java - 保存客人详细信息时酒店预订系统运行时出错

Question

我正在设计一个 Java 酒店预订系统作为学校项目。当我尝试保存客人的详细信息时，程序给出运行时错误(线程“AWT-EventQueue-0”java.lang.NullPointerException 中的异常)。我该如何解决这个问题？

提前非常感谢。

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;
import java.util.*;

public class GuestDetailsForm extends JFrame implements ActionListener{
    JTextField guestId = new JTextField(20);
    JTextField name = new JTextField(20);
    JTextField surname = new JTextField(20);
    JTextField mobileNo = new JTextField(20);
    JTextField email = new JTextField(20);
    JTextField passportNo = new JTextField(20);
    JPanel p1 = new JPanel();
    JPanel p2 = new JPanel();
    JPanel p3 = new JPanel();
    JButton insert = new JButton("Insert");
    JButton cancel = new JButton("Cancel");
    MainMenu mainMenu;

    // This constructor will create a panel that allows the user to input the clients details
    public GuestDetailsForm() {
        this.setTitle("Guest Details");
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setLayout(new BorderLayout());
        this.setVisible(true);
        this.setSize(500,300);

        p1.setLayout(new FlowLayout());
        p1.add(new JLabel("Guest Details"));

        p2.setLayout(new GridLayout(6,2));
        p2.add(new JLabel("Guest ID:", JLabel.LEFT));
        p2.add(guestId);
        guestId.setEditable(false);
        p2.add(new JLabel("Name:", JLabel.LEFT));
        p2.add(name);  
        p2.add(new JLabel("Surname:", JLabel.LEFT));
        p2.add(surname);  
        p2.add(new JLabel("Mobile No:", JLabel.LEFT));
        p2.add(mobileNo);  
        p2.add(new JLabel("Email:", JLabel.LEFT));
        p2.add(email);  
        p2.add(new JLabel("Passport No:", JLabel.LEFT));
        p2.add(passportNo);  

        try {
            guestId.setText((mainMenu.getGuests().get(mainMenu.getGuests().size() - 1).getId() + 1) + "");
        } catch(Exception e) {
            guestId.setText("1");
        }

        p3.setLayout(new FlowLayout());
        p3.add(insert);
        p3.add(cancel);
        insert.addActionListener(this);
        cancel.addActionListener(this);

        this.add(p1, BorderLayout.NORTH);
        this.add(p2, BorderLayout.CENTER);
        this.add(p3, BorderLayout.SOUTH);
    }

    // This method will clear all the fields
    public void clearFields() {
        name.setText("");
        surname.setText("");
        mobileNo.setText("");
        email.setText("");
        passportNo.setText("");

        try {
            guestId.setText((mainMenu.getGuests().get(mainMenu.getGuests().size() - 1).getId() + 1) + "");
        } catch(Exception e){
            guestId.setText("1");
        }
    }  

    // This method checks that none of the fields are empty
    private boolean validation() {
        if (name.getText().trim().length() == 0) {
            JOptionPane.showMessageDialog(this, "Name cannot be empty", "Error", JOptionPane.ERROR_MESSAGE);
            return false;
        }

        if (surname.getText().trim().length() == 0) {
            JOptionPane.showMessageDialog(this, "Surname cannot be empty", "Error", JOptionPane.ERROR_MESSAGE);
            return false;
        }

        if (mobileNo.getText().trim().length() == 0) {
            JOptionPane.showMessageDialog(this, "Mobile number cannot be empty", "Error", JOptionPane.ERROR_MESSAGE);
            return false;
        }

        if (email.getText().trim().length() == 0) {
            JOptionPane.showMessageDialog(this, "Email cannot be empty", "Error", JOptionPane.ERROR_MESSAGE);
            return false;
        }

        if (passportNo.getText().trim().length() == 0) {
            JOptionPane.showMessageDialog(this, "Passport number cannot be empty", "Error", JOptionPane.ERROR_MESSAGE);
            return false;
        }

        return true;
    }

    @Override
    public void actionPerformed(ActionEvent e) {
       JButton btn = (JButton) e.getSource();
        if (btn == insert) {
            if (validation() == true) {
                MyGuests newGuest = new MyGuests(Integer.parseInt(guestId.getText()), 
                name.getText(),
                surname.getText(),
                mobileNo.getText(),
                email.getText(),
                passportNo.getText());

                mainMenu.getGuests().add(newGuest);

                JOptionPane.showConfirmDialog(this, "Guest has been successfully added!", "Message", JOptionPane.PLAIN_MESSAGE);

                clearFields();
            }    
        }
        if (btn == cancel) {
            this.dispose();
            Guests guests = new Guests();
        }
    }

    public static void main(String args[]) {
        new GuestDetailsForm();
    }
}

主菜单类

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;
import java.util.*;

public class MainMenu extends JFrame implements ActionListener, WindowListener {
    String FILE_PATH = "HotelReservationSystem.hrs";
    JButton guest = new JButton("Guests");
    JButton reservation = new JButton("Reservations");
    JButton bill = new JButton("Bill");
    JButton exit = new JButton("Exit");
    JPanel p1 = new JPanel();
    JPanel p2 = new JPanel();
    ArrayList<MyGuests> guests = new ArrayList<MyGuests>();
    ArrayList<MyReservations> reservations = new ArrayList<MyReservations>();
    ArrayList<MyRooms> rooms = new ArrayList<MyRooms>();

    // This constructor creates the main menu of the program
    public MainMenu() {
        this.setTitle("Hotel Reservation System");
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setLayout(new BorderLayout());
        this.setVisible(true);
        this.setSize(500,300);

        p1.setLayout(new BoxLayout(p1, BoxLayout.Y_AXIS));

        p1.add(new JLabel("Welcome to the Panorama's Hotel Reservation System"));
        p1.add(Box.createVerticalStrut(20));
        p1.add(guest);
        p1.add(Box.createVerticalStrut(20));
        p1.add(reservation);
        p1.add(Box.createVerticalStrut(20));
        p1.add(bill);
        guest.addActionListener(this);
        reservation.addActionListener(this);
        bill.addActionListener(this);

        p2.setLayout(new FlowLayout());

        p2.add(exit);
        exit.addActionListener(this);

        this.add(p1, BorderLayout.NORTH);
        this.add(p2, BorderLayout.SOUTH);
    }

    // Creates a method to return the values that are stored in the MyGuests arraylist
    public ArrayList<MyGuests> getGuests() {
        return guests;
    }

    // Creates a method to return the values that are stored in the MyReservations arraylist
    public ArrayList<MyReservations> getReservations() {
         return reservations;
    }

    // Creates a method to return the values that are stored in the MyRooms arraylist
    public ArrayList<MyRooms> getRooms() {
        return rooms;
    }

    // Creates a method that searches the guest details by id
    public MyGuests searchGuestById(int searchId) {
        for (int i = 0; i < guests.size(); i++) {
            if (guests.get(i).getId() == searchId) {
                return guests.get(i);
            }
        }
        return null;
    }

    // Creates a method that searches the reservation details by id
    public MyReservations searchReservationById(String searchId) {
        for (int i = 0; i < reservations.size(); i++) {
            if (reservations.get(i).getId().equals(searchId)) {
                return reservations.get(i);
            }
        }
        return null;
    }

    // Creates a method that searches the reservation details by room number
    public MyReservations searchReservationByRoom(String searchRoomNo) {
        for (int i = 0; i < reservations.size(); i++) {
            if (reservations.get(i).getRoomNo().equals(searchRoomNo)) {
                return reservations.get(i);
            }
        }
        return null;
    }

    // Load all data from file
    public void windowOpened(WindowEvent e) {
       ToSave save = new ToSave();
       save.load(FILE_PATH);
       guests = save.getGuests();
       reservations = save.getReservations();
       rooms = save.getRooms();
    }

    // Save all data to file
    public void windowClosing(WindowEvent e) {
        ToSave save = new ToSave();
        save.setGuests(guests);
        save.setReservations(reservations);
        save.setRooms(rooms);
        save.save(FILE_PATH);
    }

    public void windowClosed(WindowEvent e) {
    }    

    public void windowIconified(WindowEvent e) {
    }

    public void windowDeiconified(WindowEvent e) {
    }

    public void windowActivated(WindowEvent e) {
    }

    public void windowDeactivated(WindowEvent e) {
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        JButton clicker = (JButton) e.getSource();
        if (clicker == guest) {
            this.dispose();
            Guests guests = new Guests();
        }    
        if (clicker == reservation) {
            this.dispose();
            Reservations reservations = new Reservations();
        }
        if (clicker == bill) {
            this.dispose();
        }    
        if (clicker == exit) {
            System.exit(0);
        }
    }     

    public static void main(String args[]) {
        new MainMenu();
    }
}

Answer 1

您尚未实例化 MainMenu 对象，因此当您尝试向其中添加 guest 时所引用的对象尚不存在。

添加

mainMenu = new MainMenu();

在构造函数中