ParseQuery NullPointer异常悖论

Question

这是我之前提问的后续问题：Parse RecyclerView Image Query

我现在有一个矛盾的nullpointer异常。我试图通过下面概述的方式从内部方法中提取列表。

public List<ParseObject> objectList;

public List<String> getData(String filter){

    ParseQuery<ParseObject> query = ParseQuery.getQuery("Images");
    query.whereEqualTo("category", filter);
    query.findInBackground(new FindCallback<ParseObject>() {

        public void done(List<ParseObject> downloads, com.parse.ParseException e) {
            if (e == null) {
                objectList=downloads;
            } else {
                Log.d("score", "Error: " + e.getMessage());
            }
        }
    });

    int size = objectList.size();

    for (int i = 0; i<size; i++){
        ParseFile file = (ParseFile) objectList.get(i).get("Image");
        urlList.add(file.getUrl());
    }

    return urlList;

}

我该如何解决这个问题？有哪些方法比我使用的更好？

编辑：

这是它正在做的事情。目前没有例外，但Recyclerview没有显示任何内容。

  @Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.activity_events, container, false);
    setList("sport", rootView);
    return rootView;
}

public void set() {

}

public void setList(String filter, View view){
    RecyclerView recyclerView = (RecyclerView) view.findViewById(R.id.rv);
    LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getActivity());
    recyclerView.setLayoutManager(linearLayoutManager);
    MyAdapter myAdapter = new MyAdapter(getData(filter));
    recyclerView.setAdapter(myAdapter);
}

编辑2：更新的代码：

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.activity_events, container, false);
    setList("sport", rootView);
    return rootView;
}

public void set() {

}

public void setList(String filter, View view){
    RecyclerView recyclerView = (RecyclerView) view.findViewById(R.id.rv);
    LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getActivity());
    recyclerView.setLayoutManager(linearLayoutManager);
    MyAdapter myAdapter = new MyAdapter(getData(filter));
    recyclerView.setAdapter(myAdapter);
}


private List<String> urlList = new ArrayList<>();



public List<ParseObject> objectList = new ArrayList<>();
public int size;

public List<String> getData(String filter){


    ParseQuery<ParseObject> query = ParseQuery.getQuery("Images");
    query.whereEqualTo("category", filter);
    query.findInBackground(new FindCallback<ParseObject>() {

        public void done(List<ParseObject> downloads, com.parse.ParseException e) {
            if (e == null) {
                Toast.makeText(getActivity(), "Downloads retrieved", Toast.LENGTH_SHORT).show();
                objectList.addAll(downloads);
            } else {
                Log.d("score", "Error: " + e.getMessage());
            }

            size = objectList.size();
        }
    });



    for (int i = 0; i<size; i++){
        ParseFile file = (ParseFile) objectList.get(i).get("Image");
        urlList.add(file.getUrl());
    }

    return urlList;

}

编辑3：更新了，我添加了适配器代码。

    View rootView;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    rootView = inflater.inflate(R.layout.activity_events, container, false);
    new GetUrlListTask(this).execute("sport");
    return rootView;
}

public void set() {

}



private List<String> urlList = new ArrayList<>();



public List<ParseObject> objectList = new ArrayList<>();
public int size;

public List<String> getData(String filter){


    ParseQuery<ParseObject> query = ParseQuery.getQuery("Images");
    query.whereEqualTo("category", filter);
    query.findInBackground(new FindCallback<ParseObject>() {

        public void done(List<ParseObject> downloads, com.parse.ParseException e) {
            if (e == null) {
                Toast.makeText(getActivity(), "Downloads retrieved", Toast.LENGTH_SHORT).show();
                objectList.addAll(downloads);
            } else {

            }

            size = objectList.size();
        }
    });



    for (int i = 0; i<size; i++){
        ParseFile file = (ParseFile) objectList.get(i).get("Image");
        urlList.add(file.getUrl());
    }

    return urlList;

}

private class GetUrlListTask extends AsyncTask<String,Void,List<String>>
{
    private WeakReference<Fragment> weakRef;

    public GetUrlListTask(Fragment frag){
        weakRef = new WeakReference< Fragment >(frag);
    }

    @Override
    protected List<String> doInBackground(String... filters) {
        return getData(filters[0]);
    }

