RecyclerView 上的空指针

Posted 2023-04-18

【中文标题】RecyclerView 上的空指针

【英文标题】：Null Pointer on RecyclerView

【发布时间】：2020-05-25 22:52:05

【问题描述】：

我的 recyclersetup 方法中的以下语句出现空指针异常。

recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));

这是由于下面的语句没有在前面的语句中设置指向recyclerView的指针

recyclerView = getView().findViewById(R.id.check_in_recent_row);

我无法确定为什么我没有获得通过此语句建立的 recyclerView 指针。数据的 ID 是正确的。我错过了什么？

这是整个片段代码

CheckInRecentList.java

package com.example.checkingin;
import android.content.Context;
import android.net.Uri;
import android.nfc.Tag;
import android.os.Bundle;
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.recycler_view_item, 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:");
        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) 

                        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));
    

以下是片段的xml文件 fragment_check_in_recent_list.xml

<?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_
    android:layout_
    tools:context=".CheckInRecentList">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/check_in_recent_recycler_view"
        android:layout_
        android:layout_
        tools:listitem="@layout/recycler_view_item"/>


</FrameLayout>

和recycler_view_item.xml

<?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:orientation="horizontal"
              android:padding="8dp"
              android:layout_
              android:layout_>

    <TableLayout
        android:id="@+id/check_in_recent_row"
        android:layout_
        android:layout_
        android:layout_gravity="center"
        android:padding="30dp">

        <TableRow
            android:layout_
            android:layout_>

            <TextView
                android:id="@+id/checkInLastDateTime"
                android:layout_
                android:layout_
                android:text="TextView"/>

            <TextView
                android:id="@+id/checkInTitle"
                android:layout_
                android:layout_
                android:text="TextView" />

            <TextView
                android:id="@+id/checkInDestinationName"
                android:layout_
                android:layout_
                android:text="TextView" />

            <ImageButton
                android:id="@+id/checkInEditButton"
                android:layout_
                android:layout_
                android:src="@android:drawable/ic_menu_edit"
                android:tooltipText="Edit Check In" />

            <ImageButton
                android:id="@+id/checkInResendButton"
                android:layout_
                android:layout_
                android:src="@android:drawable/ic_menu_share"
                android:tooltipText="Resend Check In" />
        </TableRow>

        <TableRow
            android:layout_
            android:layout_>

            <TextView
                android:id="@+id/checkInComments"
                android:layout_
                android:layout_
                android:text="TextView" />

        </TableRow>
    </TableLayout>
</LinearLayout>

CheckInListAdapter

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) 
        checkInListLayout = layoutId;
    

    public void setCheckInList(List<CheckInTable> allCheckIn) 
        checkInList = allCheckIn;
        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) 

                
            );
        
    

【问题讨论】：

这能回答你的问题吗？ What is a NullPointerException, and how do I fix it?

【参考方案1】：

您的 RecyclerView 的 ID 是 check_in_recent_recycler_view。您的findViewById 呼叫正在使用check_in_recent_row。这些需要匹配。

【讨论】：

我试过了，但不幸的是遇到了同样的问题，我把这两行改成了。还添加了上面的适配器代码。适配器 = 新 CheckInListAdapter（R.layout.fragment_check_in_recent_list）； RecyclerView recyclerView = getView().findViewById(R.id.check_in_recent_recycler_view);issue.

我找到了。还需要更改 OnCreateView 中的布局 ID。感谢您的帮助。