我之前发布了此代码，但被告知要清理它并重新发布。我得到的错误是：
Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at FVolume.createCustomers(FVolume.java:74)
at FVolume.<init>(FVolume.java:21)
at FVolume.main(FVolume.java:270)
以下是代码：
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.util.*;
import java.io.*;
public class FVolume extends JFrame implements ActionListener{
private JTabbedPane jtabbedPane;
private JPanel Customers;
JTextArea NameTextCustomers, ExistTextCustomers, NameTextContractors, ExistTextContractors;
public FVolume(){
setTitle("Volume Calculator");
setSize (300, 200);
JPanel topPanel = new JPanel();
topPanel.setLayout( new BorderLayout() );
getContentPane().add( topPanel );
createCustomers();
jtabbedPane = new JTabbedPane();
jtabbedPane.addTab("Customers", Customers);
topPanel.add(jtabbedPane, BorderLayout.CENTER);
}
/* CREATE CUSTOMERS */
public JPanel createCustomers(){
Customers = new JPanel();
Customers.setLayout(null);
NameTextCustomers = new JTextArea();
NameTextCustomers.setBounds(10, 10, 350, 150);
NameTextCustomers.setLineWrap(true);
Customers.add(NameTextCustomers);
JButton Exit = new JButton("Exit");
Exit.setBounds(30,170,80,20);
Exit.addActionListener(this);
Exit.setBackground(Color.white);
Customers.add(Exit);
JButton AddCustomers = new JButton("Add Customer");
AddCustomers.setBounds(130,170,120,20);
AddCustomers.setBackground(Color.white);
Customers.add(AddCustomers);
JButton Refresh = new JButton("Refresh");
Refresh.setBounds(260,170,80,20);
Refresh.setBackground(Color.white);
Customers.add(Refresh);
ExistTextCustomers = new JTextArea();
ExistTextCustomers.setBounds(10, 200, 350, 60);
ExistTextCustomers.setLineWrap(true);
Customers.add(ExistTextCustomers);
final JTextArea custArea = new JTextArea(6, 30);
final JTextArea custMessage = null;
AddCustomers.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
new Customer("Customer");
}
});
Customers.add(custArea);
Customers.add(AddCustomers);
Customers.add(Refresh);
Customers.add(custMessage);
Refresh.setMnemonic('R');
Refresh.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
custMessage.setText("");
try
{
File custOpen = new File("customer.txt");
FileReader custAreaIn = new FileReader(custOpen);
custArea.read(custAreaIn, custOpen.toString());
custMessage.setText("The file exists and can be read from.");
}
catch (IOException e3){
custMessage.setText("The file could not be read. " + e3.getMessage());
}
}
}
);
return Customers;
}
class Customer extends JFrame
{
private String[] states = {"AL", "AK", "AZ", "AR", "CA", "CO", "CT", "DE",
"FL", "GA", "HI", "ID", "IL", "IN", "IA", "KS", "KY", "LA", "ME",
"MD", "MA", "MI", "MN", "MS", "MO", "MT", "NE", "NV", "NH", "NJ",
"NM", "NY", "NC", "ND", "OH", "OK", "OR", "PA", "RI", "SC", "SD",
"TN", "TX", "UT", "VT", "VA", "WA", "WV", "WI", "WY"};
private JComboBox StateList = new JComboBox(states);
private JTextField NameText = new JTextField(25);
private JTextField AddressText = new JTextField(25);
private JTextField CityText = new JTextField(25);
private JTextField ZipText = new JTextField(9);
private JTextField PhoneText = new JTextField(10);
private JTextField PopMessageText = new JTextField(30);
private static final long serialVersionUID = 1L;
private AddCustButtonHandler addCusHandler = new AddCustButtonHandler();
public Customer(String who)
{
popUpWindow (who);
}
public void popUpWindow(final String who) {
final JFrame popWindow;
popWindow = new JFrame(who);
popWindow.setSize(425, 350);
popWindow.setLocation(100, 100);
popWindow.setVisible(true);
setDefaultCloseOperation(EXIT_ON_CLOSE);
Container c = new Container();
popWindow.add(c);
c.setLayout(new FlowLayout());
