我相信onCreateViewHolder和OnBindViewHolder不会运行来填充数据。
空指针异常位于CheckInRecentList.java中以下代码的第21行。调试时，allCheckIn.size（）中有4个条目。
该行... checkInLastDateTime.setText（allCheckIn.get（0）.getCheckInLastDateTime（））;获取由于checkInLastDateTime为null而导致的空指针异常。
private void observerSetup() {
Log.i(TAG, "observerSetup:");
checkInLastDateTime = getView().findViewById(R.id.checkInLastDateTime);
checkInTitle = getView().findViewById(R.id.checkInTitle);
checkInDestinationName = getView().findViewById(R.id.checkInDestinationName);
checkInComments = getView().findViewById(R.id.checkInComments);
if(mViewModel.getAllCheckIn() != null)
mViewModel.getAllCheckIn().observe(getViewLifecycleOwner(), new Observer<List<CheckInTable>>(){
@Override
public void onChanged(@Nullable final List<CheckInTable> allCheckIn) {
adapter.setCheckInList(allCheckIn);
}
});
mViewModel.getAllCheckIn().observe(getViewLifecycleOwner(), new Observer<List<CheckInTable>>() {
@Override
public void onChanged(@Nullable final List<CheckInTable> allCheckIn) {
adapter.setCheckInList(allCheckIn);
if (allCheckIn.size() > 0) {
Log.i(TAG, "onChanged: all check in size greater than zero");
checkInLastDateTime.setText(allCheckIn.get(0).getCheckInLastDateTime());
Log.i(TAG, "onChanged: running again");
checkInDestinationName.setText(allCheckIn.get(0).getCheckInDestinationName());
checkInTitle.setText(allCheckIn.get(0).getCheckInTitle());
checkInComments.setText(allCheckIn.get(0).getCheckInComments());
} else {
checkInLastDateTime.setText("None Found");
}
}
异常的logcat条目是
02-11 09:29:07.888 14867-14867/com.example.checkingin E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.checkingin, PID: 14867
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at com.example.checkingin.CheckInRecentList$4.onChanged(CheckInRecentList.java:240)
at com.example.checkingin.CheckInRecentList$4.onChanged(CheckInRecentList.java:234)
at androidx.lifecycle.LiveData.considerNotify(LiveData.java:131)
at androidx.lifecycle.LiveData.dispatchingValue(LiveData.java:149)
at androidx.lifecycle.LiveData.setValue(LiveData.java:307)
at androidx.lifecycle.LiveData$1.run(LiveData.java:91)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
该片段的完整代码是
package com.example.checkingin;
import android.content.Context;
import android.net.Uri;
import android.nfc.Tag;
import android.os.Bundle;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.lifecycle.LiveData;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;
import androidx.lifecycle.ViewModelProvider.Factory;
import androidx.recyclerview.widget.DividerItemDecoration;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.ImageButton;
import android.widget.TextView;
import java.util.List;
import static androidx.constraintlayout.widget.Constraints.TAG;
/**
* A simple {@link Fragment} subclass.
* Activities that contain this fragment must implement the
* {@link CheckInRecentList.OnFragmentInteractionListener} interface
* to handle interaction events.
* Use the {@link CheckInRecentList#newInstance} factory method to
* create an instance of this fragment.
*/
public class CheckInRecentList extends Fragment {
// TODO: Rename parameter arguments, choose names that match
// the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
private static final String ARG_PARAM1 = "param1";
private static final String ARG_PARAM2 = "param2";
private RecyclerView recyclerView;
private RecyclerView.Adapter checkInListAdapter;
//private RecyclerView.LayoutManager layoutManager;
// TODO: Rename and change types of parameters
private String mParam1;
private String mParam2;
private MainViewModel mViewModel;
private CheckInListAdapter adapter;
private MainViewModelProviderFactory viewModelFactory;
private TextView checkInLastDateTime;
private TextView checkInTitle;
private TextView checkInDestinationName;
private TextView checkInComments;
private OnFragmentInteractionListener mListener;
public CheckInRecentList() {
// Required empty public constructor
}
/**
* Use this factory method to create a new instance of
* this fragment using the provided parameters.
*
* @param param1 Parameter 1.
* @param param2 Parameter 2.
* @return A new instance of fragment CheckInRecentList.
*/
// TODO: Rename and change types and number of parameters
public static CheckInRecentList newInstance(String param1, String param2) {
CheckInRecentList fragment = new CheckInRecentList();
Bundle args = new Bundle();
args.putString(ARG_PARAM1, param1);
args.putString(ARG_PARAM2, param2);
fragment.setArguments(args);
return fragment;
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Log.i(TAG, "onCreate: On Create");
if (getArguments() != null) {
mParam1 = getArguments().getString(ARG_PARAM1);
mParam2 = getArguments().getString(ARG_PARAM2);
}
// These were originally set up from the recycler view add to the fragment
// recyclerView = findViewById(R.id.check_in_recent_recycler_view);
// use this setting to improve performance if you know that changes
// in content do not change the layout size of the RecyclerView
//recyclerView.setHasFixedSize(true);
/*
// use a linear layout manager
layoutManager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(layoutManager);
*/
// specify an adapter (see also next example)
//checkInListAdapter = new CheckInListAdapter();
// recyclerView.setAdapter(checkInListAdapter);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
mViewModel = new ViewModelProvider(this, viewModelFactory).get(MainViewModel.class);
Log.i(TAG, "onCreateView: On Create View");
// Inflate the layout for this fragment
return inflater.inflate(R.layout.fragment_check_in_recent_list, container, false);
}
// TODO: Rename method, update argument and hook method into UI event
public void onButtonPressed(Uri uri) {
Log.i(TAG, "onButtonPressed: ");
if (mListener != null) {
mListener.onFragmentInteraction(uri);
}
}
@Override
public void onAttach(Context context) {
super.onAttach(context);
Log.i(TAG, "onAttach: OnAttach");
viewModelFactory = new MainViewModelProviderFactory(context.getApplicationContext());
mViewModel = new ViewModelProvider(this, viewModelFactory).get(MainViewModel.class);
if (context instanceof OnFragmentInteractionListener) {
mListener = (OnFragmentInteractionListener) context;
} else {
throw new RuntimeException(context.toString()
+ " must implement OnFragmentInteractionListener");
}
Log.i(TAG,"OnAttach completed");
}
@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
Log.i(TAG, "onActivityCreated: On Activity Created");
mViewModel = new ViewModelProvider(this, viewModelFactory).get(MainViewModel.class);
checkInLastDateTime = getView().findViewById(R.id.checkInLastDateTime);
checkInTitle = getView().findViewById(R.id.checkInTitle);
checkInDestinationName = getView().findViewById(R.id.checkInDestinationName);
checkInComments = getView().findViewById(R.id.checkInComments);
recyclerSetup();
Log.i(TAG,"OnActivityCreated: Recycler SetUp");
//listenerSetup();
//Log.i(TAG, "onActivityCreated: Listener SetUp");
observerSetup();
Log.i(TAG, "onActivityCreated: Observer SetUp");
}
@Override
public void onDetach() {
super.onDetach();
Log.i(TAG, "onDetach: ");
mListener = null;
}
/**
* This interface must be implemented by activities that contain this
* fragment to allow an interaction in this fragment to be communicated
* to the activity and potentially other fragments contained in that
* activity.
* <p>
* See the Android Training lesson <a href=
* "http://developer.android.com/training/basics/fragments/communicating.html"
* >Communicating with Other Fragments</a> for more information.
