Question

所以我有一个函数从sql数据库获取一些坐标并将它们存储在一个arraylist中。我正在从光标读取它时打印值，它们很好。但是，当我在循环后立即打印出存储在数组列表中的值时，所有值似乎都设置为我添加到arraylist的最后一个值。我在这里做错了什么？

代码：

public ArrayList<Coord> getCoordMarkers(int myUserId)
{
    Log.d("getCoordMarkers()", "Called");
Coord myCoord = new Coord();
ArrayList<Coord> markerArray = new ArrayList<Coord>();
String sql = "SELECT "+ userId + " , " + timestamp + " , " + coordX + " , " +
                 coordY + " , " + coordType + " , " + coordId + " , " + coordTypeTable+ "." +
                 coordTypeDesc + " FROM " + coordTable + " JOIN " + coordTypeTable + " ON " +
                 coordTable + "." + coordType + " = " + coordTypeTable + "." + coordTypeAbbr +
                 " WHERE " + userId + " = '" + myUserId + "' AND " + coordTypeTable + "." +
                 coordTypeDesc + " != 'User Location';";
    // AND it is a marker
    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cur = db.rawQuery(sql, null);
    if( cur.moveToFirst()) {
        do {
            myCoord.userId = cur.getInt(0);
            myCoord.timestamp = cur.getLong(1);
            myCoord.x = cur.getDouble(2);
            myCoord.y = cur.getDouble(3);
            myCoord.coordType = cur.getInt(4);
            myCoord.id = cur.getInt(5);
            myCoord.coordTypeDesc = cur.getString(6);
            markerArray.add(myCoord);
            Log.d("getCoordMarkers()", "X: " + myCoord.x + " Y: " + myCoord.y);
        } while( cur.moveToNext());
    }
    cur.close();
    db.close();
    for( int i = 0; i < markerArray.size(); i++ )
    {
        Coord myCoord2 = markerArray.get(i);
        Log.d("Test Marker", "I: "+ i + " X: " + myCoord2.x + " Y: "+ myCoord2.y);

    }
    return markerArray;
}

以下是上述代码中的日志打印输出

06-27 01:46:06.588: DEBUG/getCoordMarkers()(541): Called
06-27 01:46:06.638: DEBUG/getCoordMarkers()(541): X: -122.093451 Y: 37.420599
06-27 01:46:06.638: DEBUG/getCoordMarkers()(541): X: -122.093451 Y: 37.420599
06-27 01:46:06.648: DEBUG/getCoordMarkers()(541): X: -122.094695 Y: 37.419679
06-27 01:46:06.648: DEBUG/getCoordMarkers()(541): X: -122.094695 Y: 37.419679
06-27 01:46:06.648: DEBUG/getCoordMarkers()(541): X: -122.090104 Y: 37.423633
06-27 01:46:06.658: DEBUG/getCoordMarkers()(541): X: -122.090104 Y: 37.423633
06-27 01:46:06.658: DEBUG/getCoordMarkers()(541): X: -122.092078 Y: 37.420464
06-27 01:46:06.658: DEBUG/getCoordMarkers()(541): X: -122.092078 Y: 37.420464
06-27 01:46:06.658: DEBUG/getCoordMarkers()(541): X: -122.091992 Y: 37.420668
06-27 01:46:06.668: DEBUG/getCoordMarkers()(541): X: -122.091992 Y: 37.420668
06-27 01:46:06.668: DEBUG/getCoordMarkers()(541): X: -122.09594 Y: 37.42077
06-27 01:46:06.678: DEBUG/getCoordMarkers()(541): X: -122.09594 Y: 37.42077
06-27 01:46:06.678: DEBUG/getCoordMarkers()(541): X: -122.09255 Y: 37.42118
06-27 01:46:06.678: DEBUG/getCoordMarkers()(541): X: -122.09255 Y: 37.42118
06-27 01:46:06.688: DEBUG/getCoordMarkers()(541): X: -122.095897 Y: 37.423633
06-27 01:46:06.698: DEBUG/getCoordMarkers()(541): X: -122.095897 Y: 37.423633
06-27 01:46:06.708: DEBUG/Test Marker(541): I: 0 X: -122.095897 Y: 37.423633
06-27 01:46:06.708: DEBUG/Test Marker(541): I: 1 X: -122.095897 Y: 37.423633
06-27 01:46:06.708: DEBUG/Test Marker(541): I: 2 X: -122.095897 Y: 37.423633
06-27 01:46:06.708: DEBUG/Test Marker(541): I: 3 X: -122.095897 Y: 37.423633
06-27 01:46:06.718: DEBUG/Test Marker(541): I: 4 X: -122.095897 Y: 37.423633
06-27 01:46:06.718: DEBUG/Test Marker(541): I: 5 X: -122.095897 Y: 37.423633
06-27 01:46:06.728: DEBUG/Test Marker(541): I: 6 X: -122.095897 Y: 37.423633
06-27 01:46:06.739: DEBUG/Test Marker(541): I: 7 X: -122.095897 Y: 37.423633
06-27 01:46:06.739: DEBUG/Test Marker(541): I: 8 X: -122.095897 Y: 37.423633
06-27 01:46:06.739: DEBUG/Test Marker(541): I: 9 X: -122.095897 Y: 37.423633
06-27 01:46:06.748: DEBUG/Test Marker(541): I: 10 X: -122.095897 Y: 37.423633
06-27 01:46:06.748: DEBUG/Test Marker(541): I: 11 X: -122.095897 Y: 37.423633
06-27 01:46:06.748: DEBUG/Test Marker(541): I: 12 X: -122.095897 Y: 37.423633
06-27 01:46:06.748: DEBUG/Test Marker(541): I: 13 X: -122.095897 Y: 37.423633
06-27 01:46:06.758: DEBUG/Test Marker(541): I: 14 X: -122.095897 Y: 37.423633
06-27 01:46:06.758: DEBUG/Test Marker(541): I: 15 X: -122.095897 Y: 37.423633

