ParseQuery空指针异常悖论 [英] ParseQuery NullPointer Exception Paradox
本文介绍了ParseQuery空指针异常悖论的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
这是一个后续行动，我的previous问题：解析RecyclerView图像查询
我现在有一个矛盾的空指针异常。我试图从内部方法提取的清单，下面列出的方式。
公开名单＆LT;＆的parseObject GT;链表类;公开名单＆LT;串GT;的getData（字符串过滤器）{ ParseQuery＆LT;＆的parseObject GT;查询= ParseQuery.getQuery（图像）;
query.whereEqualTo（类别，过滤器）;
query.findInBackground（新FindCallback＆LT;＆的parseObject GT;（）{ 公共无效完成（列表＆LT;＆的parseObject GT;下载，com.parse.ParseException E）{
如果（E == NULL）{
链表类=下载;
}其他{
Log.d（得分，错误：+ e.getMessage（））;
}
}
}）; INT大小= objectList.size（）; 的for（int i = 0; I＆LT;大小;我++）{
ParseFile文件=（ParseFile）objectList.get（I）获得（图像）;
urlList.add（file.getUrl（））;
} 返回urlList;}
我该如何解决这个问题？什么样的方式存在，这是优于我所用？
编辑：
下面是它在做什么。在present没有异常，但recyclerview不显示任何内容。
@覆盖
公共查看onCreateView（LayoutInflater充气器，容器的ViewGroup，
捆绑savedInstanceState）{
查看rootView = inflater.inflate（R.layout.activity_events，集装箱，FALSE）;
演出曲目（运动，rootView）;
返回rootView;
}公共无效集（）{}公共无效SETLIST（字符串过滤器，查看视图）{
RecyclerView recyclerView =（RecyclerView）view.findViewById（R.id.rv）;
LinearLayoutManager linearLayoutManager =新LinearLayoutManager（getActivity（））;
recyclerView.setLayoutManager（linearLayoutManager）;
MyAdapter myAdapter =新MyAdapter（的getData（过滤器））;
recyclerView.setAdapter（myAdapter）;
}
编辑2：更多更新的code：
@覆盖
公共查看onCreateView（LayoutInflater充气器，容器的ViewGroup，
捆绑savedInstanceState）{
查看rootView = inflater.inflate（R.layout.activity_events，集装箱，FALSE）;
演出曲目（运动，rootView）;
返回rootView;
}公共无效集（）{}公共无效SETLIST（字符串过滤器，查看视图）{
RecyclerView recyclerView =（RecyclerView）view.findViewById（R.id.rv）;
LinearLayoutManager linearLayoutManager =新LinearLayoutManager（getActivity（））;
recyclerView.setLayoutManager（linearLayoutManager）;
MyAdapter myAdapter =新MyAdapter（的getData（过滤器））;
recyclerView.setAdapter（myAdapter）;
}
私人列表＆LT;串GT; urlList =新的ArrayList＆LT;＆GT;（）;公开名单＆LT;＆的parseObject GT;链表类=新的ArrayList＆LT;＆GT;（）;
公众诠释大小;公开名单＆LT;串GT;的getData（字符串过滤器）{
ParseQuery＆LT;＆的parseObject GT;查询= ParseQuery.getQuery（图像）;
query.whereEqualTo（类别，过滤器）;
query.findInBackground（新FindCallback＆LT;＆的parseObject GT;（）{ 公共无效完成（列表＆LT;＆的parseObject GT;下载，com.parse.ParseException E）{
如果（E == NULL）{
Toast.makeText（getActivity（），下载检索，Toast.LENGTH_SHORT）.show（）;
objectList.addAll（下载）;
}其他{
Log.d（得分，错误：+ e.getMessage（））;
} 大小= objectList.size（）;
}
}）; 的for（int i = 0; I＆LT;大小;我++）{
ParseFile文件=（ParseFile）objectList.get（I）获得（图像）;
urlList.add（file.getUrl（））;
} 返回urlList;}
编辑3：更新，我已经加入我的适配器code
查看rootView;@覆盖
公共查看onCreateView（LayoutInflater充气器，容器的ViewGroup，
捆绑savedInstanceState）{
rootView = inflater.inflate（R.layout.activity_events，集装箱，FALSE）;
新GetUrlListTask（本）.execute（运动）;
返回rootView;
}公共无效集（）{}私人列表＆LT;串GT; urlList =新的ArrayList＆LT;＆GT;（）;公开名单＆LT;＆的parseObject GT;链表类=新的ArrayList＆LT;＆GT;（）;
公众诠释大小;公开名单＆LT;串GT;的getData（字符串过滤器）{
ParseQuery＆LT;＆的parseObject GT;查询= ParseQuery.getQuery（图像）;
query.whereEqualTo（类别，过滤器）;
query.findInBackground（新FindCallback＆LT;＆的parseObject GT;（）{ 公共无效完成（列表＆LT;＆的parseObject GT;下载，com.parse.ParseException E）{
如果（E == NULL）{
Toast.makeText（getActivity（），下载检索，Toast.LENGTH_SHORT）.show（）;
objectList.addAll（下载）;
}其他{ } 大小= objectList.size（）;
}
}）; 的for（int i = 0; I＆LT;大小;我++）{
ParseFile文件=（ParseFile）objectList.get（I）获得（图像）;
urlList.add（file.getUrl（））;
} 返回urlList;}私有类GetUrlListTask扩展的AsyncTask＆LT;弦乐，太虚，列表与LT;弦乐＆GT;＆GT;
{
私人的WeakReference＆LT;片断＆gt; weakRef; 公共GetUrlListTask（片段FRAG）{
weakRef =新的WeakReference＆LT;片断＆gt;（破片）;
} @覆盖
保护列表与LT;弦乐＆GT; doInBackground（字符串...过滤器）{
返回的getData（过滤器[0]）;
} @覆盖
保护无效onPostExecute（列表＆LT;串GT; urlList）{
片段FRAG = weakRef.get（）;
如果（FRAG！= NULL）{
RecyclerView recyclerView =（RecyclerView）rootView.findViewById（R.id.rv）;
LinearLayoutManager linearLayoutManager =新LinearLayoutManager（getActivity（））;
recyclerView.setLayoutManager（linearLayoutManager）;
MyAdapter myAdapter =新MyAdapter（urlList）;
recyclerView.setAdapter（myAdapter）;
}
}
}
和我的适配器：
列表＆LT;串GT;网址;
父母的ViewGroup;公共MyAdapter（列表＆LT;串GT;的网址）{
this.urls =网址;
}@覆盖
公共MyViewHolder onCreateViewHolder（ViewGroup中的父母，INT viewType）{
查看查看= LayoutInflater.from（parent.getContext（））膨胀（R.layout.image_entry，父母）。
MyViewHolder myViewHolder =新MyViewHolder（视图）;
this.parent =父母;
返回myViewHolder;
}@覆盖
公共无效onBindViewHolder（MyViewHolder持有人，INT位置）{
Picasso.with（parent.getContext（））
.load（urls.get（位置））
.into（holder.photo）;
Toast.makeText（parent.getContext（），urls.get（位置），Toast.LENGTH_LONG）.show（）;
}@覆盖
公众诠释getItemCount（）{
返回urls.size（）;
}公共静态类MyViewHolder扩展RecyclerView.ViewHolder { ImageView的照片; 公共MyViewHolder（查看ItemView控件）{
超（ItemView控件）;
照片=（ImageView的）itemView.findViewById（R.id.listImage）; }
}
解决方案
所以我假设你想要做的是，一旦你拿到urlList，要更新一些看法。如果是这样的话，那么我想你想这样做运行在后台的任务，整个方法，而不仅仅是像这样的查询是什么：
首先改变你的getData（）方法：
公开名单＆LT;串GT;的getData（字符串过滤器）{ ParseQuery＆LT;＆的parseObject GT;查询= ParseQuery.getQuery（图像）;
query.whereEqualTo（类别，过滤器）; 链表类= query.find（）; //我觉得你有一个尝试捕捉围绕这个还是有方法抛出一个异常 INT大小= objectList.size（）;
的for（int i = 0; I＆LT;大小;我++）{
ParseFile文件=（ParseFile）objectList.get（I）获得（图像）;
urlList.add（file.getUrl（））;
} 返回urlList;
}
注意，因为你正在做一个网络调用，上述方法现在必须从后台线程运行。
然后你可以使用类似的AsyncTask的更新视图：
私有类GetUrlListTask扩展的AsyncTask＆LT;弦乐，太虚，列表与LT;弦乐＆GT;＆GT;
{
私人的WeakReference＆LT;片断＆gt; weakRef; 公共PhotoTagsFetchTask（片段FRAG）{
weakRef =新的WeakReference＆LT;片断＆gt;（破片）;
} @覆盖
保护无效doInBackground（字符串...过滤器）{
返回的getData（过滤器[0]）;
} @覆盖
保护无效onPostExecute（列表＆LT;串GT; urlList）{
片段FRAG = weakRef.get（）;
如果（FRAG！= NULL）{
