sqlite中的游标保持在-1位置

Question

我正在编写一个Android应用程序，我的数据库中有3列，我已经使用游标将值放入数据库，我想检索它们并显示它。但是在检索它时我得到一个例外，即CursorIndexOutofBounds异常，索引-1请求大小为1.在调试过程中我意识到光标指针根本没有移动，即使在使用moveToFirst（），moveToNext（）等方法之后，它保持在-1位置。这是我的代码

main activity.Java

public class MainActivity extends ActionBarActivity
{
    @Override
    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        TableLayout t1=(TableLayout)findViewById(R.id.activity_main);
        TableRow tr_head=new TableRow(this);
        tr_head.setId(TableRow.generateViewId());
        tr_head.setBackgroundColor(Color.BLUE);
        tr_head.setLayoutParams(new TableLayout.LayoutParams(TableLayout.LayoutParams.FILL_PARENT,TableLayout.LayoutParams.WRAP_CONTENT));

        TextView label_bus=new TextView(this);
        label_bus.setId(TextView.generateViewId());
        label_bus.setText("BUS");
        label_bus.setTextColor(Color.WHITE);
        label_bus.setPadding(5, 5, 5, 5);
        tr_head.addView(label_bus);

        TextView label_arrival = new TextView(this);
        label_arrival.setId(TextView.generateViewId());
        label_arrival.setText("Expected Arrival Time");
        label_arrival.setTextColor(Color.WHITE);
        label_arrival.setPadding(5, 5, 5, 5);
        tr_head.addView(label_arrival);

        TextView fare = new TextView(this);
        fare.setId(TextView.generateViewId());
        fare.setText("Fare");
        fare.setTextColor(Color.WHITE);
        fare.setPadding(5, 5, 5, 5);
        tr_head.addView(fare);
        t1.addView(tr_head, new TableLayout.LayoutParams(
        ViewGroup.LayoutParams.FILL_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT));

        try {
            databaseHelper.addFriend("C23", "5 mins", "10");
            databaseHelper.addFriend("DN46", "7 mins", "10");
            databaseHelper.addFriend("C8A", "2 mins", "10");
        }
        catch (SQLException e)
        {
            System.out.println("Exception Caught"+e);
        } 

        MyDatabaseHelper ob=new MyDatabaseHelper(this);
        ob.getFriends().moveToNext();
        int a;
        String b;
        a=ob.getFriends().getColumnCount();
        System.out.println("No. of columns "+a);
        a=ob.getFriends().getCount();
        System.out.println("No. of rows "+a);
        b=ob.getFriends().getColumnName(2);
        System.out.println("The column is "+b);

        boolean c;

        String BUS="",Expected_Arrival_Time="",f="";
        if(ob.getFriends()!=null) {
            System.out.println("Something is present");
            c=ob.getFriends().moveToFirst();
            System.out.println("The truth value is "+c);
            a=ob.getFriends().getColumnIndex("BUS");
            System.out.println("The index of BUS "+a);
            a=ob.getFriends().getColumnIndex("Expected_Arrival_Time");
            System.out.println("The index of expected arrival time "+a);
            a=ob.getFriends().getColumnIndex("Fare");
            System.out.println("The index of Fare "+a);
            System.out.println(ob.getFriends().getCount());
            a=ob.getFriends().getPosition();
            System.out.println("The position of the cursor is "+a);
            c=ob.getFriends().moveToNext();
            System.out.println("Truth value of c "+c);
            a=ob.getFriends().getPosition();
            System.out.println("The position of the cursor is "+a);

            TableRow tr = new TableRow(this);
            tr.setLayoutParams(new TableLayout.LayoutParams(
                    TableLayout.LayoutParams.FILL_PARENT,
                    TableLayout.LayoutParams.WRAP_CONTENT));

            TextView labelBUS = new TextView(this);
            labelBUS.setText(BUS);
            labelBUS.setTextColor(Color.WHITE);
            tr.addView(labelBUS);

            TextView labelARRIVAL = new TextView(this);
            labelARRIVAL.setText(Expected_Arrival_Time);
            labelARRIVAL.setTextColor(Color.WHITE);
            tr.addView(labelARRIVAL);

            TextView labelFARE=new TextView(this);
            labelFARE.setText(f);
            labelFARE.setTextColor(Color.WHITE);
            tr.addView(labelFARE);

            t1.addView(tr, new TableLayout.LayoutParams(
                    TableLayout.LayoutParams.FILL_PARENT,
                    TableLayout.LayoutParams.WRAP_CONTENT));

            if(!ob.getFriends().moveToNext())

        }
        @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            // Inflate the menu; this adds items to the action bar if it is present.
            getMenuInflater().inflate(R.menu.menu_main, menu);
            return true;
        }

        @Override
        public boolean onOptionsItemSelected(MenuItem item) {
            // Handle action bar item clicks here. The action bar will
            // automatically handle clicks on the Home/Up button, so long
            // as you specify a parent activity in AndroidManifest.xml.
            int id = item.getItemId();

            //noinspection SimplifiableIfStatement
            if (id == R.id.action_settings) {
                return true;
            }

            return super.onOptionsItemSelected(item);
        }
    }
}

my database helper.Java

public class MyDatabaseHelper extends SQLiteOpenHelper
{
    private static final String DATABASE_NAME="Bus_Database";

    public MyDatabaseHelper(Context context) 
    {

        super(context,DATABASE_NAME,null,1);
    }

    public void onCreate(SQLiteDatabase database) 
    {
        database.execSQL("CREATE TABLE busdb (BUS TEXT, Expected_Arrival_Time TEXT, Fare INTEGER);");
    }
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) 
    {
        db.execSQL("DROP TABLE IF EXISTS busdb");

        onCreate(db);
    }

    public void addFriend(String BUS,String Expected_Arrival_Time,String fare) throws SQLException
    {
        ContentValues values=new ContentValues(3);

        values.put("BUS", BUS);

        values.put("Expected_Arrival_Time", Expected_Arrival_Time);

        values.put("fare",fare);

        getWritableDatabase().insert("busdb","BUS",values);
    }

    public Cursor getFriends()
    {
        Cursor cursor = getReadableDatabase().query("busdb",new String[] { "BUS", "Expected_Arrival_Time", "Fare"},null, null, null, null, null);
        return cursor;
    }
}

Answer 1

    System.out.println("Something is present");
    c=ob.getFriends().moveToFirst();
    System.out.println("The truth value is "+c);
    a=ob.getFriends().getColumnIndex("BUS");
    System.out.println("The index of BUS "+a);
    a=ob.getFriends().getColumnIndex("Expected_Arrival_Time");
    System.out.println("The index of expected arrival time "+a);
    a=ob.getFriends().getColumnIndex("Fare");

每次调用getFriends()方法时，你都会得到新的Cursor。所以很明显你不能对它做任何事情。将光标保存在某个变量中，然后使用它。

Cursor friendsCursor = ob.getFriends();
if (friendsCursor != null) {
    friendsCursor.moveToFirst();
    // you code
}