Question

//function which is in my DatabaseHelper class which extends SQLiteOpenHelper
public String getCoordinatesLatitude(int id) {
    String rowLat = "";
    SQLiteDatabase db = this.getReadableDatabase();
    String latQuery = "SELECT " + KEY_LATITUDE + "FROM " + TABLE_COORDINATES + "WHERE " + KEY_ID + "=" + id;
    Cursor cursorr = db.rawQuery(latQuery,null);
    if (cursorr != null){
        cursorr.moveToPosition(id);
        rowLat = cursorr.getString(cursorr.getColumnIndex(KEY_LATITUDE));
    }

    // return coordinates
    return rowLat;
}

//function in my main activity
public void onListItemClick(ListView l, View v, int position, long id) {

      selectedFromList = (String) (l.getItemAtPosition(position));
      selectedItem = (int) l.getItemIdAtPosition(position);
      String rowLat = helper.getCoordinatesLatitude(selectedItem);
      Toast.makeText(getApplicationContext(), rowLat, Toast.LENGTH_SHORT).show();

      }

我有listview，其中包含locations。每个位置都包含latitude，longitude和日期。我基本上想要的是点击listview项，并根据其latitude ID从我的数据库中检索项listview值。我设置了getCoordinatesLatitude()函数，但我不知道我的代码是否正确，因为当我点击listview项时，我强行关闭，logcat显示nullPointerException。我怎样才能做到这一点？谢谢

我的列表视图是什么样的 what my listview looks like

UPDATE !!!! 我设法摆脱了nullPointerException错误，并将我的代码更改为此

 public String getLatitudeFromId(long id) {
    String rowLat = "not found";
    SQLiteDatabase db = this.getReadableDatabase();
    //String latQuery = "SELECT " + KEY_LATITUDE + " FROM " + TABLE_COORDINATES + " WHERE " + KEY_ID + "=" + id;
    //Cursor cursor = db.rawQuery(latQuery,null);
    Cursor cursor = db.query(TABLE_COORDINATES, new String[] { "latitude" },"id="+id, null, null, null,null);
    if (cursor.moveToFirst()){
        cursor.moveToPosition((int) id);
        rowLat = cursor.getString(cursor.getColumnIndex("latitude"));
    }
    cursor.close();
    db.close();

    // return coordinates
    return rowLat;
}

和

 public void onListItemClick(ListView l, View v, int position, long id) {
    DatabaseHelper helper = new DatabaseHelper(this);
    selectedFromList = (String) (l.getItemAtPosition(position));
    selectedItem = l.getItemIdAtPosition(position);
    String rowLat = helper.getLatitudeFromId(selectedItem);
    Toast.makeText(getApplicationContext(), rowLat, Toast.LENGTH_SHORT).show();
}

但现在，当我点击listview项目时，找不到toast，因此它会在if（cursor.moveToFirst（））语句中停止，因为游标为空。当我的listview充满项目时，如何将光标置空？：P

UPDATE2 我只是通过将查询更改为此Cursor游标= db.query（TABLE_COORDINATES，new String [] {KEY_DATE}，null，null，null，null，null）来修复问题;其中KEY_DATE是您在列表视图中单击项目时要显示的列名

Answer 1

如果已分配给listview适配器的数组是“array”，则可以检索array.get（position）.getLatitude（）; （我假设您使用了自定义适配器）

Answer 2

首先，您的表必须将列设置为名为_id而不是id的主键。然后你可以传递给getLatitudeFromId你从onItemClick获得的id。

将getLatitudeFromId（long id）更改为：

 public String getLatitudeFromId(long id) {
    String rowLat = "not found";

    SQLiteDatabase db = this.getReadableDatabase();
    Cursor cursor = db.query(TABLE_COORDINATES, new String[] { "latitude" },"_id=" + id, null, null, null,null);
    if (cursor.moveToFirst()){
        rowLat = cursor.getString(cursor.getColumnIndex("latitude"));
    }
    cursor.close();
    db.close();

    // return coordinates
    return rowLat;
}

将onListItemClick更改为：

public void onListItemClick(ListView l, View v, int position, long id) {

    DatabaseHelper helper = new DatabaseHelper(this);
   // ??? selectedFromList = (String) (l.getItemAtPosition(position));
   // ??? selectedItem = l.getItemIdAtPosition(position);
    String rowLat = helper.getLatitudeFromId(id);
    Toast.makeText(getApplicationContext(), rowLat, Toast.LENGTH_SHORT).show();
}

注意：l.getItemAtPosition（position）返回的视图不是字符串，而不是int

Answer 3

我实际上已经解决了问题。奇怪的是，我改变了我的查询游标cursor = db.query（TABLE_COORDINATES，new String [] {KEY_DATE}，null，null，null，null，null）; 只是让选择为null并且只有我想要的列，并且它有效！这个问题可以标记为已回答。

Android基于listview id从数据库中获取单行

3 个答案: