错误：尝试在空对象引用“android.database.Cursor com.myapplication.DatabaseHeleper.getData()”上调用虚拟方法

Question

This the error

我在执行下面提到的代码后附加了上面的错误，下面给出的代码不是完整代码，但它针对我的问题所在的主要代码。我试图通过一个变量发送短信，其中数字存储在从 sqlite 数据库中获取。所以下面的代码显示我试图从 dbhelper.java 类中获取并存储在 sendms.java 类中的变量 num 中，但我认为它没有获取因此我会要求您查看代码并指导我在哪里改进错误。我希望现在的问题很清楚，并且给出了足够的描述，所以请帮忙。

sendsms.java

   try {

            Cursor cursor = databaseHelper.getdata();
            while (cursor.moveToNext())

            {

                String num = cursor.getString(0);
                String numm = cursor.getString(1);
                String nummm = cursor.getString(2);

            }

       }
        catch(Exception e){
            Toast.makeText(getBaseContext(), e.getMessage(),
                    Toast.LENGTH_SHORT).show();
        }

            public void run() {

                sms.sendTextMessage(num, null, "Help! I've met with an accident at http://maps.google.com/?q=" + String.valueOf(latitude) + "" +
                        "," + String.valueOf(longitude), null, null);

dbhelper.java

    public static final String TABLE_REGISTER = "signin";
    public static final String COL_ID = "USER_ID";
    public static final String COL_NAME = "NAME";
    public static final String COL_PHONE = "PHONE_NUMBER";
    public static final String COL_EMAIL = "EMAIL";
    public static final String COL_PASSWORD = "PASSWORD";
    public static final String COL_CONFIRM_PASSWORD = "CONFIRM_PASSWORD";

    public static final String COL_NAMEone_CON = "NAMEONE";
    public static final String COL_NUMBERone_CON = "NUMBERONE";
    public static final String COL_NAMEtwo_CON = "NAMETWO";
    public static final String COL_NUMBERtwo_CON = "NUMBERTWO";
    public static final String COL_NAMEthree_CON = "NAMETHREE";
    public static final String COL_NUMBERthree_CON = "NUMBERTHREE";

    public SQLiteDatabase db;

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE " + TABLE_REGISTER + "(" + COL_ID + " INTEGER PRIMARY KEY AUTOINCREMENT , "
                + COL_NAME + " TEXT , " + COL_PHONE + "  LONG  UNIQUE  ," + COL_EMAIL + " VARCHAR UNIQUE," + COL_PASSWORD + " VARCHAR , "
                + COL_CONFIRM_PASSWORD + " VARCHAR ," + COL_NAMEone_CON + " TEXT , "
                + COL_NUMBERone_CON + "  LONG  UNIQUE ," + COL_NAMEtwo_CON + " TEXT ," + COL_NUMBERtwo_CON + "  LONG  UNIQUE , "
                + COL_NAMEthree_CON + " TEXT ," + COL_NUMBERthree_CON + " LONG  UNIQUE " + ")");

    }

    public Cursor getdata(){
        SQLiteDatabase db = this.getWritableDatabase();

        Cursor cursor = db.rawQuery("select NUMBERONE, NUMBERTWO,NUMBERTHREE  from signin ", null);

        return cursor;
    }

Answer 1

看来你得先调用getReadableDatabase或者getWritableDatabase才需要数据，这是因为创建数据库需要时间。所以，你必须意识到这一点。

另外，我想说明你必须先cursor.moveToFirst()，然后才能在 sendms.java 上执行 cursor.moveToNext()

Answer 2

请像这样更改您的代码：-

 try {

        Cursor cursor = databaseHelper.getdata();
        if (cursor != null && cursor.getCount() > 0) {
            while (cursor.moveToNext()) {

               String num = cursor.getString(0);
               String numm = cursor.getString(1);
               String nummm = cursor.getString(2);

           }
     }

   }
    catch(Exception e){
        Toast.makeText(getBaseContext(), e.getMessage(),
                Toast.LENGTH_SHORT).show();
    }