NullPointerException异常延长SimpleCursorAdapter [英] nullPointerException with extended SimpleCursorAdapter
问题描述
我正在学习有关自定义提供商和装载机。举一个简单的例子，我想实现一个的GridView 显示存储在外部SD卡中的图片。虽然我已经读了很多（在SO文件，线程，谷歌群，论坛......），我不能让我的code工作。我知道的几个问题可能会在$此如对$ psent但我要一步一步来。第一个错误停止code是一个 NullPointerException异常，所以我的问题是如何解决它。
I'm learning about custom providers and loaders. As a simple example I'm trying to implement a GridView that shows the pictures stored in the external SD card. Although I've read a lot (documentation, threads on SO, Google groups, forums...) I'm not able to get my code working. I know that several issues may be present on this example but I want to go step by step. The first error stopping the code is a NullPointerException, so my question is how to fix it.
这是我的活动的最小版本（它使用支持库V4）：
This is a minimal version of my Activity (it uses support library v4):
public class MainActivity extends FragmentActivity implements LoaderManager.LoaderCallbacks<Cursor> {
private String[] columns = {
MediaStore.Images.Media.DATA, MediaStore.Images.Media._ID
};
private String orderBy = MediaStore.Images.Media._ID;
private ImageAdapter mAdapter;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Prepare the loader
getSupportLoaderManager().initLoader(0, null, this);
// Initialize the layout Adapter
String[] cols = new String[] {MediaStore.Images.Media.DATA};
int[] views = new int[] {R.id.thumbImage};
mAdapter = new ImageAdapter(getApplication(), R.layout.galleryitem,
null, cols, views, 0);
// Set the layout Adapter
GridView gridview = (GridView) findViewById(R.id.gallery_gridview);
gridview.setAdapter(mAdapter);
}
@Override
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
Uri baseUri = MediaStore.Images.Media.EXTERNAL_CONTENT_URI;
return new CursorLoader(getApplication(), baseUri, columns,
null, null, orderBy);
}
@Override
public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
mAdapter.swapCursor(data);
}
@Override
public void onLoaderReset(Loader<Cursor> loader){
mAdapter.swapCursor(null);
}
}
这是我的延长 SimpleCursorAdapter ：
public class ImageAdapter extends SimpleCursorAdapter {
private int columnIndex;
private LayoutInflater mInflater;
private class ViewHolder {
ImageView imageView;
}
public ImageAdapter(Context c, int layout, Cursor aCursor, String[] from,
int[] to, int flags) {
super(c, layout, aCursor, from, to, flags);
this.mContext = c;
this.mCursor = aCursor;
mInflater = (LayoutInflater)
c.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}
@Override
public int getCount() {return mCursor.getCount();}
@Override
public Object getItem(int position) {return position;}
@Override
public long getItemId(int position) {return position;}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder vh;
View cell = convertView;
if (cell == null) {
vh = new ViewHolder();
// No View passed, create one.
cell = mInflater.inflate(R.layout.galleryitem, null);
// Populate the ViewHolder
vh.imageView = (ImageView) cell.findViewById(R.id.thumbImage);
// Store the ViewHolder inside the layout
cell.setTag(vh);
// Setup the View behavior by setting some listeners...
} else {
vh = (ViewHolder) cell.getTag();
}
// Update the cell View state
// Move the cursor to the current position
mCursor.moveToPosition(position);
// Get the current value for the requested position
columnIndex = mCursor.getColumnIndexOrThrow(MediaStore.Images.Media.DATA);
int imageID = mCursor.getInt(columnIndex);
// Set the content of the image based on the provided Uri
vh.imageView.setImageURI(Uri.withAppendedPath(
MediaStore.Images.Thumbnails.EXTERNAL_CONTENT_URI, "" +
imageID));
return cell;
}
}
当我运行code中的logcat中显示了一个 NullPointerException异常在 getCount将（）方法适配器。似乎光标不被传递给适配器
When I run the code the logcat shows a NullPointerException at the getCount() method of the adapter. It seems that the cursor is not being passed to the adapter.
更新
在活动我传递null作为指针参数到适配器构造函数，因为：
In the activity I'm passing null as the cursor argument to the adapter constructor because:
- 在我不知道如何，因为我使用
getSupportLoaderManager（）来得到一个参考光标。initLoader（） - 同样的事情做对其他SO线程，由于同样的原因，它似乎好工作为他们
更新
这是logcat的输出
This is the logcat output
08-22 09:19:44.754: I/Process(5758): Sending signal. PID: 5758 SIG: 9
08-22 09:23:16.041: D/AndroidRuntime(8691): Shutting down VM
08-22 09:23:16.041: W/dalvikvm(8691): threadid=3: thread exiting with uncaught exception (group=0x4001b188)
08-22 09:23:16.041: E/AndroidRuntime(8691): Uncaught handler: thread main exiting due to uncaught exception
08-22 09:23:16.061: E/AndroidRuntime(8691): java.lang.RuntimeException: Unable to start activity ComponentInfo{uvesoft.com.mycustomadapter/uvesoft.com.mycustomadapter.MainActivity}: java.lang.NullPointerException
08-22 09:23:16.061: E/AndroidRuntime(8691): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2496)
08-22 09:23:16.061: E/AndroidRuntime(8691): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512)
08-22 09:23:16.061: E/AndroidRuntime(8691): at android.app.ActivityThread.access$2200(ActivityThread.java:119)
08-22 09:23:16.061: E/AndroidRuntime(8691): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863)
08-22 09:23:16.061: E/AndroidRuntime(8691): at android.os.Handler.dispatchMessage(Handler.java:99)
08-22 09:23:16.061: E/AndroidRuntime(8691): at android.os.Looper.loop(Looper.java:123)
08-22 09:23:16.061: E/AndroidRuntime(8691): at android.app.ActivityThread.main(ActivityThread.java:4363)
08-22 09:23:16.061: E/AndroidRuntime(8691): at java.lang.reflect.Method.invokeNative(Native Method)
08-22 09:23:16.061: E/AndroidRuntime(8691): at java.lang.reflect.Method.invoke(Method.java:521)
08-22 09:23:16.061: E/AndroidRuntime(8691): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
08-22 09:23:16.061: E/AndroidRuntime(8691): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
08-22 09:23:16.061: E/AndroidRuntime(8691): at dalvik.system.NativeStart.main(Native Method)
08-22 09:23:16.061: E/AndroidRuntime(8691): Caused by: java.lang.NullPointerException
08-22 09:23:16.061: E/AndroidRuntime(8691): at uvesoft.com.mycustomadapter.ImageAdapter.getCount(ImageAdapter.java:34)
08-22 09:23:16.061: E/AndroidRuntime(8691): at android.widget.GridView.setAdapter(GridView.java:128)
08-22 09:23:16.061: E/AndroidRuntime(8691): at uvesoft.com.mycustomadapter.MainActivity.onCreate(MainActivity.java:37)
08-22 09:23:16.061: E/AndroidRuntime(8691): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
08-22 09:23:16.061: E/AndroidRuntime(8691): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2459)
08-22 09:23:16.061: E/AndroidRuntime(8691): ... 11 more
08-22 09:23:16.081: I/dalvikvm(8691): threadid=7: reacting to signal 3
08-22 09:23:16.081: E/dalvikvm(8691): Unable to open stack trace file '/data/anr/traces.txt': Permission denied
我真的AP preciate任何帮助。 TIA
I would really appreciate any help. TIA
推荐答案
第一：摆脱这一行：
private Cursor mCursor;
和代替 mCursor 使用 getCursor（），因为你没有覆盖 swapCursor 方法和 mCursor 将指向旧光标。
and instead of mCursor use getCursor() since you didnt override swapCursor method and mCursor will be point to the old cursor.
第二：变化
@Override
public int getCount() {return mCursor.getCount();}
到
@Override
public int getCount() {
if(getCursor()==null)
return 0;
return getCursor().getCount();
}
或最好不要覆盖 getCount将，的getItem 和 getItemId 所有
这篇关于NullPointerException异常延长SimpleCursorAdapter的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
