setImageBitmap（）/ NullpointerException

Question

我尝试使用Bitmap和ImageView下载并显示图片。 我使用的AsyncTask以url和view作为参数，但我总是得到一个

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setImageBitmap(android.graphics.Bitmap)' on a null object reference

doInBackground：

protected Bitmap doInBackground(Void... params) {
        try {
            Log.d("MyActivity", url.toString());
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            InputStream is = connection.getInputStream();
            Bitmap res = BitmapFactory.decodeStream(is);
            is.close();
            if(res!= null)
                Log.d("MyActivity", "I'm here");
            return res;
        } catch(IOException ioe){

        }
        return null;
    }

onPostExecute：

 protected void onPostExecute(Bitmap bitmap) {
       ImageView imageView = (ImageView) view.findViewById(R.id.imageView);
        if(bitmap == null)
            Snackbar.make(view, "Error", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        else if(bitmap instanceof Bitmap) {
            imageView.setImageBitmap(bitmap);
            Snackbar.make(view, "Downloaded", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    }

Answer 1

您传递给asynctask的视图不包含带有R.id.imageView id的ImageView，因此当您尝试在View中找到ImageView时，您将获得一个null ImageView。然后，当调用setImageBitmap方法时，由于ImageView == null，会得到nullPointerException。

ImageView imageView = (ImageView) view.findViewById(R.id.imageView);
if(null == imageView) {
  Log.e("Error", "Ouh! there is no there is no child view with R.id.imageView ID within my parent view View.");
}

尝试上面的代码，您将看到在LogCat中打印错误。