Why does this Java program throw a NullPointerExce

Question

In this program, I need my program to put a class's status into a JTextArea that is uneditable, but I keep getting a NullPointerException. I have a feeling that this has something to do with the fact that the object is deserialized every time it starts. If I remove this and replace it with an actual String, it works fine. What do I do? I'll post both classes below.

BankGUI class:

package GUIs;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.GridBagLayout;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JCheckBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JList;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.ListSelectionModel;
import javax.swing.ScrollPaneConstants;
import javax.swing.UIManager;

public class BankGUI {

    BankAccount account;

    private void deserializeAccount() {
        try {
            ObjectInputStream objectStream2 = new ObjectInputStream(
                    new FileInputStream("bankAccounts.txt"));
            account = (BankAccount) objectStream2.readObject();
            objectStream2.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        System.out.println("");
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (Exception e) {
            e.printStackTrace();
        }
        BankGUI gui = new BankGUI();
        gui.deserializeAccount();
        gui.displayGUI();
    }

    // all global components for JFrame
    JTextArea statusArea;
    JCheckBox isLockedCheckBox;
    JList depositAmount;
    JList withdrawAmount;
    JButton depositButton;
    JButton withdrawButton;
    JButton saveAccountButton;

    private void displayGUI() {
        JFrame frame = new JFrame("Virtual Bank v3.3");

        Integer[] intList = { 1, 2, 5, 10, 20, 50 };

        JPanel rightPanel = new JPanel();
        rightPanel.setLayout(new BoxLayout(rightPanel, BoxLayout.Y_AXIS));
        rightPanel.setBackground(Color.GREEN);

        JPanel centerPanel = new JPanel();
        centerPanel.setBackground(Color.GREEN);
        centerPanel.setLayout(new GridBagLayout());

        frame.add(BorderLayout.CENTER, centerPanel);
        frame.add(BorderLayout.EAST, rightPanel);

        // add some JLabel's
        JLabel depositAmountLabel = new JLabel("Deposit Amount:");
        JLabel withdrawAmountLabel = new JLabel("Withdraw Amount:");
        JLabel isLockedLabel = new JLabel("Lock account(True/False)");

        // finish components(center panel)
        statusArea = new JTextArea(25, 25);
        statusArea.setEditable(false);
        statusArea.setText(account.status);

        centerPanel.add(statusArea);

        // add this to panel
        isLockedCheckBox = new JCheckBox();
        // add this to panel

        // scrollers and Jlists
        // ***********************************************************************
        depositAmount = new JList(intList);
        JScrollPane scroller1 = new JScrollPane(depositAmount);
        scroller1
                .setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
        scroller1
                .setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
        depositAmount.setVisibleRowCount(1);
        depositAmount.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);

        withdrawAmount = new JList(intList);
        JScrollPane scroller2 = new JScrollPane(depositAmount);
        scroller2
                .setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
        scroller2
                .setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
        depositAmount.setVisibleRowCount(1);
        depositAmount.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        // ***********************************************************************

        depositButton = new JButton("Deposit Amount.");
        withdrawButton = new JButton("Withdraw Amount");
        saveAccountButton = new JButton("Save your Account");

        rightPanel.add(depositAmount);
        rightPanel.add(depositButton);

        frame.setSize(425, 650);
        frame.setVisible(true);
    }

    private void serializeAccount() {
        try {
            ObjectOutputStream objectStream1 = new ObjectOutputStream(
                    new FileOutputStream("bankAccounts.txt"));
            objectStream1.writeObject(account);
            objectStream1.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

}

BankAccount class:

package GUIs;

import java.io.Serializable;

public class BankAccount implements Serializable {

    private static final long serialVersionUID = -5341449653011848470L;

    int balance = 0;
    int userWallet = 0;
    String owner = "Foo Bar";

    String status = "Account Owner: " + owner + "\nAccount balance: $"
            + balance + "\nOwner Wallet Balance: $" + userWallet;
    boolean isLocked = false;

    public int withdraw(int amount) {
        balance -= amount;
        userWallet += amount;
        return userWallet;
    }

    public int deposit(int amount) {
        balance += amount;
        userWallet -= amount;
        return balance;
    }

    public int depositCashIntoWallet(int amount) {
        userWallet += amount;
        return userWallet;
    }

}

Here's the stack trace:

Exception in thread "main" java.lang.NullPointerException
    at GUIs.BankGUI.displayGUI(BankGUI.java:85)
    at GUIs.BankGUI.main(BankGUI.java:49)

Answer 1

One possibility is that the file used to serialize BankAccount, bankAccounts.txt, may be corrupt. This could have happened due to the class itself being modified. In that case you could temporarily replace

account = (BankAccount) objectStream2.readObject();

with

account = new BankAccount();

in deserializeAccount. This will allow the application to load without throwing an NPE.

Add an ActionListener to call serializeAccount and re-save the file. The deserializeAccount method can now be reverted and the application should be able to load the bankAccounts.txt.

Answer 2

The stack trace almost tells you exactly what the problem is. You have a NullPointerException at line 85 in your BankGUI class. If my IDE is correct line 85 is this line

statusArea.setText(account.status);

Only 2 things can be null here. statusArea and account. Since you initialize statusArea right before you use it, it must be account.

You need to initialize account before you use it.

BankAccount account = new BankAccount();

I can see you try to initialize account in a try catch block here:

try {
        ObjectInputStream objectStream2 = new ObjectInputStream(
                new FileInputStream("bankAccounts.txt"));
        account = (BankAccount) objectStream2.readObject();
        objectStream2.close();
} catch (Exception e) {
        e.printStackTrace();
}

If an exception is thrown account is not initialized. Make sure this does not throw an exception or make sure you check for null before you use account.