Question

我有一个主要活动，它有标签主机。我将tabhost类膨胀到我的主要活动中。

我的问题是其中一个标签有一个gridview。单击选项卡时，其类获得NullPointerException。

我的主要活动滚动到达底部，我调用提供asyc任务数据的内部类，但notifyDataSetChanged给出异常。

这是logcat。

04-07 14:36:51.520: E/AndroidRuntime(8503): FATAL EXCEPTION: main
04-07 14:36:51.520: E/AndroidRuntime(8503): java.lang.NullPointerException
04-07 14:36:51.520: E/AndroidRuntime(8503):     at com.example.profile.Tab2$GetDataTask.onPostExecute(Tab2.java:176)
04-07 14:36:51.520: E/AndroidRuntime(8503):     at com.example.profile.Tab2$GetDataTask.onPostExecute(Tab2.java:1)
04-07 14:36:51.520: E/AndroidRuntime(8503):     at android.os.AsyncTask.finish(AsyncTask.java:631)
04-07 14:36:51.520: E/AndroidRuntime(8503):     at android.os.AsyncTask.access$600(AsyncTask.java:177)
04-07 14:36:51.520: E/AndroidRuntime(8503):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
04-07 14:36:51.520: E/AndroidRuntime(8503):     at android.os.Handler.dispatchMessage(Handler.java:99)
04-07 14:36:51.520: E/AndroidRuntime(8503):     at android.os.Looper.loop(Looper.java:176)
04-07 14:36:51.520: E/AndroidRuntime(8503):     at android.app.ActivityThread.main(ActivityThread.java:5419)
04-07 14:36:51.520: E/AndroidRuntime(8503):     at java.lang.reflect.Method.invokeNative(Native Method)
04-07 14:36:51.520: E/AndroidRuntime(8503):     at java.lang.reflect.Method.invoke(Method.java:525)
04-07 14:36:51.520: E/AndroidRuntime(8503):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046)
04-07 14:36:51.520: E/AndroidRuntime(8503):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862)
04-07 14:36:51.520: E/AndroidRuntime(8503):     at dalvik.system.NativeStart.main(Native Method)

以下是我调用该类的主要活动代码：

Tab2 newObj = new Tab2(); newObj.LoadMore(); //

这是我的标签类代码：

    public class GetDataTask extends AsyncTask<Void, Void, List<String>> {
    @Override
    protected List<String> doInBackground(Void... params) {
        // Simulates a background job.
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
        }
        return products_x220;
    }

    @Override
    protected void onPostExecute(List<String> result) {
        products_x220.addAll(GetDataService.List_x220);

        Products_imgAdapter.notifyDataSetChanged(); // this line gives error.
        super.onPostExecute(result);
    }
}

Tab2类的整个代码：

公共类Tab2扩展了Fragment {

List<String> products_x220 = new ArrayList<String>();
ImageAdapter Products_imgAdapter;
DisplayImageOptions options;


@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    options = new DisplayImageOptions.Builder()
    .showImageOnLoading(R.drawable.ic_stub)
    .showImageForEmptyUri(R.drawable.ic_empty)
    .showImageOnFail(R.drawable.ic_error)
    .cacheInMemory(false)
    .cacheOnDisk(true)
    .considerExifParams(true)
    .bitmapConfig(Bitmap.Config.ARGB_8888)
    .build();

    new GetDataService();
    new GetDataTask().execute();
    products_x220.addAll(GetDataService.List_x220);

}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.collectionproducts_layout, container, false);


    ExpandableHeightGridView mGridViewProducts = (ExpandableHeightGridView)v.findViewById(R.id.gridview_products);
    mGridViewProducts.setExpanded(true);

    Products_imgAdapter = new ImageAdapter();
    mGridViewProducts.setAdapter(Products_imgAdapter);

    loadMoreBtn = (Button)v.findViewById(R.id.loadMoreBtn);

    loadMoreBtn.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {

            GetDataService.List_x220.clear();
            new GetDataService();
            new GetDataTask().execute();
        }
    });

    return v;
}

@Override
public void setUserVisibleHint(boolean visible)
{
    super.setUserVisibleHint(visible);
    if (visible && isResumed())
    {
        onResume();
    }
}
@Override
public void onResume()
{
    super.onResume();
    if (!getUserVisibleHint())
    {
        isVisible = false;
        return;
    }
    Tab1.isVisible = false;
    isVisible = true;

}

public void LoadMore(){
    new GetDataService();
    new GetDataTask().execute();
}

public class GetDataTask extends AsyncTask<Void, Void, List<String>> {
    @Override
    protected List<String> doInBackground(Void... params) {
        // Simulates a background job.
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
        }
        return products_x220;
    }

    @Override
    protected void onPostExecute(List<String> result) {
        products_x220.addAll(GetDataService.List_x220);

        Products_imgAdapter.notifyDataSetChanged();
        super.onPostExecute(result);
    }
}
public class ImageAdapter extends BaseAdapter {

    private LayoutInflater inflater;
    ImageLoader imageLoader = ImageLoader.getInstance();
    ImageAdapter() {
        inflater = LayoutInflater.from(getActivity());
    }

    @Override
    public int getCount() {
        return products_x220.size();
    }

    @Override
    public Object getItem(int position) {
        return null;
    }

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

    @Override
    public View getView(final int position, View convertView, ViewGroup parent) {
        final ViewHolder2 holder2;
        View view = convertView;
        if (view == null) {
            view = inflater.inflate(R.layout.item_grid_products, parent, false);
            holder2 = new ViewHolder2();
            assert view != null;
            holder2.imageView = (ImageView) view.findViewById(R.id.image);
            holder2.progressBar = (ProgressBar) view.findViewById(R.id.progress);
            view.setTag(holder2);
        } else {
            holder2 = (ViewHolder2) view.getTag();
        }

        ImageLoader.getInstance()
                .displayImage(products_x220.get(position), holder2.imageView, options, new SimpleImageLoadingListener() {
                    @Override
                    public void onLoadingStarted(String imageUri, View view) {
                        holder2.progressBar.setProgress(0);
                        holder2.progressBar.setVisibility(View.VISIBLE);
                    }

                    @Override
                    public void onLoadingFailed(String imageUri, View view, FailReason failReason) {
                        holder2.progressBar.setVisibility(View.GONE);
                    }

                    @Override
                    public void onLoadingComplete(String imageUri, View view, Bitmap loadedImage) {
                        holder2.progressBar.setVisibility(View.GONE);
                    }
                }, new ImageLoadingProgressListener() {
                    @Override
                    public void onProgressUpdate(String imageUri, View view, int current, int total) {
                        holder2.progressBar.setProgress(Math.round(100.0f * current / total));
                    }
                });

        return view;
    }

}
static class ViewHolder2 {
    ImageView imageView;
    ProgressBar progressBar;
}

}

请问为什么我会得到nullpointerexception。谢谢大家。

Answer 1

在调用GetDataTask异步任务时，适配器不存在，因此，在PostExecute上，适配器尚不存在 - 因此您的选项卡未获取适配器的对象....

Answer 2

你应该在setAdapter之后调用asyncTask。只需删除

new GetDataTask().execute();

来自oncreate并在

之后设置

mGridViewProducts.setAdapter(Products_imgAdapter);

从另一个类调用内部类并且notifyDataSetChanced调用NullPointerException

2 个答案: