java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.ArrayList.add(java.lang.Object)' on a null object reference

Question

I was trying to build a messenger-ish app, when trying to add this part to display the chats a user has it just crashes on startup, before adding this part of code it was fine.

    initializeRecyclerView();
    getUserChatList();
}

private void getUserChatList(){
    DatabaseReference mUserChatDB = FirebaseDatabase.getInstance().getReference().child("user").child(FirebaseAuth.getInstance().getUid()).child("chat");

    mUserChatDB.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            if(dataSnapshot.exists()){
                for (DataSnapshot childSnapshot : dataSnapshot.getChildren()){
                    ChatObject mChat = new ChatObject(childSnapshot.getKey());
                    chatList.add(mChat);
                    mChatListAdapter.notifyDataSetChanged();
                }
            }
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) { } //I don't need this yet
    });
}

  private void initializeRecyclerView() {
        mChatList= findViewById(R.id.chatList);
        mChatList.setNestedScrollingEnabled(false);
        mChatList.setHasFixedSize(false);
        mChatListLayourManager = new LinearLayoutManager(getApplicationContext(), RecyclerView.VERTICAL, false);
        mChatList.setLayoutManager(mChatListLayourManager);
        mChatListAdapter = new ChatListAdapter(chatList);
        mChatList.setAdapter(mChatListAdapter);
    }

MainPageActivity.java

package com.example.myapplication;

import android.Manifest;
import android.content.Intent;
import android.os.Build;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

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

import com.example.myapplication.Chat.ChatListAdapter;
import com.example.myapplication.Chat.ChatObject;
import com.google.firebase.auth.FirebaseAuth;
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;

public class MainPageActivity extends AppCompatActivity {

    private RecyclerView mChatList;
    private RecyclerView.Adapter mChatListAdapter;
    private RecyclerView.LayoutManager mChatListLayourManager;

   ArrayList<ChatObject> chatList;


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

        Button mLogout = findViewById(R.id.logout);
        Button mFindUser = findViewById(R.id.findUser);
        mFindUser.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                startActivity(new Intent(getApplicationContext(), FindUserActivity.class));
            }
        });
        mLogout.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                FirebaseAuth.getInstance().signOut();
                Intent intent = new Intent(getApplicationContext(), LoginActivity.class);
                intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                startActivity(intent);
                finish();
                return;
            }
        });

        getPermissions();
        initializeRecyclerView();
        getUserChatList();
    }

    private void getUserChatList(){
        DatabaseReference mUserChatDB = FirebaseDatabase.getInstance().getReference().child("user").child(FirebaseAuth.getInstance().getUid()).child("chat");

        mUserChatDB.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                if(dataSnapshot.exists()){
                    for (DataSnapshot childSnapshot : dataSnapshot.getChildren()){
                        ChatObject mChat = new ChatObject(childSnapshot.getKey());
                        chatList.add(mChat);
                        mChatListAdapter.notifyDataSetChanged();
                    }
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) { } //I don't need this yet
        });
    }

      private void initializeRecyclerView() {
            mChatList= findViewById(R.id.chatList);
            mChatList.setNestedScrollingEnabled(false);
            mChatList.setHasFixedSize(false);
            mChatListLayourManager = new LinearLayoutManager(getApplicationContext(), RecyclerView.VERTICAL, false);
            mChatList.setLayoutManager(mChatListLayourManager);
            mChatListAdapter = new ChatListAdapter(chatList);
            mChatList.setAdapter(mChatListAdapter);
        }
   private void getPermissions() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            requestPermissions(new String[]{Manifest.permission.WRITE_CONTACTS, Manifest.permission.READ_CONTACTS}, 1);
            }
        }
    }

This is the run log, the debug log shows the same thing. Build output shows no errors.

    V/FA: Processing queued up service tasks: 5
E/eglCodecCommon: glUtilsParamSize: unknow param 0x00008cdf
    glUtilsParamSize: unknow param 0x00008824
D/EGL_emulation: eglMakeCurrent: 0xad105600: ver 3 0 (tinfo 0xad1037a0)
V/FA: Activity resumed, time: 1289503
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.myapplication, PID: 5987
    java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.ArrayList.add(java.lang.Object)' on a null object reference
        at com.example.myapplication.MainPageActivity$3.onDataChange(MainPageActivity.java:74)
        at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@19.2.0:75)
        at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@19.2.0:63)
        at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@19.2.0:55)
        at android.os.Handler.handleCallback(Handler.java:751)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6119)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Process 5987 terminated.

Tried Cleaning and Rebuilding the project. I'll provide more information if needed. Thanks in advance.

Answer 1

You never assign a value to chatList. You have to initialize your variable before trying to add a value to it.

Also, I would recommend against naming your recyclerView mChatList and your list of ChatObjects chatList. Super confusing for maintenance down the road. Your recyclerview could be mChatRecycler or something...