*/
public interface OnFragmentInteractionListener {
// TODO: Update argument type and name
void onFragmentInteraction(Uri uri);
}
private void clearFields() {
checkInLastDateTime.setText("");
checkInDestinationName.setText("");
checkInTitle.setText("");
checkInComments.setText("");
}
private void listenerSetup() {
ImageButton editCheckInButton = getView().findViewById(R.id.checkInEditButton);
ImageButton resendCheckInButton = getView().findViewById(R.id.checkInResendButton);
mViewModel = new ViewModelProvider(this, viewModelFactory).get(MainViewModel.class);
Log.i(TAG, "listenerSetup: ");
editCheckInButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
//put in edit check in logic
}
});
resendCheckInButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
//put in resend logic
}
});
}
private void observerSetup() {
Log.i(TAG, "observerSetup:");
checkInLastDateTime = getView().findViewById(R.id.checkInLastDateTime);
checkInTitle = getView().findViewById(R.id.checkInTitle);
checkInDestinationName = getView().findViewById(R.id.checkInDestinationName);
checkInComments = getView().findViewById(R.id.checkInComments);
if(mViewModel.getAllCheckIn() != null)
mViewModel.getAllCheckIn().observe(getViewLifecycleOwner(), new Observer<List<CheckInTable>>(){
@Override
public void onChanged(@Nullable final List<CheckInTable> allCheckIn) {
adapter.setCheckInList(allCheckIn);
}
});
mViewModel.getAllCheckIn().observe(getViewLifecycleOwner(), new Observer<List<CheckInTable>>() {
@Override
public void onChanged(@Nullable final List<CheckInTable> allCheckIn) {
adapter.setCheckInList(allCheckIn);
if (allCheckIn.size() > 0) {
Log.i(TAG, "onChanged: all check in size greater than zero");
checkInLastDateTime.setText(allCheckIn.get(0).getCheckInLastDateTime());
Log.i(TAG, "onChanged: running again");
checkInDestinationName.setText(allCheckIn.get(0).getCheckInDestinationName());
checkInTitle.setText(allCheckIn.get(0).getCheckInTitle());
checkInComments.setText(allCheckIn.get(0).getCheckInComments());
} else {
checkInLastDateTime.setText("None Found");
}
}
});
}
private void recyclerSetup() {
Log.i(TAG, "recyclerSetup: ");
adapter = new CheckInListAdapter(R.layout.fragment_check_in_recent_list);
RecyclerView recyclerView = getView().findViewById(R.id.check_in_recent_recycler_view);
recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));
recyclerView.setAdapter(adapter);
//recyclerView.addItemDecoration(new DividerItemDecoration(this, LinearLayoutManager.VERTICAL));
}
}
适配器代码是
package com.example.checkingin;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageButton;
import android.widget.ImageView;
import android.widget.TextView;
import android.app.Activity;
import androidx.lifecycle.ViewModelProvider;
import androidx.recyclerview.widget.RecyclerView;
import java.util.List;
import static androidx.constraintlayout.widget.Constraints.TAG;
public class CheckInListAdapter extends RecyclerView.Adapter<CheckInListAdapter.ViewHolder>{
private int checkInListLayout;
private List<CheckInTable> checkInList;
public CheckInListAdapter(int layoutId) {
Log.i(TAG, "CheckInListAdapter: ");
checkInListLayout = layoutId;
}
public void setCheckInList(List<CheckInTable> allCheckIn) {
checkInList = allCheckIn;
Log.i(TAG, "setCheckInList: ");
notifyDataSetChanged();
}
@Override
public int getItemCount() {
return checkInList == null ? 0 : checkInList.size();
}
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
Log.i(TAG, "onCreateViewHolder: ");
View view = LayoutInflater.from(
parent.getContext()).inflate(checkInListLayout, parent, false);
ViewHolder checkInListViewHolder = new ViewHolder(view);
return checkInListViewHolder;
}
@Override
public void onBindViewHolder(final ViewHolder holder, final int listPosition) {
TextView checkInLastDateTime = holder.checkInLastDateTime;
TextView checkInTitle = holder.checkInTitle;
TextView checkInDestinationName = holder.checkInDestinationName;
TextView checkInComments = holder.checkInComments;
ImageView checkInEditButton = holder.checkInEditButton;
ImageView checkInResendButton = holder.checkInResendButton;
Log.i(TAG, "onBindViewHolder: ");
checkInLastDateTime.setText(checkInList.get(listPosition).getCheckInLastDateTime());
checkInTitle.setText(checkInList.get(listPosition).getCheckInTitle());
checkInDestinationName.setText(checkInList.get(listPosition).getCheckInDestinationName());
checkInComments.setText(checkInList.get(listPosition).getCheckInComments());
holder.checkInEditButton.setImageResource(R.drawable.ic_menu_edit);