JPanel one = new JPanel();
JPanel two = new JPanel();
JPanel three = new JPanel();
JPanel four = new JPanel();
JPanel five = new JPanel();
JPanel six = new JPanel();
one.add(new JLabel(who + " Name "));
one.add(NameText);
two.add(new JLabel("Address "));
two.add(AddressText);
three.add(new JLabel("City "));
three.add(CityText);
four.add(new JLabel("State "));
StateList.setSelectedIndex(0);
four.add(StateList);
four.add(new JLabel("ZIP"));
four.add(ZipText);
four.add(new JLabel("Phone"));
four.add(PhoneText);
JButton addwho = new JButton("Add " + who);
addwho.setMnemonic('A');
JButton close = new JButton("Close");
close.setMnemonic('C');
JButton deleteFile = new JButton("Delete File");
deleteFile.setMnemonic('D');
five.add(addwho);
five.add(close);
five.add(deleteFile);
PopMessageText.setEditable(false);
PopMessageText.setHorizontalAlignment(JTextField.CENTER);
six.add(PopMessageText);
c.add(one);
c.add(two);
c.add(three);
c.add(four);
c.add(five);
c.add(six);
deleteFile.setToolTipText("Delete File");
addwho.setToolTipText("Add "+ who);
close.setToolTipText("Close");
if (who == "Customer")
addwho.addActionListener(addCusHandler);
close.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
NameText.setText("");
AddressText.setText("");
CityText.setText("");
ZipText.setText("");
PhoneText.setText("");
PopMessageText.setText("");
popWindow.dispose();
}
}
);
deleteFile.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
PopMessageText.setText("");
if (who == "Customer") {
File file = new File("Customer.txt");
boolean cusFileDeleted = file.delete();
if (cusFileDeleted) {
PopMessageText
.setText("Customer file has been deleted");
} else {
PopMessageText
.setText("There was an error in deleting file");
}
}
}
}
);
}
class AddCustButtonHandler implements ActionListener {
public void actionPerformed(ActionEvent addCusHandler) {
int StateIndex;
try {
File file = new File("Customer.txt");
boolean success = file.createNewFile();
if (success) {
PopMessageText
.setText("Customer.txt file created file added");
} else if (file.canWrite()) {
PopMessageText
.setText("Writing data to Customer.txt, file added");
} else {
PopMessageText.setText("Cannot create file: Customer.txt");
}
try {
FileWriter fileW = new FileWriter("Customer.txt", true);
fileW.write(NameText.getText());
fileW.write(",");
fileW.write(AddressText.getText());
fileW.write(",");
fileW.write(CityText.getText());
fileW.write(",");
StateIndex = StateList.getSelectedIndex();
fileW.write(states[StateIndex]);
fileW.write(",");
fileW.write(ZipText.getText());
fileW.write(",");
fileW.write(PhoneText.getText());
fileW.write("\r\n");
fileW.close();
PopMessageText.setText("A new Customer has been added!");
FileReader fileR = new FileReader("Customer.txt");
BufferedReader buffIn = new BufferedReader(fileR);
String textData = buffIn.readLine();
buffIn.close();
}
catch (IOException e1) {
JOptionPane.showMessageDialog(null, e1.getMessage(), "ERROR", 2);
}
NameText.setText("");
AddressText.setText("");
CityText.setText("");
ZipText.setText("");
PhoneText.setText("");
}
catch (IOException e1) {
}
}
}
public void actionPerformed(ActionEvent event){
}
private void Exit_pressed(){
System.exit(0);
}
}
public void actionPerformed(ActionEvent e) {
}
public static void main(String[] args){
JFrame frame = new FVolume();
frame.setSize(380, 350);
frame.setVisible(true);
}
}
答案 0 :(得分：6)
这是问题所在：
final JTextArea custMessage = null;
...
Customers.add(custMessage);
你希望这会做什么？基本上，不允许传递对add方法的空引用。
为了将来参考，自己诊断的线索是找到引用代码的堆栈跟踪的第一位 - 在这种情况下是行：
Customers.add(custMessage);
然后查看它正在使用的内容（Customers和custMessage）并确定它们是否正常。在这种情况下，很明显custMessage的值为null，这是不允许的。