重新运行Excel文件中的写入程序会抛出空指针异常

Question

当我重复运行此代码时 - 除了第一次，即使文件已经存在，也会抛出空指针异常。

static public class writeexcel {

    public void write(){
    try{
        File f = new File("C:/Users/acer/Desktop/2.xls");
        Workbook existingbook=null;
        WritableWorkbook ww;
        WritableSheet  sr;
        if(!f.exists()){
            ww = Workbook.createWorkbook(f);
            sr = ww.createSheet("mysheet", 0);

        }else{

            existingbook = Workbook.getWorkbook(f);
            ww = Workbook.createWorkbook(f, existingbook);
            sr = ww.getSheet("Mysheet");
        }
        Label lb = new Label(0, 0, " success");
        Label lb1 = new Label (1,5,"new task");
        sr.addCell(lb);
        sr.addCell(lb1);
        ww.write();
        ww.close();
    }catch(Exception e){
        e.printStackTrace();
    }

    }
}

public static void main(String[] args) {
    // TODO Auto-generated method stub
    writeexcel exl = new writeexcel() ;
    exl.write();    
    }

}

Answer 1

尝试beloe代码，它将在100个单元格和行中写入“Success”。然后，您可以根据您的要求对其进行修改。

public class Test {

    public static void main(final String[] args) {
        Test exl = new Test();
        exl.write();
    }

    public void write() {
        try {
            File f = new File("fileLocation.xls");
            Workbook existingbook = null;
            WritableWorkbook ww;
            WritableSheet sr;
            if (!f.exists()) {
                ww = Workbook.createWorkbook(f);
                sr = ww.createSheet("mysheet", 0);

            } else {

                existingbook = Workbook.getWorkbook(f);
                ww = Workbook.createWorkbook(f, existingbook);
                sr = ww.getSheet("Mysheet");
            }
            for (int i = 0; i < 100; i++) {
                for (int x = 0; x < 100; x++) {
                    Label lb1 = new Label(i + 1, x + 1, "new task");
                    // Label lb = new Label(i, x, " success");

                    // sr.addCell(lb);
                    sr.addCell(lb1);

                }
            }
            ww.write();
            ww.close();
        } catch (Exception e) {
            e.printStackTrace();
        }

    }
}