    @Override
    protected void onPostExecute(List<String> urlList) {
        Fragment frag = weakRef.get();
        if (frag != null){
            RecyclerView recyclerView = (RecyclerView) rootView.findViewById(R.id.rv);
            LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getActivity());
            recyclerView.setLayoutManager(linearLayoutManager);
            MyAdapter myAdapter = new MyAdapter(urlList);
            recyclerView.setAdapter(myAdapter);
        }
    }
}

我的适配器：

   List<String> urls;
ViewGroup parent;

public MyAdapter(List<String> urls){
    this.urls=urls;
}

@Override
public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.image_entry, parent);
    MyViewHolder myViewHolder = new MyViewHolder(view);
    this.parent=parent;
    return myViewHolder;
}

@Override
public void onBindViewHolder(MyViewHolder holder, int position) {
    Picasso.with(parent.getContext())
            .load(urls.get(position))
            .into(holder.photo);
    Toast.makeText(parent.getContext(), urls.get(position), Toast.LENGTH_LONG).show();
}

@Override
public int getItemCount() {
    return urls.size();
}

public static class MyViewHolder extends RecyclerView.ViewHolder{

    ImageView photo;

    public MyViewHolder(View itemView) {
        super(itemView);
        photo = (ImageView) itemView.findViewById(R.id.listImage);

    }
}

Answer 1

所以我假设你想要做的是一旦你得到urlList，你想要更新一些视图。如果是这样，那么我认为你想要做的就是在后台任务中运行整个方法，而不仅仅是这样的查询：

首先更改你的getData（）方法：

public List<String> getData(String filter){

    ParseQuery<ParseObject> query = ParseQuery.getQuery("Images");
    query.whereEqualTo("category", filter);

    objectList = query.find(); //I think you have to surround this by a try catch or have the method throw an exception

    int size = objectList.size();
    for (int i = 0; i<size; i++){
        ParseFile file = (ParseFile) objectList.get(i).get("Image");
        urlList.add(file.getUrl());
   }

    return urlList;
}

请注意，由于您正在进行网络调用，因此上述方法现在必须从后台线程运行。

然后你可以使用像AsyncTask这样的东西来更新视图：

private class GetUrlListTask extends AsyncTask<String,Void,List<String>>
{
    private WeakReference<Fragment> weakRef;

    public PhotoTagsFetchTask(Fragment frag){
        weakRef = new WeakReference< Fragment >(frag);
    }

    @Override
    protected Void doInBackground(String... filters) {
        return getData(filters[0]);
    }

    @Override
    protected void onPostExecute(List<String> urlList) {
        Fragment frag = weakRef.get();
        if (frag != null){
            RecyclerView recyclerView = (RecyclerView) view.findViewById(R.id.rv);
            LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getActivity());
            recyclerView.setLayoutManager(linearLayoutManager);
            MyAdapter myAdapter = new MyAdapter(urlList);
            recyclerView.setAdapter(myAdapter);
        }
    }
}

请注意，使用上面的弱引用是为了避免上下文泄漏（基本上避免了不必要的内存泄漏）。

你也可以这样做：

public void getData(String filter){

ParseQuery<ParseObject> query = ParseQuery.getQuery("Images");
query.whereEqualTo("category", filter);
query.findInBackground(new FindCallback<ParseObject>() {

    public void done(List<ParseObject> downloads, com.parse.ParseException e) {
        if (e == null) {
            objectList=downloads;
            int size = objectList.size();

            for (int i = 0; i<size; i++){
              ParseFile file = (ParseFile) objectList.get(i).get("Image");
              urlList.add(file.getUrl());
            }

            RecyclerView recyclerView = (RecyclerView) view.findViewById(R.id.rv);
            LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getActivity());
            recyclerView.setLayoutManager(linearLayoutManager);
            MyAdapter myAdapter = new MyAdapter(urlList);
            recyclerView.setAdapter(myAdapter);
        } else {
            Log.d("score", "Error: " + e.getMessage());
        }
    }
});



}

Answer 2

将objectList=downloads更改为objectList.addAll(downloads)并在第一行添加objectList = new ArrayList<>()。

同时从查询回调的objectList.size()方法中的done开始移动该部分