使用异步任务将数据显示到列表视图时强制关闭应用程序

Posted 2023-04-19

【中文标题】使用异步任务将数据显示到列表视图时强制关闭应用程序

【英文标题】：Force Close App when display data into listview using asynctask

【发布时间】：2018-03-01 04:14:15

【问题描述】：

当我运行 applicatiom 时被强制关闭。此应用程序功能是使用 asynctask 在 listview 中显示来自 sqlite 的数据。但我在

处收到空指针错误

collegeList.add(地图); --> 空指针

然后，我想要的是，数据将按 id 显示。 id 是外键。

这是我的代码：

private class GetHttpResponse extends AsyncTask<Void, Void, Void> 
    ArrayList<HashMap<String, String>> collegeList;
    protected void onPreExecute()
     super.onPreExecute();

    protected Void doInBackground(Void... arg0) 
        List<Rainday> raindaydetail = new LinkedList<Rainday>();
        raindaydetail=dbHelper.getAllRainday();

        for(Rainday temp_vg: raindaydetail)
            temp_vg.getId();
            temp_vg.getYear_rainday();
            temp_vg.getJan();
            temp_vg.getFeb();
            temp_vg.getMar();
            temp_vg.getApr();
            temp_vg.getMay();
            temp_vg.getJun();
            temp_vg.getJul();
            temp_vg.getAug();
            temp_vg.getSep();
            temp_vg.getOct();
            temp_vg.getNov();
            temp_vg.getDec();

            // creating new HashMap
            HashMap<String, String> map = new HashMap<String, String>();
            // adding each child node to HashMap key => value
            map.put("id", String.valueOf(temp_vg.getId()));
            map.put("year_rainfall", temp_vg.getYear_rainday());
            map.put("jan", temp_vg.getJan());
            map.put("feb", temp_vg.getFeb());
            map.put("mar", temp_vg.getMar());
            map.put("apr", temp_vg.getApr());
            map.put("may", temp_vg.getMay());
            map.put("jun", temp_vg.getJun());
            map.put("jul", temp_vg.getJul());
            map.put("aug", temp_vg.getAug());
            map.put("sep", temp_vg.getSep());
            map.put("oct", temp_vg.getOct());
            map.put("nov", temp_vg.getNov());
            map.put("dec", temp_vg.getDec());

          collegeList.add(map);
    

    return null;



protected void onPostExecute(Void result)


    proCollageList.setVisibility(View.GONE);
    listCollege.setVisibility(View.VISIBLE);
    if (collegeList != null) 
        ListAdapterRainday adapter = new ListAdapterRainday(RaindayReport.this, collegeList);
        listCollege.setAdapter(adapter);

mysqliteHelper：

 public List<Rainday> getAllRainday() 
    List<Rainday> rainday = new LinkedList<Rainday>();
    // 1. build the query
    String query = "SELECT  * FROM " + TABLE_RAINDAY + " WHERE id = " + KEY_ID_RAINDAY;
    // 2. get reference to writable DB
    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cursor = db.rawQuery(query, null);
    // 3. go over each row, build book and add it to list
    Rainday rainday1 = null;
    if (cursor.moveToFirst()) 
        do 
            rainday1 = new Rainday();
            rainday1.setId(Integer.parseInt(cursor.getString(0)));
            rainday1.setYear_rainday(cursor.getString(1));
            rainday1.setJan(cursor.getString(2));
            rainday1.setFeb(cursor.getString(3));
            rainday1.setMar(cursor.getString(4));
            rainday1.setApr(cursor.getString(5));
            rainday1.setMay(cursor.getString(6));
            rainday1.setJun(cursor.getString(7));
            rainday1.setJul(cursor.getString(8));
            rainday1.setAug(cursor.getString(9));
            rainday1.setSep(cursor.getString(10));
            rainday1.setOct(cursor.getString(11));
            rainday1.setNov(cursor.getString(12));
            rainday1.setDec(cursor.getString(13));
            // Add vgm
            rainday.add(rainday1);
         while (cursor.moveToNext());
    
    return rainday;

我需要你的帮助。如果有人帮助我，我将不胜感激。

【问题讨论】：

它只是一个空指针.. 查找为什么您的列表为空。你初始化了吗？

【参考方案1】：

我想你忘了初始化你的 collegeList

像这样初始化它

ArrayList<HashMap<String, String>> collegeList;
        collegeList= new ArrayList<>();

【参考方案2】：

在你的代码中，我在下面一行找到了

ArrayList<HashMap<String, String>> collegeList;

而且您没有在代码中的任何位置初始化collegeList