NullPointerException on RecyclerView, there is no id mistake

Question

I have checked several times the name of my layout RecyclerView as I show on other StackOverflow questions but it is correct. The error of the logcat is this:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.selfcial/com.example.selfcial.Models.MessageActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.LinearLayoutManager.setStackFromEnd(boolean)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3308)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3457)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2044)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7562)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.LinearLayoutManager.setStackFromEnd(boolean)' on a null object reference
        at com.example.selfcial.Models.MessageActivity.onCreate(MessageActivity.java:69)
        at android.app.Activity.performCreate(Activity.java:7893)
        at android.app.Activity.performCreate(Activity.java:7880)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1307)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3283)

This is my my MessageActivity:

package com.example.selfcial.Models;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.ImageButton;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

import androidx.appcompat.widget.Toolbar;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import com.bumptech.glide.Glide;
import com.example.selfcial.Adapters.MessageAdapter;
import com.example.selfcial.R;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

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

public class MessageActivity extends AppCompatActivity {


    TextView username;
    ImageView profile;
    RecyclerView recyclerViewy;
    EditText sendMsg;
    ImageButton sendBtn;

    FirebaseUser firebaseUser;
    DatabaseReference reference;
    Intent intent;


    MessageAdapter messageAdapter;
    List<Chat> chats;

    RecyclerView recyclerView;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_message);


        profile = findViewById(R.id.imageview_profile);
        username = findViewById(R.id.usernameLogin);
        sendMsg = findViewById(R.id.writeMsg);
        sendBtn = findViewById(R.id.sendBtn);


        //RecyclerViewChat
        recyclerView.findViewById(R.id.recycler_view_chat);
        recyclerView.setHasFixedSize(true);

        LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getApplicationContext());
        linearLayoutManager.setStackFromEnd(true);
        recyclerView.setLayoutManager(linearLayoutManager);



        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayShowTitleEnabled(false);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        getSupportActionBar().setDisplayShowHomeEnabled(true);


        intent = getIntent();
        String id = intent.getStringExtra("id");
        firebaseUser = FirebaseAuth.getInstance().getCurrentUser();
        reference = FirebaseDatabase.getInstance().getReference("users").child(id);




        sendBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //Convert edittext to string
                String msg = sendMsg.getText().toString();

                //Send message
                if (!msg.equals("")) {
                    sendMessage(firebaseUser.getUid(), id, msg);
                } else {
                    Toast.makeText(MessageActivity.this, "It's an empty message", Toast.LENGTH_SHORT).show();
                }
                sendMsg.setText("");
            }
        });







        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                Users user = snapshot.getValue(Users.class);
                username.setText(user.getUsername());

                if (user.getImageUrl().equals("default")) {
                    profile.setImageResource(R.drawable.avatar);
                } else {
                    Glide.with(MessageActivity.this)
                            .load(user.getImageUrl())
                            .into(profile);
                }
                readMessages(firebaseUser.getUid(), id, user.getImageUrl());

            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });









    }




    private void sendMessage(String sender, String receiver, String message) {
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference();

        HashMap<String, Object> hashMap = new HashMap<>();
        hashMap.put("sender", sender);
        hashMap.put("receiver", receiver);
        hashMap.put("message", message);


        reference.child("chats").push().setValue(hashMap);
    }







    private void readMessages(String myid, String id, String imgUrl) {
        chats = new ArrayList<>();

        reference = FirebaseDatabase.getInstance().getReference("chats");
        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                chats.clear();

                for (DataSnapshot snapshot1 : snapshot.getChildren()) {

                    Chat chat = snapshot1.getValue(Chat.class);

                    if (chat.getReceiver().equals(myid) && chat.getSender().equals(id) ||
                            chat.getReceiver().equals(id) && chat.getSender().equals(myid)) {
                        chats.add(chat);
                    }
                    messageAdapter = new MessageAdapter(MessageActivity.this, chats, imgUrl);
                    recyclerView.setAdapter(messageAdapter);
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });
    }















}

This is the MessageAdapter:

ackage com.example.selfcial.Adapters;

import android.content.Context;
import android.content.Intent;
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.bumptech.glide.Glide;
import com.example.selfcial.Models.Chat;
import com.example.selfcial.Models.MessageActivity;
import com.example.selfcial.Models.Users;
import com.example.selfcial.R;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;

import java.util.List;

public class MessageAdapter extends RecyclerView.Adapter<MessageAdapter.ViewHolder> {

    private Context context;
    private List<Chat> mChat;
    private String imgUrl;


    public static final int MSG_TYPE_LEFT = 0;
    public static final int MSG_TYPE_RIGHT = 1;


    //Firebase
    FirebaseUser firebaseUser;



    //Constructor
    public MessageAdapter(Context context, List<Chat> mChat, String imgUrl) {
        this.context = context;
        this.mChat = mChat;
        this.imgUrl = imgUrl;
    }


    @NonNull
    @Override
    public MessageAdapter.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view;
        if (viewType == MSG_TYPE_RIGHT) {
            view = LayoutInflater.from(context).inflate(R.layout.item_sent,
                    parent,
                    false);

        }else {
            view = LayoutInflater.from(context).inflate(R.layout.item_received,
                    parent,
                    false);

        }
        return new ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull MessageAdapter.ViewHolder holder, int position) {
        Chat chat = mChat.get(position);
        holder.show_message.setText(chat.getMessage());

        if (imgUrl.equals("default")) {
            holder.profile_image.setImageResource(R.drawable.avatar);
        }else {
            Glide.with(context)
                    .load(imgUrl)
                    .into(holder.profile_image);
        }
    }

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

    public class ViewHolder extends RecyclerView.ViewHolder{
        private TextView show_message;
        private ImageView profile_image;

        public ViewHolder(@NonNull View itemView) {
            super(itemView);

            show_message = itemView.findViewById(R.id.message);
            profile_image = itemView.findViewById(R.id.profileImgReceive);
        }
    }


    @Override
    public int getItemViewType(int position) {
        firebaseUser = FirebaseAuth.getInstance().getCurrentUser();
        if (mChat.get(position).getSender().equals(firebaseUser.getUid())) {
            return MSG_TYPE_RIGHT;
        } else {
            return MSG_TYPE_LEFT;
        }
    }
}

And this is the activity_message.xml:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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:id="@+id/linearLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".Models.MessageActivity">

    <androidx.appcompat.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:background="?attr/colorPrimary"
        android:minHeight="?attr/actionBarSize"
        android:theme="?attr/actionBarTheme"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <de.hdodenhof.circleimageview.CircleImageView
            android:id="@+id/imageview_profile"
            android:layout_width="40dp"
            android:layout_height="40dp"
            tools:layout_editor_absoluteX="16dp"
            tools:layout_editor_absoluteY="28dp"
            tools:srcCompat="@drawable/avatar" />

        <TextView
            android:id="@+id/usernameLogin"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Username"
            android:textColor="@color/white"
            android:layout_marginLeft="23dp" />

    </androidx.appcompat.widget.Toolbar>


    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recycler_view_chat"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:background="#F8E1E1"
        app:layout_constraintBottom_toTopOf="@+id/cardView5"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/toolbar" />

    <androidx.cardview.widget.CardView
        android:id="@+id/cardView5"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="40dp"
            android:gravity="center_vertical"
            android:orientation="horizontal">


            <ImageButton
                android:id="@+id/sendVoice"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginStart="5dp"
                android:layout_weight="1"
                android:background="@android:color/transparent"
                android:contentDescription="TODO"
                app:srcCompat="@drawable/ic_mic" />

            <ImageButton
                android:id="@+id/photoBtn"
                android:layout_width="wrap_content"
                android:layout_height="30dp"
                android:layout_marginLeft="5dp"
                android:layout_weight="1"
                android:background="#00FFFFFF"
                android:scrollbarThumbHorizontal="@color/black"
                android:scrollbarThumbVertical="@color/black"
                app:srcCompat="@drawable/ic_baseline_camera_alt_24" />


            <ImageButton
                android:id="@+id/gallery"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginLeft="5dp"
                android:layout_weight="1"
                android:background="@android:color/transparent"
                app:srcCompat="@drawable/ic_gallery" />

            <EditText
                android:id="@+id/writeMsg"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginLeft="7dp"
                android:layout_marginBottom="0dp"
                android:layout_weight="1"
                android:background="@drawable/msg_shape"
                android:ems="10"
                android:hint="Bb"
                android:inputType="textCapSentences"
                android:padding="5dp" />

            <ImageButton
                android:id="@+id/sendBtn"
                android:layout_width="wrap_content"
                android:layout_height="30dp"
                android:layout_marginLeft="5dp"
                android:layout_weight="1"
                android:background="#00FFFFFF"
                android:scrollbarThumbHorizontal="@color/black"
                android:scrollbarThumbVertical="@color/black"
                app:srcCompat="@drawable/ic_send" />


        </LinearLayout>
    </androidx.cardview.widget.CardView>


</androidx.constraintlayout.widget.ConstraintLayout>

Is the error occured by a mistake on the MessageAdapter or is it a bug somewhere else? I also tried to change LinearLayoutManager line to LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this); but this didn't work too.

Answer 1

You call findViewById() here:

recyclerView.findViewById(R.id.recycler_view_chat);

But you are calling the findViewById() method on your recyclerView variable, not assigning the result of your Activity's findViewById() to your variable, so your variable is always null.

recyclerView = findViewById(R.id.recycler_view_chat);