为什么这个Java程序抛出NullPointerException?
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,但我一直得到一个NullPointerException。我觉得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,它就可以正常工作。我该怎么办?我将在下面发布这两个类 BankGUI类:为什么这个Java程序抛出NullPointerException?,java,swing,user-interface,nullpointerexception,Java,Swing,User Interface,Nullpointerexception,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,但我一直得到一个NullPointerException。我觉得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,它就可以正常工作。我该怎么办?我将在下面发布这两个类 BankGUI类: package GUIs; import java.awt.BorderLayout; import java.awt.Color; import java.awt.GridBagLayout; impor
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();
}
}
}
银行账户类别:
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;
}
}
以下是堆栈跟踪:
Exception in thread "main" java.lang.NullPointerException
at GUIs.BankGUI.displayGUI(BankGUI.java:85)
at GUIs.BankGUI.main(BankGUI.java:49)
堆栈跟踪几乎可以准确地告诉您问题所在。在BankGUI类的第85行有一个
NullPointerException
。如果我的IDE是正确的,那么第85行是这一行吗
statusArea.setText(account.status);
这里只有两个东西可以为null。状态区域和帐户。由于您在使用statusArea
之前就初始化了它,因此它必须是account
您需要先初始化帐户,然后才能使用它
BankAccount account = new BankAccount();
我可以看到您在try-catch块中尝试初始化帐户
:
try {
ObjectInputStream objectStream2 = new ObjectInputStream(
new FileInputStream("bankAccounts.txt"));
account = (BankAccount) objectStream2.readObject();
objectStream2.close();
} catch (Exception e) {
e.printStackTrace();
}
如果引发异常,
帐户
未初始化。在使用帐户之前,请确保不会引发异常或检查null
一种可能是用于序列化银行帐户
,bankAccounts.txt
的文件可能已损坏。这可能是因为类本身被修改了。在这种情况下,您可以临时替换
account = (BankAccount) objectStream2.readObject();
与
在反序列化帐户中
。这将允许应用程序加载而不抛出NPE
添加一个ActionListener
调用serializeAccount
并重新保存文件。现在可以还原反序列化Account
方法,并且应用程序应该能够加载NPE插入的显示GUI
行的bankAccounts.txt
?使用account==null
反序列化后,您是否检查了account
是否为null
?或者在try块中使用该帐户,或者在使用它之前检查它是否为null。@SteveB。在答案中添加了null检查。@HashshilingHacker尝试找出它为null的原因。当您尝试反序列化时会发生什么?(查看打印出来的stacktrace,使用断点等)为什么会发生这种情况?(例如,如果bankAccounts.txt不存在,则可能是FileNotFoundException
)。正确的解决方案取决于发生了什么。@MAV我已经阅读了堆栈跟踪,并且理解了它。我对序列化非常陌生,对Java一般来说也很陌生,我只是想知道为什么它会变成null。它是在储存过程中死亡还是什么?帮帮我。。。“我迷路了!”散播黑客没有更多的信息,我不可能说任何具体的话。如果您从反序列化(而不是空指针)中了解stacktrace,请查找您使用的方法以了解抛出它的原因。
account = new BankAccount();