java.lang.NullPointerException : Attempt to invoke virtual method . .. 空对象引用

Question

我有一个名为 ResultActivity 的类，它显示地点信息列表。此 Activity 向服务器请求并接收一个 json 对象。但是我的 Activity 不起作用，我发现了这个错误

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.shameal/com.example.shameal.ResultActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'com.android.volley.toolbox.ImageLoader com.example.shameal.controller.AppController.getImageLoader()' on a null object reference

结果 Activity

public class ResultActivity extends ListActivity{
private ListView listView;
private FeedListAdapter listAdapter;
private List<FeedItem> feedItems;

@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    ...

    listView = (ListView) findViewById(R.id.list);
    feedItems = new ArrayList<FeedItem>();
    listAdapter = new FeedListAdapter(this, feedItems);
    listView.setAdapter(listAdapter);

    ...
}

FeedListAdapter

public class FeedListAdapter extends BaseAdapter {  
private Activity activity;
private LayoutInflater inflater;
private List<FeedItem> feedItems;
ImageLoader imageLoader = AppController.getInstance().getImageLoader();

public FeedListAdapter(Activity activity, List<FeedItem> feedItems) {
    this.activity = activity;
    this.feedItems = feedItems;
}

@Override
public int getCount() {
    return feedItems.size();
}

@Override
public Object getItem(int location) {
    return feedItems.get(location);
}

@Override
public long getItemId(int position) {
    return position;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {

    if (inflater == null)
        inflater = (LayoutInflater) activity
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    if (convertView == null)
        convertView = inflater.inflate(R.layout.feed_item, null);

    if (imageLoader == null)
        imageLoader = AppController.getInstance().getImageLoader();

    TextView name = (TextView) convertView.findViewById(R.id.name);
    TextView address = (TextView) convertView
            .findViewById(R.id.address);
    TextView description = (TextView) convertView
            .findViewById(R.id.description);
    TextView rate = (TextView) convertView.findViewById(R.id.rate);
    NetworkImageView image = (NetworkImageView) convertView
            .findViewById(R.id.image);
    FeedImageView feedImageView = (FeedImageView) convertView
            .findViewById(R.id.feedImage1);

    FeedItem item = feedItems.get(position);

    name.setText(item.getName());

    // Converting timestamp into x ago format
    address.setText(item.getAddress());

    // Chcek for empty status message
    if (!TextUtils.isEmpty(item.getDescription())) {
        description.setText(item.getDescription());
        description.setVisibility(View.VISIBLE);
    } else {
        // status is empty, remove from view
        description.setVisibility(View.GONE);
    }

    rate.setText(item.getRate());
    image.setImageUrl(item.getImage(), imageLoader);

    // Feed image
    if (item.getMenu() != null || item.getMenu() != "0") {
        feedImageView.setImageUrl(item.getMenu(), imageLoader);
        feedImageView.setVisibility(View.VISIBLE);
        feedImageView
                .setResponseObserver(new FeedImageView.ResponseObserver() {
                    @Override
                    public void onError() {
                    }

                    @Override
                    public void onSuccess() {
                    }
                });
    } else {
        feedImageView.setVisibility(View.GONE);
    }

    return convertView;
}

我认为错误在这一行:listAdapter = new FeedListAdapter(this, feedItems); 但我找不到修复它的解决方案。

感谢您的帮助!

已编辑:我遵循了本教程:http://www.androidhive.info/2014/06/android-facebook-like-custom-listview-feed-using-volley/ .我只是使用本教程的代码，但我的应用程序无法运行。

Answer 1

在 list 中添加一行

<application
    android:name=".AppController" //this one
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme">