为什么这个 Java 程序会抛出 NullPointerException? [英] Why does this Java program throw a NullPointerException?

问题描述

在这个程序中，我需要我的程序将一个类的状态放入一个不可编辑的 JTextArea 中，但我不断收到 NullPointerException.我有一种感觉，这与对象每次启动时都被反序列化有关.如果我删除它并用实际的字符串替换它，它工作正常.我该怎么办?我将在下面发布这两个类.

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 类:

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 类:

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;
    }

}

这是堆栈跟踪:

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

推荐答案

堆栈跟踪几乎可以准确地告诉您问题是什么.您在 BankGUI 类的第 85 行有一个 NullPointerException.如果我的 IDE 是正确的，第 85 行就是这一行

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);

这里只有两件事可以为空.状态区域和帐户.由于您在使用 statusArea 之前就对其进行了初始化，因此它必须是 account.

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

使用前需要先初始化账号.

You need to initialize account before you use it.

BankAccount account = new BankAccount();

我可以看到您尝试在此处的 try catch 块中初始化 account:

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();
}

如果抛出异常 account 不会被初始化.确保这不会引发异常或确保在使用 account 之前检查 null.

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.

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