如果文件不存在，那么这个code创建新的文件，还创建示例工作表Sheet1，但如果文件存在，那么它增加了新的工作表，以现有的Excel文件。

If file does not exist then this code creates new file and also creates sample sheet1 but if file exists then it adds new sheet to existing excel file.

    HSSFWorkbook workbook = null;
    File file = new File(context.getExternalFilesDir(null), "Sample.xls");
    FileOutputStream fileOut = new FileOutputStream(file);

    if (file.exists()) {
        try {
            workbook = (HSSFWorkbook)WorkbookFactory.create(file);
        } catch (InvalidFormatException e) {
            e.printStackTrace();
        }
        HSSFSheet sheet = workbook.createSheet("Sample sheet2");
    }
    else{
        workbook = new HSSFWorkbook();
        HSSFSheet sheet = workbook.createSheet("Sample sheet1");
    }
    workbook.write(fileOut);
    fileOut.close();