Answer 1

尝试重新验证myCord。它看起来像一个简单的重新分配问题。即使使用Java，也要学习指针！

    do {
        myCoord = new Coord(); //**********
        myCoord.userId = cur.getInt(0);
        myCoord.timestamp = cur.getLong(1);
        myCoord.x = cur.getDouble(2);
        myCoord.y = cur.getDouble(3);
        myCoord.coordType = cur.getInt(4);
        myCoord.id = cur.getInt(5);
        myCoord.coordTypeDesc = cur.getString(6);
        markerArray.add(myCoord);
        Log.d("getCoordMarkers()", "X: " + myCoord.x + " Y: " + myCoord.y);
        myCoord = null; //for the garbage collector *******
    } while( cur.moveToNext());

Answer 2

需要为每次迭代构造新对象，而不是修改相同的对象，如下：

do {
     myCoord = new Coord();

不要在每次循环迭代时声明它，只重新实现

Answer 3

那是因为你一遍又一遍地向数组中添加同一个对象，每次都要覆盖它的值！每次尝试创建一个新对象，如果可以，请immutable object。

Answer 4

请错误地初始化此代码。使用此代码

ArrayList<Uri> ImageList = new ArrayList<Uri>();

内部

@Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        ArrayList<Uri> ImageList = new ArrayList<Uri>();
        if(requestCode == PICK_FILE)
        {
            if(resultCode == RESULT_OK)
            {
                if(data.getClipData()!=null)
                {
                    int countClipData = data.getClipData().getItemCount();
                    int currentImageSelected = 0;

                    while (currentImageSelected < countClipData)
                    {
                        ImageUri = data.getClipData().getItemAt(currentImageSelected).getUri();
                        ImageList.add(ImageUri);
                        currentImageSelected = currentImageSelected +1;
                    }

                    Alert.setVisibility(View.VISIBLE);
                    Alert.setText("You Have Selected"+ImageList.size()+"Images");
                    Btn_Choose.setVisibility(View.GONE);
                }

                else
                    {
                        Toast.makeText(this, "Please select your car images", Toast.LENGTH_SHORT).show();
                    }
            }
        }
    }
}

ArrayList.add（）无法正常工作

4 个答案: