Question

我想创建一个测验应用程序，其中包含1个问题和4个选项，我创建了一个数据库，使用ListView将问题和选项从中提取到5个文本视图。我浏览了许多教程并使用stackoverflow中的页面使用ArrayList和Cursor创建了它，但我得到了一个错误。

java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.Cursor android.database.sqlite.SQLiteDatabase.query(java.lang.String, java.lang.String[], java.lang.String, java.lang.String[], java.lang.String, java.lang.String, java.lang.String)' on a null object reference
            at com.example.shark.shopapp.merge.getData(merge.java:26)
            at com.example.shark.shopapp.MainActivity.onCreate(MainActivity.java:68)

它显示在Cursor.query（）行中。 MainActivity：

setContentView(R.layout.ques_layout);  db=openOrCreateDatabase("check1.db",SQLiteDatabase.CREATE_IF_NECESSARY,null);
db.execSQL("CREATE TABLE que(ques TEXT,optn1 TEXT,optn2 TEXT,optn3 TEXT,optn4 TEXT)");
ListView l1=(ListView)findViewById(R.id.listView2);
merge mer=new merge(this);
ArrayList<String> data=mer.getData();
l1.setAdapter(new ArrayAdapter<String>(this,R.layout.ques_layout,data));
db.close();

Merge.java：

public merge(Context c){ourContext=c;}
    public ArrayList<String> getData() {
        String[] columns = new String[]{ques, optn1, optn2, optn3, optn4};
        Cursor c2=db.query(DATABASE_TABLE,columns,null,null,null,null,null);
        ArrayList<String> result = new ArrayList<String>();
        int ques = c2.getColumnIndex("_id");
        int optn1 = c2.getColumnIndex("optn1");
        int optn2 = c2.getColumnIndex("optn2");
        int optn3 = c2.getColumnIndex("optn3");
        int optn4 = c2.getColumnIndex("optn4");
        for (c2.moveToFirst(); !c2.isAfterLast(); c2.moveToNext()) {
            result.add(c2.getString(ques) + c2.getString(optn1) + c2.getString(optn2) + c2.getString(optn3) + c2.getString(optn4));
        }
        db.close();
        return result;
        }
    }

Answer 1

试试这个：
String strQuery =＆＃34; select from（选择option1作为表
中的选项工会全部从表格中选择option2作为选项工会全部从表格中选择option3作为选项工会全部选择option4作为表格中的选项）作为结果＆＃34;;
游标c2 = db.SelectQuery（strQuery）;
String [] from = {＆＃34; option＆＃34;}; //您可以根据需要添加任意数量在[] to to = {＆＃34; R.id.TextViewID＆＃34;}; //您可以根据需要添加任意数量。
SimpleCursorAdapter sca = new SimpleCursorAdapter（youractivityname.this，R.layout.listviewlayout，c2，from，to）; yourlistview.setAdapter（SCA）;

Answer 2

你可以这样做。

_cursor = _database.rawQuery("select * from "+YOUR_TABLE_NAME, null);

                _cursor.moveToFirst(); 
                while(_cursor.isAfterLast() == false){

                    _idDB =  _cursor.getInt(_cursor.getColumnIndex(MySqliteHelper.READING_DETAILS_COLUMN_ID));
                    _ivrsDB =  _cursor.getString(_cursor.getColumnIndex(MySqliteHelper.READING_DETAILS_COLUMN_IVRS_NUM));
                    _readingDB = _cursor.getString(_cursor.getColumnIndex(MySqliteHelper.READING_DETAILS_COLUMN_METER_READING)); 
                    _imageDB = _cursor.getString(_cursor.getColumnIndex(MySqliteHelper.READING_DETAILS_COLUMN_IMAGE));
                    _userIdDB = _cursor.getString(_cursor.getColumnIndex(MySqliteHelper.READING_DETAILS_COLUMN_USER_ID));
                    _latDB = _cursor.getString(_cursor.getColumnIndex(MySqliteHelper.READING_DETAILS_COLUMN_LAT));
                    _longDB = _cursor.getString(_cursor.getColumnIndex(MySqliteHelper.READING_DETAILS_COLUMN_LONG));
                    _dateDB = _cursor.getString(_cursor.getColumnIndex(MySqliteHelper.READING_DETAILS_COLUMN_DATE_TIME));
                    _mob1DB = _cursor.getString(_cursor.getColumnIndex(MySqliteHelper.READING_DETAILS_COLUMN_MOB_FIRST));
                    _mob2DB = _cursor.getString(_cursor.getColumnIndex(MySqliteHelper.READING_DETAILS_COLUMN_MOB_SECOND));
                    _reasonSelected  = _cursor.getString(_cursor.getColumnIndex(MySqliteHelper.READING_DETAILS_COLUMN_REASON));

                    //need to call UPDATE WS
                    updateDataFromDB(context, _idDB , _ivrsDB , _readingDB, _imageDB, _userIdDB , _latDB , _longDB, _dateDB , _mob1DB , _mob2DB
                            ,_reasonSelected);


                    System.out.println("Id from DB=" + _idDB);
                    System.out.println("IVRS NUMBER="+ _ivrsDB);
                    System.out.println("Reading="+ _readingDB);
                    //System.out.println("Image Path="+_cursor.getString(_cursor.getColumnIndex(MySqliteHelper.READING_DETAILS_COLUMN_IMAGE)));
                    System.out.println("USer ID="+_userIdDB);
                    System.out.println("LAT="+_latDB);
                    System.out.println("LONG="+_longDB);
                    System.out.println("DATE="+ _dateDB);
                    System.out.println("MOb 1="+_mob1DB);
                    System.out.println("Mob 2="+_mob2DB);
                    System.out.println("Reason from db =" + _reasonSelected);




                    _cursor.moveToNext();
                }
                _cursor.close();

Answer 3

您的db变量未在merge初始化。

假设您使用SQLiteOpenHelper，请致电，例如帮助对象上的getWritableDatabase()获取可以分配给SQLiteDatabase的{{1}}对象。

我试图从数据库中获取数据并将其放在android

3 个答案: