android asynctask-imageView獲取圖片空指針異常

imageView獲取圖片空指針異常

通過URL獲取圖片時，然後在imageView中設置，結果系統給我報出空指針異常。

請高手幫忙解決一下，謝謝。

public class DetailsActivity extends Activity {
    ImageView thumbnail = (ImageView) findViewById(R.id.btnThumbnail);
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.detail);

        final Bundle extras = getIntent().getExtras();
        String img = extras.getString("Thumbnail");
        new DownloadImageTask((ImageView) findViewById(R.id.btnThumbnail))
                .execute("http://mysite.com/images/"
                        + img);

        }
    class DownloadImageTask extends AsyncTask<String, Void, Bitmap> {

        public DownloadImageTask(ImageView bmImage) {
            thumbnail = bmImage;
        }

        protected Bitmap doInBackground(String... urls) {
            String urldisplay = urls[0];
            Log.e("URL",urldisplay);
            Bitmap mIcon11 = null;
            try {
                InputStream in = new java.net.URL(urldisplay).openStream();
                mIcon11 = BitmapFactory.decodeStream(in);
            } catch (Exception e) {
                Log.e("Error", e.getMessage());
                e.printStackTrace();
            }
            return mIcon11;
        }

        protected void onPostExecute(Bitmap result) {
            thumbnail.setImageBitmap(result);
        }
    }

}

最佳回答：

寫法沒問題，只不過thumbnail不應該在外部去查找指向他的id
ImageView thumbnail = (ImageView) findViewById(R.id.btnThumbnail);
改成：
ImageView thumbnail即可