I write a Library login page with Java Swing and try to run it. The page can run properly but when I type in any username, choose a type and press login, it raises an AWT-EventQueue-0: NullPointerException
.
The Library class deserializes two files containing the user and book info, and initialize them into objects.
User is the parent class of Member and Staff, Book class represents a book with its title, description, copy. The methods of these classes are all correct.
LoginWindow.java
import java.awt.EventQueue;
import javax.swing.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
public class LoginWindow {
private JFrame frmLogIn;
private JTextField textField;
/**
* Launch the application.
*/
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
LoginWindow window = new LoginWindow();
window.frmLogIn.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
/**
* Initialize the contents of the frame.
*/
public LoginWindow() {
frmLogIn = new JFrame();
JLabel lblName = new JLabel("Name:");
lblName.setBounds(23, 25, 46, 14);
frmLogIn.getContentPane().add(lblName);
JLabel lblType = new JLabel("Type:");
lblType.setBounds(23, 56, 46, 14);
frmLogIn.getContentPane().add(lblType);
JTextField textField = new JTextField();
textField.setBounds(71, 22, 158, 20);
frmLogIn.getContentPane().add(textField);
textField.setColumns(10);
JComboBox comboBox = new JComboBox();
comboBox.setModel(new DefaultComboBoxModel(new String[] {"Staff", "Member"}));
comboBox.setBounds(71, 53, 158, 20);
frmLogIn.getContentPane().add(comboBox);
JButton btnNewButton = new JButton("Login");
btnNewButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
if (!Library.users.containsKey(textField.getText())) {
new CreateUserWindow();
frmLogIn.dispose();
}
else if (!Library.users.get(textField.getText()).getUserType().equals(comboBox.getActionCommand())) {
JOptionPane.showMessageDialog(null,
"The user name and user type do not match, please try again.",
"User information mismatch", JOptionPane.ERROR_MESSAGE);
}
else {
String type = comboBox.getActionCommand();
if (type.equals("Staff")) new StaffWindow((Staff)Library.users.get(textField.getText()));
else new MemberWindow((Member)Library.users.get(textField.getText()));
}
}
});
btnNewButton.setBounds(23, 88, 206, 23);
frmLogIn.getContentPane().add(btnNewButton);
frmLogIn.setTitle("Log in");
frmLogIn.setBounds(100, 100, 268, 185);
frmLogIn.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frmLogIn.getContentPane().setLayout(null);
frmLogIn.setVisible(true);
}
}
Library.java
import java.io.*;
import java.text.ParseException;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.*;
public class Library implements Serializable{
public static HashMap<String, User> users;
public static HashMap<String, Book> books;
public Library(boolean readFromSerialized) throws IOException, ClassNotFoundException {
users = new HashMap<>();
books = new HashMap<>();
if (readFromSerialized) {
ObjectInputStream u_in = new ObjectInputStream(new FileInputStream("./Assignment/data/Users.txt"));
ObjectInputStream b_in = new ObjectInputStream(new FileInputStream("./Assignment/data/Books.txt"));
users = (HashMap<String, User>) u_in.readObject();
books = (HashMap<String, Book>) b_in.readObject();
u_in.close();
b_in.close();
}
}
public static void serializeToFile() {
try {
ObjectOutputStream u_out = new ObjectOutputStream(new FileOutputStream("./Assignment/data/Users.txt"));
ObjectOutputStream b_out = new ObjectOutputStream(new FileOutputStream("./Assignment/data/Books.txt"));
u_out.writeObject(Library.users);
b_out.writeObject(Library.books);
u_out.close();
b_out.close();
} catch (Exception ex) {}
}
}
Error message shows that the following code snippet contains some error, the code is about user info validation:
if (!Library.users.containsKey(textField.getText())) {
new CreateUserWindow();
frmLogIn.dispose();
}