android AsyncTask 和 RecyclerView 的 NullPointerException

Question

我有一个 AsyncTask class 从网络下载图像并将其设置到 ImageView 中（引用自 here）

相关Activity代码：

    public class ListAdoptions extends AppCompatActivity {
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_list_adoptions);

            recyclerView = (RecyclerView) findViewById(R.id.recycler_view);

            mAdapter = new PetsAdapter(petList);
            RecyclerView.LayoutManager mLayoutManager = new LinearLayoutManager(getApplicationContext());
            recyclerView.setLayoutManager(mLayoutManager);
            recyclerView.setItemAnimator(new DefaultItemAnimator());
            recyclerView.setAdapter(mAdapter);

            preparePetData();
        }


        private void preparePetData() {

            DownloadImageTask imgtask = new DownloadImageTask((ImageView) findViewById(R.id.picture));
            imgtask.execute("https://i.ytimg.com/vi/opKg3fyqWt4/hqdefault.jpg");

            mAdapter.notifyDataSetChanged();
        }

        private class DownloadImageTask extends AsyncTask<String, Void, Bitmap> {
            ImageView bmImage;

            public DownloadImageTask(ImageView bmImage) {
                this.bmImage = bmImage;
            }

            protected Bitmap doInBackground(String... urls) {
                String urldisplay = urls[0];
                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) {
                bmImage.setImageBitmap(result);
            }
        }
    }

错误：

Process: com.example.test3, PID: 8427
    java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setImageBitmap(android.graphics.Bitmap)' on a null object reference
        at com.example.petadoption.ListAdoptions$DownloadImageTask.onPostExecute(ListAdoptions.java:171)
        at com.example.petadoption.ListAdoptions$DownloadImageTask.onPostExecute(ListAdoptions.java:150)
        at android.os.AsyncTask.finish(AsyncTask.java:695)
        at android.os.AsyncTask.-wrap1(Unknown Source:0)
        at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:712)
        at android.os.Handler.dispatchMessage(Handler.java:105)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6541)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

Answer 1

看不到完整代码，但消息是空对象... 确保你在 ListAdoptions 中定义了 bmImage，比如 findViewById(R.id.bmImage) 并检查这个对象是否存在于 xml 中，id bmImage

Answer 2

处理 null 时要做的事情：

确保你的名字拼写正确
添加 if 语句以查看事物何时变为空
使用调试器并单步执行代码以查看对象为空的原因

由于某些原因 bmImage 不存在，我建议查看您的布局和此 class 本身以找到 null

Answer 3

您的 bmImage 似乎始终为空。很可能 findViewById(R.id.picture) 总是空的。确保您使用 id "picture" 定义了 ImageView 并且没有在 XML 布局文件中遗漏 plus 。 android:id="@+id/picture"

android AsyncTask 和 RecyclerView 的 NullPointerException

NullPointerException with android AsyncTask and RecyclerView

java

android

nullpointerexception

android-asynctask

android-recyclerview