Null Pointer Exception | 易学教程

问题

I am getting a null pointer exception, but I dont know why. I checked to see if the cell was null before I read it to a string. So, why is that string null?

private void fillArray() 
{
    try 
    {
        readBook = new HSSFWorkbook(readFile);
    } 
    catch (IOException e) 
    {
        System.out.println("If we know what we're doing, no one should ever see this line.");
    }
    if (readBook != null) 
    {HSSFSheet infoSheet = readBook.getSheetAt(0);
        HSSFRow headingsRow = infoSheet.getRow(0);
        int i = 0;
        HSSFCell cell = headingsRow.getCell(i);
        String columnHeading = cell.toString();
        while (cell != null && !(cell.toString().equals(""))) 
        {
            cell = headingsRow.getCell(i);
            columnHeading = cell.toString();
            columnHeadings.add(columnHeading);
            i++;
        }
        if(columnListIsSetup == false)
        {
            createList();
            columnListIsSetup = true;
        }
    }

回答1:

I think this is the problem:

while (cell != null && !(cell.toString().equals(""))) 
{
    // We know that cell isn't null before this line...
    cell = headingsRow.getCell(i);

    // ... but now we've got a new value for cell, which could be null
    columnHeading = cell.toString();
    columnHeadings.add(columnHeading);
    i++;
}

I suspect you want to change it to:

while (cell != null && !(cell.toString().equals(""))) 
{
    // We know cell isn't null for this...
    columnHeading = cell.toString();
    columnHeadings.add(columnHeading);

    i++;
    // It's fine to set cell to null here... we'll be
    // checking again in a second...
    cell = headingsRow.getCell(i);
}

回答2:

while (cell != null && !(cell.toString().equals("")))  {
    cell = headingsRow.getCell(i);      // here, cell gets reassigned so the 
                                        // "cell != null" check in the while 
                                        // loop condition loses it's value,
                                        // you need to check again

    if (cell == null)       // add the following to make sure the NEW cell value is not null
        break;              //

    columnHeading = cell.toString();
    columnHeadings.add(columnHeading);
    i++;
}

回答3:

Before making sure cell is null (or) empty, you are doing String columnHeading = cell.toString(); which could be causing the NullPointerException in case where cell is null.

来源：https://stackoverflow.com/questions/11369678/null-pointer-exception

标签

java

nullpointerexception

poi-hssf