Java Firestore片段内的RecyclerView中出现空指针异常
我正在尝试使用Firestore用Java对象填充Java Firestore片段内的RecyclerView中出现空指针异常,java,android,android-fragments,android-recyclerview,google-cloud-firestore,Java,Android,Android Fragments,Android Recyclerview,Google Cloud Firestore,我正在尝试使用Firestore用Java对象填充recyclerView。RecyclerView位于片段中。我正在使用一个RecycleServiceAdapter,它扩展了FireStoreRecycleAdapter类。当我运行代码时,应用程序立即崩溃 这里是错误 .NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setLayout
recyclerView
。RecyclerView位于片段中。我正在使用一个RecycleServiceAdapter,它扩展了FireStoreRecycleAdapter
类。当我运行代码时,应用程序立即崩溃
这里是错误
.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setLayoutManager(androidx.recyclerview.widget.RecyclerView$LayoutManager)' on a null object reference
下面是一些片段Java代码:
public class EventsFragment extends Fragment{
private FirebaseFirestore db = FirebaseFirestore.getInstance();
private CollectionReference eventRef = db.collection("Events");
private EventRecyclerAdapter adapter;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
final ViewGroup rootView = (ViewGroup) inflater.inflate(
R.layout.events_fragment, container, false);
setUpRecyclerView();
return rootView;
}
private void setUpRecyclerView(){
Query query = eventRef.limit(8);
FirestoreRecyclerOptions<Event> options = new FirestoreRecyclerOptions.Builder<Event>()
.setQuery(query, Event.class)
.build();
adapter = new EventRecyclerAdapter(options);
RecyclerView recyclerView = (RecyclerView) getActivity().findViewById(R.id.events_recycler_view);
recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
recyclerView.setHasFixedSize(true);
recyclerView.setAdapter(adapter);
}
}
公共类事件片段扩展片段{
私有FirebaseFirestore db=FirebaseFirestore.getInstance();
私有集合引用eventRef=db.collection(“事件”);
私有事件回收适配器;
@凌驾
创建视图上的公共视图(布局、充气机、视图组容器、,
Bundle savedInstanceState){
最终视图组根视图=(视图组)充气器。充气(
R.layout.events_片段,容器,假);
setUpRecyclerView();
返回rootView;
}
私有void setUpRecyclerView(){
查询=eventRef.limit(8);
FirestoreRecyclerOptions选项=新建FirestoreRecyclerOptions.Builder()
.setQuery(查询,事件.class)
.build();
适配器=新的EventRecyclerAdapter(选项);
RecyclerView RecyclerView=(RecyclerView)getActivity().findViewById(R.id.events\u recycler\u视图);
setLayoutManager(新的LinearLayoutManager(getActivity());
recyclerView.setHasFixedSize(true);
recyclerView.setAdapter(适配器);
}
}
以下是xml片段:
<androidx.coordinatorlayout.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
xmlns:app="http://schemas.android.com/apk/res-auto">
<com.google.android.material.appbar.AppBarLayout
android:id="@+id/app_bar"
android:layout_width="match_parent"
android:layout_height="wrap_content">
<com.google.android.material.appbar.MaterialToolbar
android:layout_width="match_parent"
android:layout_height="?attr/actionBarSize"
app:title="Upcoming Events"
app:menu="@menu/main_menu"
android:background="@android:color/white"
/>
</com.google.android.material.appbar.AppBarLayout>
<androidx.recyclerview.widget.RecyclerView
app:layout_behavior="com.google.android.material.appbar.AppBarLayout$ScrollingViewBehavior"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingTop="8dp"
android:clipToPadding="false"
android:divider="@null"
android:id="@+id/events_recycler_view"/>
</androidx.coordinatorlayout.widget.CoordinatorLayout>
以下是RecyclerAdapter代码:
public class EventRecyclerAdapter extends FirestoreRecyclerAdapter<Event, EventRecyclerAdapter.EventHolder> {
public EventRecyclerAdapter(@NonNull FirestoreRecyclerOptions<Event> options) {
super(options);
}
@Override
protected void onBindViewHolder(@NonNull EventHolder holder, int position, @NonNull Event model) {
holder.textViewTitle.setText(model.getTitle());
holder.textViewDescription.setText(model.getDescription());
