How to check if database reference child exists

Question

I'm making a chat, and wondering how to set one id for one conversation. When you are writing to someone the id is "yourId+friendId" so if your friend want to answer you, his conversationId should be still "myId+hisId".
I have to set databasereference child id, but before that i need to check if id "yourId+friendId" exist. I don't know how. Here's my code:

databaseMessages= FirebaseDatabase.getInstance().getReference("conversation");
firebaseAuth=FirebaseAuth.getInstance();

if(databaseMessages.child(chattingUser.getUserId()+", "+firebaseAuth.getCurrentUser().getUid())!=null) {
        databaseMessagesChild = databaseMessages.child(chattingUser.getUserId() + ", " + firebaseAuth.getCurrentUser().getUid());
    }
else {
        databaseMessagesChild = databaseMessages.child(firebaseAuth.getCurrentUser().getUid() + ", " + chattingUser.getUserId());
    }

and it does't work. How to check if databaseMessages.child(chattingUser.getUserId()+", "+firebaseAuth.getCurrentUser().getUid()) exists?

Here's my whole code:

public class Chat extends AppCompatActivity implements View.OnClickListener {

private User chattingUser;
FirebaseAuth firebaseAuth;
DatabaseReference databaseMessages, databaseMessagesChild;
ListView listViewConversation;
List<Message> messageList;


@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.chat_screen);

    databaseMessages= FirebaseDatabase.getInstance().getReference("conversation");
    firebaseAuth=FirebaseAuth.getInstance();

    Intent intent=getIntent();
    chattingUser= (User) intent.getSerializableExtra("User");
    TextView nazwa = (TextView)findViewById(R.id.chat_with);
    nazwa.setText(chattingUser.getNickname());

    listViewConversation=(ListView)findViewById(R.id.list_conversation);
    //listViewConversation.scrollTo(0, listViewConversation.getScrollY());
    messageList=new ArrayList<>();



    databaseMessagesChild = databaseMessages.child(firebaseAuth.getCurrentUser().getUid() + ", " + chattingUser.getUserId());


    DatabaseReference mref=databaseMessages.child(chattingUser.getUserId()+", "+firebaseAuth.getCurrentUser().getUid());
    mref.addValueEventListener(new ValueEventListener() {
        public void onDataChange(DataSnapshot dataSnapshot) {
            if (dataSnapshot.exists()){
                databaseMessagesChild = databaseMessages.child(chattingUser.getUserId() + ", " + firebaseAuth.getCurrentUser().getUid());
                Log.v("wybralem", "pierwszy host");
                Log.v("i ten klucz to:", databaseMessagesChild.getKey());

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

        }
    });








/*
    if(databaseMessages.child(chattingUser.getUserId()+", "+firebaseAuth.getCurrentUser().getUid())!=null) {
        databaseMessagesChild = databaseMessages.child(chattingUser.getUserId() + ", " + firebaseAuth.getCurrentUser().getUid());
        Log.v("wybralem", "pierwszy host");
    }
    else {
        databaseMessagesChild = databaseMessages.child(firebaseAuth.getCurrentUser().getUid() + ", " + chattingUser.getUserId());
        Log.v("wybralem", "drugi host");
    } */


    Button send = (Button)findViewById(R.id.send_button);
    send.setOnClickListener(this);

}





@Override
protected void onStart() {
    super.onStart();
    Log.v("KEY",databaseMessagesChild.getKey());
    databaseMessagesChild.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            messageList.clear();

            for(DataSnapshot usersSnapshot : dataSnapshot.getChildren()){
                Message messages = usersSnapshot.getValue(Message.class);
                messageList.add(messages);

            }

            MessagesList adapter = new MessagesList(Chat.this, messageList);
            listViewConversation.setAdapter(adapter);

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
}






@Override
public void onClick(View v) {
    int i = v.getId();

    if(i==R.id.send_button){
        EditText message_field= (EditText) findViewById(R.id.message_field);
        String message_text=message_field.getText().toString();
        message_field.setText("");
        String myId=firebaseAuth.getCurrentUser().getUid().toString();
        Message message = new Message(myId, chattingUser.getUserId() , message_text);
        //Toast.makeText(Chat.this, firebaseAuth.getCurrentUser().getUid(), Toast.LENGTH_SHORT ).show();

        databaseMessagesChild.push().setValue(message);


    }

}

}

Answer 1

databaseReference.orderByChild("id").equalTo(yourId + friendId).addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        if (dataSnapshot.exists()) {
            // Data exists
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
    }
});

Answer 2

Try this:

DatabaseReference mref=databaseMessages.child(chattingUser.getUserId()+", "+firebaseAuth.getCurrentUser().getUid());

mref.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    if (dataSnapshot.exists()){

                    }

                }
                @Override
                public void onCancelled(DatabaseError databaseError) {

                }
            });