ParseQuery NullPointerexception悖论

这是我上一个问题的后续问题： Parse RecyclerView图像查询

我现在有一个矛盾的nullpointerexception。 我试图以下面概述的方式从内部方法中提取列表。

public List objectList; public List getData(String filter){ ParseQuery query = ParseQuery.getQuery("Images"); query.whereEqualTo("category", filter); query.findInBackground(new FindCallback() { public void done(List 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 urlList = new ArrayList(); public List objectList = new ArrayList(); public int size; public List getData(String filter){ ParseQuery query = ParseQuery.getQuery("Images"); query.whereEqualTo("category", filter); query.findInBackground(new FindCallback() { public void done(List 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，我添加了我的适配器代码。

  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 urlList = new ArrayList(); public List objectList = new ArrayList(); public int size; public List getData(String filter){ ParseQuery query = ParseQuery.getQuery("Images"); query.whereEqualTo("category", filter); query.findInBackground(new FindCallback() { public void done(List 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> { private WeakReference weakRef; public GetUrlListTask(Fragment frag){ weakRef = new WeakReference(frag); } @Override protected List doInBackground(String... filters) { return getData(filters[0]); } @Override protected void onPostExecute(List 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); } } } 

和我的适配器：

  List urls; ViewGroup parent; public MyAdapter(List 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。 如果是这种情况，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 getData(String filter){ ParseQuery 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 

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，您可以使用类似AsyncTask的内容来更新视图：

 private class GetUrlListTask extends AsyncTask> { private WeakReference 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 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); } } } 

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 query = ParseQuery.getQuery("Images"); query.whereEqualTo("category", filter); query.findInBackground(new FindCallback() { public void done(List downloads, com.parse.ParseException e) { if (e == null) { objectList=downloads; int size = objectList.size(); for (int i = 0; i 

Answer 2

将objectList=downloads到objectList.addAll(downloads)并在第一行中添加objectList = new ArrayList<>() 。

还要在查询回调的done方法内部从objectList.size()开始移动节