFive);
panel.add(lblEmail);
txtEmail = new JTextField();
txtEmail.setBounds(450,100,150,20);
panel.add(txtEmail);
lblAddressOne = new JLabel("Address 1");
lblAddressOne.setBounds(145,150,100,20);
Font styleSix = new Font ("Arial", Font.BOLD,13);
lblAddressOne.setFont(styleSix);
panel.add(lblAddressOne);
txtAddressOne = new JTextField();
txtAddressOne.setBounds(210,150,150,20);
panel.add(txtAddressOne);
lblAddressTwo = new JLabel("Address 2");
lblAddressTwo.setBounds(145,200,100,20);
Font styleSeven = new Font ("Arial", Font.BOLD,13);
lblAddressTwo.setFont(styleSeven);
panel.add(lblAddressTwo);
txtAddressTwo = new JTextField();
txtAddressTwo.setBounds(210,200,150,20);
panel.add(txtAddressTwo);
lblCity = new JLabel("City");
lblCity.setBounds(180,250,100,20);
Font styleEight = new Font ("Arial", Font.BOLD,13);
lblCity.setFont(styleEight);
panel.add(lblCity);
txtCity = new JTextField();
txtCity.setBounds(210,250,150,20);
panel.add(txtCity);
lblPostCode = new JLabel("Post Code");
lblPostCode.setBounds(380,250,100,20);
Font styleNine = new Font ("Arial", Font.BOLD,13);
lblPostCode.setFont(styleNine);
panel.add(lblPostCode);
txtPostCode = new JTextField();
txtPostCode.setBounds(450,250,150,20);
panel.add(txtPostCode);
}
public void Image() {
ImageIcon image = new ImageIcon("icon.png");
lblPicture = new JLabel(image);
lblPicture.setBounds(600,90, 330, 270);
panel.add(lblPicture);
}
public void Buttons() {
btSave = new JButton ("Save");
btSave.setBounds(380,325,100,20);
panel.add(btSave);
btSave.addActionListener(new SaveListener(btDelete));
btDelete = new JButton ("Delete");
btDelete.setBounds(260,325,100,20);
panel.add(btDelete);
btDelete.addActionListener(new DeleteListener());
btExit = new JButton ("Exit");
btExit.setBounds(500,325,100,20);
panel.add(btExit);
btExit.addActionListener(new Action());
}
public void List() {
listModel = new DefaultListModel<String>();
listModel.addElement("Daniel Walker");
listModel.addElement("John Smith");
listModel.addElement("Bob Jones");
//JList
list = new JList<String>(listModel);
list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
list.setSelectedIndex(0);
list.setVisibleRowCount(5);
list.setBounds(0,10,125,350);
panel.add(list);
}
static class Action implements ActionListener {
public void actionPerformed(ActionEvent e) {
JFrame option = new JFrame();
int n = JOptionPane.showConfirmDialog(option,
"Are you sure you want to exit?",
"Exit?",
JOptionPane.YES_NO_OPTION);
if(n == JOptionPane.YES_OPTION){
System.exit(0);
}
}
}
class SaveListener implements ActionListener, DocumentListener {
private boolean alreadyEnabled = false;
private JButton button;
public SaveListener(JButton button) {
this.button = button;
}
//Required by ActionListener.
public void actionPerformed(ActionEvent e) {
String name = txtFirstName.getText();
//User didn't type in a unique name...
if (name.equals("") || alreadyInList(name)) {
Toolkit.getDefaultToolkit().beep();
txtFirstName.requestFocusInWindow();
txtFirstName.selectAll();
return;
}
int index = list.getSelectedIndex(); //get selected index
if (index == -1) { //no selection, so insert at beginning
index = 0;
} else { //add after the selected item
index++;
}
listModel.insertElementAt(txtFirstName.getText() +" "+ txtSurname.getText(), index);
//print to file
out.println(txtFirstName.getText());
out.println(txtSurname.getText());
out.println(txtMiddlename.getText());
out.println(txtPhone.getText());
out.println(txtAddressOne.getText());
out.println(txtAddressTwo.getText());
out.println(txtEmail.getText());
out.println(txtCity.getText());
out.println(txtPostCode.getText());
out.close();
//Reset the text field.
txtFirstName.requestFocusInWindow();
txtSurname.requestFocusInWindow();
txtMiddlename.requestFocusInWindow();
txtPhone.requestFocusInWindow();
txtAddressOne.requestFocusInWindow();
txtAddressTwo.requestFocusInWindow();
txtEmail.requestFocusInWindow();
txtCity.requestFocusInWindow();
txtPostCode.requestFocusInWindow();
txtFirstName.setText("");
txtSurname.setText("");
txtMiddlename.setText("");
txtPhone.setText("");
txtAddressOne.setText("");
txtAddressTwo.setText("");
txtEmail.setText("");
txtCity.setText("");
txtPostCode.setText("");
//Select the new item and make it visible.
list.setSelectedIndex(index);
list.ensureIndexIsVisible(index);
}
protected boolean alreadyInList(String name) {
return listModel.contains(name);
}
//Required by DocumentListener.
public void insertUpdate(DocumentEvent e) {
enableButton();
}
//Required by DocumentListener.
public void removeUpdate(DocumentEvent e) {
handleEmptyTextField(e);
}
//Required by DocumentListener.
public void changedUpdate(DocumentEvent e) {
if (!handleEmptyTextField(e)) {
enableButton();
}
}
private void enableButton() {
if (!alreadyEnabled) {
button.setEnabled(true);
}
}
private boolean handleEmptyTextField(DocumentEvent e) {
if (e.getDocument().getLength() <= 0) {
button.setEnabled(false);
alreadyEnabled = false;
return true;
}
return false;
}
}
public void valueChanged(ListSelectionEvent e) {
if (e.getValueIsAdjusting() == false) {
if (list.getSelectedIndex() == -1) {
btDelete.setEnabled(false);
} else {
btDelete.setEnabled(true);
}
}
}
class DeleteListener implements ActionListener {
public void actionPerformed(ActionEvent e) {
int index = list.getSelectedIndex();
listModel.remove(index);
int size = listModel.getSize();
if (size == 0) {
btDelete.setEnabled(false);
} else {
if (index == listModel.getSize()) {
index--;
}
list.setSelectedIndex(index);
list.ensureIndexIsVisible(index);
}
}
}
}