无法将来自 firebase 实时数据库的消息显示到 recyclerview

Question

I am trying to create a messaging app. Till now my code worked fine but now I am facing an error while sending messages to the user when I try to send a message from one user to another my messages get updated in the Realtime database but it doesn't show in the RecyclerView . But When I previously don't have any messages in my database It works fine but after clicking on sending my app crashes.

Database screenshot

It shows error in following line

               if (chat.getReciver().equals(myid) && chat.getSender().equals(userid)||
                    chat.getReciver().equals(userid)&&chat.getSender().equals(myid)){
                mChat.add(chat);
            }

And the error which I get in logcat window is

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference at com.shivam.chatapp2.MessageActivity$4.onDataChange(MessageActivity.java:148)

My full code

MessageActivity.java

    public class MessageActivity extends AppCompatActivity {
    CircleImageView profile_image;
    TextView user_name;

    FirebaseFirestore fstore;
    FirebaseAuth fAuth;
    FirebaseUser fuser;
    DatabaseReference reference;

    Toolbar toolbar;
    ImageButton imageButton;
    EditText textsend;
    String userID;

    MessageAdapter messageAdapter;
    List<Chat> mChat;
    RecyclerView recyclerView;



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

        fAuth=FirebaseAuth.getInstance();
        fstore=FirebaseFirestore.getInstance();
        userID = fAuth.getCurrentUser().getUid();


        profile_image=findViewById(R.id.profile_image);
        user_name=findViewById(R.id.username);
        imageButton=findViewById(R.id.btn_send);
        textsend=findViewById(R.id.text_send);

        recyclerView=findViewById(R.id.recycler_view2);
        recyclerView.setHasFixedSize(true);
        LinearLayoutManager linearLayoutManager= new LinearLayoutManager(getApplicationContext());
        linearLayoutManager.setStackFromEnd(true);
        recyclerView.setLayoutManager(linearLayoutManager);

        toolbar=findViewById(R.id.myToolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setTitle("");
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        toolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                finish();
            }
        });

        Intent intent=getIntent();
        fuser=FirebaseAuth.getInstance().getCurrentUser();

        String userid=intent.getStringExtra("userid");
        String username= intent.getStringExtra("UserName");
        reference = FirebaseDatabase.getInstance().getReference("Users").child(userid);

        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                User user = dataSnapshot.getValue(User.class);
                user_name.setText(user.getFirst());

                readMessages(fuser.getUid(), userid);
            }

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

            }
        });


        imageButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String msg=textsend.getText().toString();
                if (!msg.equals("")){
                    sendMessage(fuser.getUid(),userid,msg);
                }
                else {
                    Toast.makeText(MessageActivity.this,"Empty messages cant be send",Toast.LENGTH_SHORT).show();
                }
            }
        });

    }

    private void sendMessage(String sender, final String receiver, String message) {

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

        DatabaseReference reference = FirebaseDatabase.getInstance().getReference();
        reference.child("Chats").push().setValue(hashMap);

    }
    private void readMessages(String myid,String userid){
        mChat=new ArrayList<>();
        reference=FirebaseDatabase.getInstance().getReference("Chats");

        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                mChat.clear();
                for (DataSnapshot snapshot:dataSnapshot.getChildren()) {
                    Chat chat=snapshot.getValue(Chat.class);
                    assert chat != null;
                    if (chat.getReciver().equals(myid) && chat.getSender().equals(userid)||
                            chat.getReciver().equals(userid)&&chat.getSender().equals(myid)){
                        mChat.add(chat);
                    }
                    messageAdapter=new MessageAdapter(MessageActivity.this,mChat);
                    recyclerView.setAdapter(messageAdapter);

                }
            }

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

            }
        });

    }
}

MessageAdapter.java

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

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

    private Context mContext;
    private List<Chat> mChat;

    FirebaseUser fuser;

    public MessageAdapter() {
    }

    public MessageAdapter(Context mContext, List<Chat> mChat) {
        this.mContext = mContext;
        this.mChat = mChat;

    }

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

            return new MessageAdapter.ViewHolder(view);
        }
        else {
            View view = LayoutInflater.from(mContext).inflate(R.layout.chat_item_left, parent, false);

            return new MessageAdapter.ViewHolder(view);

        }
    }

    @Override
    public void onBindViewHolder(@NonNull MessageAdapter.ViewHolder holder, int position) {

        Chat chat=mChat.get(position);
        holder.show_message.setText(chat.getMessage());




    }

    @Override
    public int getItemCount() {
        return mChat.size();
        }
    public class ViewHolder extends RecyclerView.ViewHolder{

         public TextView show_message;
         //public ImageView profile_image;

        public ViewHolder(@NonNull View itemView) {
        super(itemView);
        show_message=itemView.findViewById(R.id.username);
        //profile_image=itemView.findViewById(R.id.profile_image);

        }
    }

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

Chat.java

public class Chat {
    private String sender;
    private String reciver;
    private  String message;

    public Chat(String sender, String reciver, String message) {
        this.sender = sender;
        this.reciver = reciver;
        this.message = message;
    }

    public Chat() {
    }

    public String getSender() {
        return sender;
    }

    public void setSender(String sender) {
        this.sender = sender;
    }

    public String getReciver() {
        return reciver;
    }

    public void setReciver(String reciver) {
        this.reciver = reciver;
    }

    public String getMessage() {
        return message;
    }

Answer 1

The problem in your code is the reciver property in your Chat class. This property should be named receiver , as it is in your database:

So to solve this, please change your Chat class to:

public class Chat {
    private String sender;
    private String receiver;
    private String message;

    public Chat(String sender, String receiver, String message) {
        this.sender = sender;
        this.receiver = receiver;
        this.message = message;
    }

    public Chat() {
    }

    public String getSender() {
        return sender;
    }

    public void setSender(String sender) {
        this.sender = sender;
    }

    public String getReceiver() {
        return receiver;
    }

    public void setReceiver(String receiver) {
        this.receiver = receiver;
    }

    public String getMessage() {
        return message;
    }
}

Answer 2

你忘记在 textSend editText 上调用 setText