Question

我正在创建一个应用程序，它将excel文件读入程序并对其进行操作。在此之前，一切都很好。但是，当我为我的应用程序实现GUI时，它不再起作用了。

这是代码：

当程序启动时，它会创建一个带按钮的gui并打开2个新的电子表格。我在这些电子表格中输入数据，然后按下调用crossReferenceButtonListener的按钮，但是无法获取数据。

下面的错误信息

public GUI () throws Exception{
    this.setTitle("NYWM Cross Reference Application");
    this.setSize(400,100);
    this.setVisible(true);
    this.setLocationRelativeTo(null);

    buttonHolder = new Panel (new BorderLayout());
    this.add(buttonHolder);

    crossReference = new Button ("CrossReference");
    generateHPD = new Button ("Generate HPD");

    buttonHolder.add(crossReference, BorderLayout.NORTH);
    buttonHolder.add(generateHPD, BorderLayout.SOUTH);

    crossReference.addActionListener(new crossReferenceButtonListener());
    generateHPD.addActionListener(new generateHPDButtonListener());

    createExcelSheet (log,"C:/Log.xlsx", "Log");
    createExcelSheet(sheet,"C:/Spreadsheet.xlsx", "Spreadsheet");


}

private void createExcelSheet (XSSFWorkbook wb, String path, String fileName) throws Exception{
        try{
        FileOutputStream out = new FileOutputStream (path);
        wb = new XSSFWorkbook ();
        XSSFSheet sheet = wb.createSheet(fileName);
        wb.write(out);

        Process p = Runtime.getRuntime().exec(
                "rundll32 url.dll, FileProtocolHandler " + path);


        }
        catch (Exception e){
            throw e;
        }
    }

    private class crossReferenceButtonListener implements ActionListener {
        public void actionPerformed (ActionEvent event){
            try {


                ref = new CrossReference (log.getSheet("Log"), sheet.getSheet("Spreadsheet"));
                ref.CrossReference();
                JOptionPane.showMessageDialog(null, "Cross Reference was successful!");
            } catch (Exception e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

        }
    }

错误消息：更具体地说，它在此行失败

 ref = new CrossReference (log.getSheet("Log"), sheet.getSheet("Spreadsheet"));

错误：

at java.awt.EventDispatchThread.run(Unknown Source)
java.lang.NullPointerException
    at nywm_crossReferenceApplication.GUI$crossReferenceButtonListener.actionPerformed(GUI.java:70)
    at java.awt.Button.processActionEvent(Unknown Source)
    at java.awt.Button.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

尝试以下方法：

public GUI () throws Exception{
  this.setTitle("NYWM Cross Reference Application");
  this.setSize(400,100);
  this.setVisible(true);
  this.setLocationRelativeTo(null);

  buttonHolder = new Panel (new BorderLayout());
  this.add(buttonHolder);

  crossReference = new Button ("CrossReference");
  generateHPD = new Button ("Generate HPD");

  buttonHolder.add(crossReference, BorderLayout.NORTH);
  buttonHolder.add(generateHPD, BorderLayout.SOUTH);

  // NOTE the order below has been flipped!!!!
  createExcelSheet (log,"C:/Log.xlsx", "Log");
  createExcelSheet(sheet,"C:/Spreadsheet.xlsx", "Spreadsheet");

  crossReference.addActionListener(new crossReferenceButtonListener());
  generateHPD.addActionListener(new generateHPDButtonListener());
}

在原始代码中，创建按钮，然后添加动作侦听器：

crossReference.addActionListener(new crossReferenceButtonListener());

但是，只有在添加crossReferenceButtonListener之后才会调用createExcelSheet，这是Workbook实际被实例化的地方。并且crossReferenceButtonListener想要使用Workbook上的字段，但它们为空，因此您的NullPointerException。

在添加createExcelSheet之前调用crossReferenceButtonListener方法，您将确保在log之前创建了sheet和crossReferenceButtonListener试图使用'em。

Answer 2

Java没有按引用调用语义。您将两个变量log和sheet传递给方法createExcelSheet，显然希望它们在该方法中初始化但不起作用。方法调用后它们仍为null。

createExcelSheet (log,"C:/Log.xlsx", "Log");
createExcelSheet(sheet,"C:/Spreadsheet.xlsx", "Spreadsheet");

您必须重写该方法以返回创建的XSSFWorkbook并将调用更改为

log = createExcelSheet ("C:/Log.xlsx", "Log");
sheet = createExcelSheet("C:/Spreadsheet.xlsx", "Spreadsheet");

在程序执行POI，JAVA时从excel电子表格中读取插入的数据

2 个答案: