java - 写入 RandomAccessFile 时出现 NullPointerException

Question

我可以在创建随机访问文件后直接写入它，但一旦我尝试从我的函数写入，它就会中断。

private void openFile()
{
    JFileChooser fileChooser = new JFileChooser();

    fileChooser.setFileSelectionMode(JFileChooser.FILES_ONLY );
    int result = fileChooser.showSaveDialog( this );

    // user clicked Cancel button on dialog
    if ( result == JFileChooser.CANCEL_OPTION )
    {
        return;
    }

    File fileName = fileChooser.getSelectedFile();

    if ( fileName == null || fileName.getName().equals( "" ) )
    {
    JOptionPane.showMessageDialog( this,
        "Invalid File Name",
        "Invalid File Name",
        JOptionPane.ERROR_MESSAGE );
    }
    else 
    {
     // Open the file
        try 
        {
            RandomAccessFile output = new RandomAccessFile( fileName, "rw" );
            createRecord();
        }
        catch ( IOException e ) 
        {
            JOptionPane.showMessageDialog( this,
            "File does not exist",
            "Invalid File Name",
            JOptionPane.ERROR_MESSAGE );
        }      
    }
}

private void createRecord()
{
    String firstName = tfdFirst.getText();
    String lastName = tfdLast.getText();
    String emailAddress = tfdEmail.getText();
    String homeAddress = tfdAddress.getText();
    Long phoneNumber = new Long(tfdPhone.getText());
    char gender = tfdGender.getText().charAt(0);

    System.out.println(firstName + lastName + emailAddress + homeAddress + phoneNumber + gender);

    try
    {
        output.seek(0);
        output.writeChar(gender);
        writeString( output, lastName);
        writeString( output, firstName);
        writeString( output, emailAddress);
        writeString( output, homeAddress);
        output.writeLong(phoneNumber);
    }
    catch(IOException ex)
    {
        ex.printStackTrace();
    }
}

抱歉，代码之墙很长，但这是两个有问题的函数。第一个尝试打开一个名为输出的 RandomAccessFile，然后调用 createRecord() 尝试将用户输入从 JFrame 写入文件。如果我在 createRecord() 的位置放置一个简单的 writeInt() 函数，那么它可以正常工作，但是一旦调用 createRecord() ，它就像如果输出从未存在过。

我已经为此工作了几个小时但无济于事，因此如果有人可以帮助找到解决方案，我将非常感激。

编辑:根据要求进行堆栈跟踪。 (警告，很长...)

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at good_gals_customer_app.NewCustomerUI.createRecord(NewCustomerUI.java:298)
at good_gals_customer_app.NewCustomerUI.openFile(NewCustomerUI.java:273)
at good_gals_customer_app.NewCustomerUI.butCreateRecordActionPerformed(NewCustomerUI.java:186)
at good_gals_customer_app.NewCustomerUI.access$100(NewCustomerUI.java:21)
at good_gals_customer_app.NewCustomerUI$2.actionPerformed(NewCustomerUI.java:87)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6527)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6292)
at java.awt.Container.processEvent(Container.java:2234)
at java.awt.Component.dispatchEventImpl(Component.java:4883)
at java.awt.Container.dispatchEventImpl(Container.java:2292)
at java.awt.Component.dispatchEvent(Component.java:4705)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
at java.awt.Container.dispatchEventImpl(Container.java:2278)
at java.awt.Window.dispatchEventImpl(Window.java:2739)
at java.awt.Component.dispatchEvent(Component.java:4705)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:746)
at java.awt.EventQueue.access$400(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:697)
at java.awt.EventQueue$3.run(EventQueue.java:691)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:719)
at java.awt.EventQueue$4.run(EventQueue.java:717)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:716)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

Answer 1

您似乎遇到了阴影问题...

您已在 openFile 中创建了一个局部变量 output...

RandomAccessFile output = new RandomAccessFile( fileName, "rw" );

但是访问，似乎是createRecord中的实例变量...

output.seek(0);

实例变量很可能为null...

通常，我建议删除重新声明并使用实例字段，但在此情况下，我认为您应该删除实例字段并将 RandomAccessFile 的引用传递给 createRecord 方法

您也没有管理您的资源，请记住，如果您打开，则必须关闭它...

try (RandomAccessFile output = new RandomAccessFile( fileName, "rw" ))
{
    createRecord(output);
}
catch ( IOException e ) 
{
    JOptionPane.showMessageDialog( this,
    "File does not exist",
    "Invalid File Name",
    JOptionPane.ERROR_MESSAGE );
}      

看看The try-with-resources Statement了解更多详情