RecyclerView recyclerView =（RecyclerView）view.findViewById（R.id.rv）;
LinearLayoutManager linearLayoutManager =新LinearLayoutManager（getActivity（））;
recyclerView.setLayoutManager（linearLayoutManager）;
MyAdapter myAdapter =新MyAdapter（urlList）;
recyclerView.setAdapter（myAdapter）;
}
}
}
请注意，上面的使用弱引用是为了避免泄漏的情况下（基本上避免了不必要的内存泄漏）。
您也可以这样做：
公共无效的getData（字符串过滤器）{ParseQuery＆LT;＆的parseObject GT;查询= ParseQuery.getQuery（图像）;
query.whereEqualTo（类别，过滤器）;
query.findInBackground（新FindCallback＆LT;＆的parseObject GT;（）{ 公共无效完成（列表＆LT;＆的parseObject GT;下载，com.parse.ParseException E）{
如果（E == NULL）{
链表类=下载;
INT大小= objectList.size（）; 的for（int i = 0; I＆LT;大小;我++）{
ParseFile文件=（ParseFile）objectList.get（I）获得（图像）;
urlList.add（file.getUrl（））;
} RecyclerView recyclerView =（RecyclerView）view.findViewById（R.id.rv）;
LinearLayoutManager linearLayoutManager =新LinearLayoutManager（getActivity（））;
recyclerView.setLayoutManager（linearLayoutManager）;
MyAdapter myAdapter =新MyAdapter（urlList）;
recyclerView.setAdapter（myAdapter）;
}其他{
Log.d（得分，错误：+ e.getMessage（））;
}
}
}）;}
This is a follow-up to my previous question: Parse RecyclerView Image Query
I am now having a paradoxical nullpointer exception. I am attempting to extract a list from an inner method, in the way outlined below.
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;
}
How do I solve this? What ways exist which are better than what I am using?
EDIT:
Here is what it's doing. At present there are no exceptions, but the recyclerview is not displaying anything.
@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);
}
EDIT 2: More updated code:
@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;
}
EDIT 3: Updated, and I have added my adapter code.
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);
}
}
}
And My adapter:
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);
}
}
解决方案
So I am assuming that what you want to do is once you get the urlList, you want to update some view. If that's the case then I think what you want to do it run that whole method in a background task rather than just the query like so:
First change your getData() method:
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;
}
Note that since you are doing a network call, the above method now HAS to run from a background thread.
Then you can use something like an AsyncTask to update a view:
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);
}
}
}
Note that the use of weak reference above is to avoid context leaks (basically avoids unnecessary memory leaks).
You could also do this:
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());
}
}
});
}
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