holder.checkInResendButton.setImageResource(R.drawable.ic_menu_share);
ImageButton editCheckInButton = checkInEditButton.findViewById(R.id.checkInEditButton);
ImageButton resendCheckInButton = checkInResendButton.findViewById(R.id.checkInResendButton);
/*
editCheckInButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
//put in edit check in logic
}
}
);
resendCheckInButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
//put in resend logic
}
});
*/
}
static class ViewHolder extends RecyclerView.ViewHolder {
TextView checkInLastDateTime;
TextView checkInTitle;
TextView checkInDestinationName;
TextView checkInComments;
ImageView checkInEditButton;
ImageView checkInResendButton;
ViewHolder(View itemView) {
super(itemView);
Log.i(TAG, "ViewHolder: ");
checkInLastDateTime = itemView.findViewById(R.id.checkInLastDateTime);
checkInTitle = itemView.findViewById(R.id.checkInTitle);
checkInDestinationName = itemView.findViewById(R.id.checkInDestinationName);
checkInComments = itemView.findViewById(R.id.checkInComments);
checkInEditButton = itemView.findViewById(R.id.checkInEditButton);
checkInResendButton = itemView.findViewById(R.id.checkInResendButton);
itemView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
}
});
}
}
}
片段布局是
<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".CheckInRecentList">
<androidx.recyclerview.widget.RecyclerView
android:id="@+id/check_in_recent_recycler_view"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:listitem="@layout/recycler_view_item"/>
</FrameLayout>
回收商列表xml是
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="horizontal"
android:padding="0dp">
<TableLayout
android:id="@+id/check_in_recent_row"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_gravity="center|top"
android:layout_weight="35">
<TableRow
android:layout_width="372dp"
android:layout_height="match_parent"
android:orientation="horizontal">
<TextView
android:layout_width="66dp"
android:layout_height="20dp"
android:layout_weight="10"
android:text="Last Used"
android:background="#00000000"
android:textColor="@color/blue"
/>
<TextView
android:layout_width="wrap_content"
android:layout_height="20dp"
android:background="#00000000"
android:layout_weight="35"
android:textColor="@color/blue"
android:text="Title" />
<TextView
android:layout_width="wrap_content"
android:layout_height="20dp"
android:background="#00000000"
android:layout_weight="35"
android:textColor="@color/blue"
android:text="Destination" />
<TextView
android:layout_width="wrap_content"
android:layout_height="20dp"
android:layout_weight="10"
android:background="#00000000"
android:textColor="@color/blue"
android:text="Edit"/>
<TextView
android:layout_height="20dp"
android:layout_weight="10"
android:background="#00000000"
android:textColor="@color/blue"
android:text="Resend"/>
</TableRow>
<TableRow
android:layout_width="372dp"
android:layout_height="match_parent"
android:orientation="horizontal">
<TextView
android:id="@+id/checkInLastDateTime"
android:layout_width="66dp"
android:layout_height="match_parent"
android:layout_weight="10"
android:gravity="center_vertical"
android:text="TestView"/>
<TextView
android:id="@+id/checkInTitle"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:gravity="center_vertical"
android:layout_weight="35"
android:text="TextView" />
<TextView
android:id="@+id/checkInDestinationName"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:gravity="center_vertical"
android:layout_weight="35"
android:text="TextView" />
<ImageButton
android:id="@+id/checkInEditButton"
android:layout_width="66dp"
android:layout_height="40dp"
android:layout_weight="10"
android:src="@android:drawable/ic_menu_edit"
android:tooltipText="Edit Check In" />
<ImageButton
android:id="@+id/checkInResendButton"
android:layout_height="40dp"
android:layout_weight="10"
android:src="@android:drawable/ic_menu_share"
android:tooltipText="Resend Check In" />
</TableRow>
<TableRow
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="@+id/checkInComments"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="100"
android:text="TextView" />
</TableRow>
</TableLayout>
</LinearLayout>
最佳答案
从我看到的代码中，您正在尝试访问属于片段类中的适配器的View小部件。这就是为什么它为空。
getView().findViewById(R.id.checkInLastDateTime);
..将为null，因为
checkInLastDateTime不属于片段的布局，而是适配器的布局。此问题源于您如何实现RecyclerView。我建议您进行函数调用，以将列表项从片段更新到适配器，并在适配器内更新它们。
进一步的建议是，谷歌搜索并阅读有关如何实现RecyclerViews的基本示例，以使其更加清晰。
我希望这有帮助。
关于java - 数据未在RecyclerView中显示。 onCreateViewHolder和onBindViewHolder无法执行，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/60172421/