}
@NonNull
@Override
public EventHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.events_list_item,
parent, false);
return new EventHolder(v);
}
class EventHolder extends RecyclerView.ViewHolder {
TextView textViewTitle;
TextView textViewDescription;
public EventHolder(@NonNull View itemView) {
super(itemView);
textViewTitle = itemView.findViewById(R.id.title);
textViewDescription = itemView.findViewById(R.id.description);
}
}
}
公共类EventRecyclerAdapter扩展了FirestoreRecyclerAdapter{
public EventRecyclerAdapter(@NonNull FirestoreRecyclerOptions选项){
超级(期权);
}
@凌驾
受保护的void onBindViewHolder(@NonNull EventHolder,int位置,@NonNull事件模型){
holder.textViewTitle.setText(model.getTitle());
holder.textViewDescription.setText(model.getDescription());
}
@非空
@凌驾
public EventHolder onCreateViewHolder(@NonNull ViewGroup父级,int viewType){
视图v=LayoutInflater.from(parent.getContext()).flate(R.layout.events\u list\u项,
父母,假);
返回新事件持有者(v);
}
类EventHolder扩展了RecyclerView.ViewHolder{
文本视图文本视图标题;
文本视图文本视图描述;
public EventHolder(@NonNull View itemView){
超级(项目视图);
textViewTitle=itemView.findViewById(R.id.title);
textViewDescription=itemView.findViewById(R.id.description);
}
}
}
我相信问题出在回收站的某个地方。我已经试着找出这个问题好几个小时了。有什么是我忽略的吗?我只需要运行它。当您尝试在
setUpRecyclerView()中设置LayoutManager
时,您已经注意到recyclerView
为null
发生这种情况的原因是,在片段视图
实际附加到片段之前,您试图在活动的视图
树中“查找”该回收视图
:
RecyclerView RecyclerView=(RecyclerView)getActivity().findViewById(R.id.events\u recycler\u视图)
第一个Fragment
生命周期方法是onResume()
(至少在运行Android 9的仿真器上的示例应用程序中)
但是您可以更早地配置回收视图
,因为您可以在片段
自己的视图
中找到它,例如在onViewCreated()
中:
请注意,onViewCreated()
中的view参数表示视图
,您可以通过调用getView()
在片段
中找到该视图。但是只有在onCreateView()
完成之后,因为此方法的目的是首先创建并“发布”片段的视图
最后同样重要的是,在onCreateView()
内部,您可以通过新膨胀的视图“找到”回收视图:
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
final ViewGroup rootView = (ViewGroup) inflater.inflate(
R.layout.events_fragment, container, false);
//setUpRecyclerView();
RecyclerView recyclerView = (RecyclerView) rootView .findViewById(R.id.events_recycler_view);
recyclerView.setLayoutManager(new LinearLayoutManager(rootView.getContext());
// etc.
return rootView;
}
因此,您最好将根视图
作为参数传递到setUpRecyclerView()
中,并按如下方式使用它:
private void setUpRecyclerView(ViewGroup root){
Query query = eventRef.limit(8);
FirestoreRecyclerOptions<Event> options = new FirestoreRecyclerOptions.Builder<Event>()
.setQuery(query, Event.class)
.build();
adapter = new EventRecyclerAdapter(options);
RecyclerView recyclerView = (RecyclerView) root.findViewById(R.id.events_recycler_view);
recyclerView.setLayoutManager(new LinearLayoutManager(root.getContext()));
recyclerView.setHasFixedSize(true);
recyclerView.setAdapter(adapter);
}
private void setUpRecyclerView(视图组根目录){
查询=eventRef.limit(8);
FirestoreRecyclerOptions选项=新建FirestoreRecyclerOptions.Builder()
.setQuery(查询,事件.class)
.build();
适配器=新的EventRecyclerAdapter(选项);
RecyclerView RecyclerView=(RecyclerView)root.findViewById(R.id.events\u recycler\u视图);
setLayoutManager(新的LinearLayoutManager(root.getContext());
recyclerView.setHasFixedSize(true);
recyclerView.setAdapter(适配器);
}
异常的完整堆栈跟踪将告诉您错误所在的行。您应该使用它来找出哪个对象是空的。这是对setLayoutManager的调用-您的recyclerview为空。
private void setUpRecyclerView(ViewGroup root){
Query query = eventRef.limit(8);
FirestoreRecyclerOptions<Event> options = new FirestoreRecyclerOptions.Builder<Event>()
.setQuery(query, Event.class)
.build();
adapter = new EventRecyclerAdapter(options);
RecyclerView recyclerView = (RecyclerView) root.findViewById(R.id.events_recycler_view);
recyclerView.setLayoutManager(new LinearLayoutManager(root.getContext()));
recyclerView.setHasFixedSize(true);
recyclerView.setAdapter(adapter);
}