android - 尝试在空对象引用错误时调用 recyclerView.setLayoutManager

问题描述

我试图在我的回收站视图中显示日记条目列表，但我收到以下错误：

Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setLayoutManager(androidx.recyclerview.widget.RecyclerView$LayoutManager)' on a null object reference

这是一段似乎调用错误的代码：

recyclerView = findViewById(R.id.myRecyclerViewOneandOnly);
        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));

我看过这篇文章：setLayoutManager NullPointException in RecyclerView，但我检查了我的 setContentView:setContentView(R.layout.activity_journal_list);它看起来很好，因为它对应于带有回收器视图的正确布局文件：

稍后在代码中，我也设置了适配器：

recyclerViewAdapter = new RecyclerViewAdapter(JournalList.this, journalsList);
                    recyclerView.setAdapter(recyclerViewAdapter);
                    recyclerViewAdapter.notifyDataSetChanged();

完整JournalList.java代码：

import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import com.google.android.gms.tasks.OnFailureListener;
import com.google.android.gms.tasks.OnSuccessListener;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.firestore.CollectionReference;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.QueryDocumentSnapshot;
import com.google.firebase.firestore.QuerySnapshot;
import com.google.firebase.storage.StorageReference;
import com.krish.journalapp.data.JournalApi;

import org.jetbrains.annotations.NotNull;

import java.util.ArrayList;
import java.util.List;

public class JournalList extends AppCompatActivity {
    private FirebaseAuth firebaseAuth;
    private FirebaseAuth.AuthStateListener myListener;
    private FirebaseUser mUser;
    private FirebaseFirestore db = FirebaseFirestore.getInstance();
    private StorageReference storageReference;


    private List<Journal> journalsList;
    private RecyclerView recyclerView;
    private RecyclerViewAdapter recyclerViewAdapter;

    private CollectionReference mCollection = db.collection("Journal");
    private TextView noJournalEntrysFound;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        firebaseAuth = FirebaseAuth.getInstance();
        mUser = firebaseAuth.getCurrentUser();

        journalsList = new ArrayList<>();
        noJournalEntrysFound = findViewById(R.id.textAddSomethingIfNot);

        recyclerView = findViewById(R.id.myRecyclerViewOneandOnly);
        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_journal_list);
        Toolbar toolbar = (Toolbar) findViewById(R.id.my_toolbar);
        setSupportActionBar(toolbar);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.menu, menu);

        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected(@NonNull MenuItem item) {
        switch (item.getItemId()){
            case R.id.addItem:
                //add another journal entry
                if(mUser != null && firebaseAuth != null){
                    startActivity(new Intent(JournalList.this, PostJournal.class));
                }

                break;
            case R.id.log_out:

                if(mUser != null && firebaseAuth != null){
                    firebaseAuth.signOut();

                    startActivity(new Intent(JournalList.this, MainActivity.class));
                    //finish();
                }
                //log out of account

                break;
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    protected void onStart() {
        mCollection.whereEqualTo("userId", JournalApi.getInstance().getId()).get().addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
            @Override
            public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
                if(!queryDocumentSnapshots.isEmpty()){
                    for(QueryDocumentSnapshot journals : queryDocumentSnapshots){
                        Journal myJournale = journals.toObject(Journal.class);
                        journalsList.add(myJournale);
                    }
                    //we are passing in JournalList.this, but that will go into the context for the recycler view to inflate FROM, isn't this a problem??

                    recyclerViewAdapter = new RecyclerViewAdapter(JournalList.this, journalsList);
                    recyclerView.setAdapter(recyclerViewAdapter);
                    recyclerViewAdapter.notifyDataSetChanged();
                }else{
                    noJournalEntrysFound.setVisibility(View.VISIBLE);
                }
            }
        }).addOnFailureListener(new OnFailureListener() {
            @Override
            public void onFailure(@NonNull @NotNull Exception e) {
                Log.d("TAG", e.toString());
            }
        });

        super.onStart();
    }
}

完整的 RecyclerViewAdapter 代码：

import android.content.Context;
import android.text.format.DateUtils;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;

import com.squareup.picasso.Picasso;

import org.jetbrains.annotations.NotNull;

import java.util.List;

public class RecyclerViewAdapter extends RecyclerView.Adapter<RecyclerViewAdapter.JournalListHolder> {

    private Context context;
    private List<Journal> journalList;

    public RecyclerViewAdapter(Context context, List<Journal> journalList) {
        this.context = context;
        this.journalList = journalList;
    }

    @NonNull
    @NotNull
    @Override
    public JournalListHolder onCreateViewHolder(@NonNull @NotNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(context).inflate(R.layout.journal_list_view, parent, false);

        return new JournalListHolder(view, context);
    }

    @Override
    public void onBindViewHolder(@NonNull @NotNull RecyclerViewAdapter.JournalListHolder holder, int position) {

        Journal myJournal = journalList.get(position);
        String imageUrl;

        holder.titleSent.setText(myJournal.getTitle());
        holder.thoughtSent.setText(myJournal.getThought());

        imageUrl = myJournal.getImagePath();
        String timeAgo = (String) DateUtils.getRelativeTimeSpanString(myJournal.getTimeAdded().getSeconds() * 1000);
        holder.dateSent.setText(timeAgo);

        //use picasso library to downlaod and show image
        Picasso.get().load(imageUrl).placeholder(R.drawable.cheese).fit().into(holder.imageSent);


    }

    @Override
    public int getItemCount() {
        return journalList.size();
    }

    class JournalListHolder extends RecyclerView.ViewHolder{
        ImageView imageSent;
        TextView titleSent, thoughtSent, dateSent;

        public JournalListHolder(@NonNull @NotNull View itemView, Context ctx) {
            super(itemView);

            ctx = context;
            imageSent = itemView.findViewById(R.id.imageSent);
            titleSent = itemView.findViewById(R.id.titleSent);
            thoughtSent = itemView.findViewById(R.id.thoughtSent);
            dateSent = itemView.findViewById(R.id.dateSent);
        }
    }
}

谢谢！

标签： androidandroid-recyclerviewandroid-view

android - 尝试在空对象引用错误时调用 recyclerView.setLayoutManager

问题描述

解决方案

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