DatabaseException: Can't convert object of type java.lang.String to type

The solution provided by Alex worked but didn't quite answer why I couldn't get store my data in an object, and for my app while the child added listener, the app is able to read data that was already on Firebase but when new child is added, the value is empty.

    onChildAdded:DataSnapshot { key = -L0JjGo-3QMYDsuTMQcN, value =  }

I've done a little more digging to find out what might be causing this and found that it is probably caused because the values were not all written at the same time. And looking at firebase it seems that the key is added first, then the values are added. This is why the error says can't convert, it is because it doesn't exist. I'm using the Python Firebase Admin SDK to add the data to Firebase, so I'm not sure if this is the reason for that.

So to fix my problem I moved my code to the onChildChanged function and added a check so that the code only runs when all of the data I need exists. This way I can get the values stored in an object straight away.

    @Override
    public void onChildChanged(DataSnapshot dataSnapshot, String previousChildName){
        if (dataSnapshot.child("text").exists() &&
                dataSnapshot.child("executed").exists() &&
                dataSnapshot.child("timestamp").exists()){
            Log.d(TAG, "onChildChanged:" + dataSnapshot.toString());
            CommandObject command = dataSnapshot.getValue(CommandObject.class);
            if ("TAKE_OFF".equals(command.text)) {
                mMiniDrone.takeOff();
            } else if ("LAND".equals(command.text)) {
                mMiniDrone.land();
            }
        }
    }

Alex posted the right solution for me, but since I was using firebase-ui and kotlin, the code used is different. More info here

    val options = FirebaseRecyclerOptions.Builder<ChatMessage>()
            .setQuery(query, ChatMessage::class.java)
            .build()

    // equivalent options object with manual parsing, use it to debug which field gives error
    val options = FirebaseRecyclerOptions.Builder<ChatMessage>()
            .setQuery(query, object : SnapshotParser<ChatMessage> {

                override fun parseSnapshot(snapshot: DataSnapshot): ChatMessage {

                    val senderId = snapshot.child("senderId").getValue(String::class.java)
                    val receiverId = snapshot.child("receiverId").getValue(String::class.java)
                    val senderName = snapshot.child("senderName").getValue(String::class.java)
                    val text = snapshot.child("text").getValue(String::class.java)
                    val timestamp = snapshot.child("timestamp").getValue(Long::class.java)

                    return ChatMessage(senderId, receiverId, senderName, text, timestamp)
                }

            })
            .build()


    adapterMessages = object : FirebaseRecyclerAdapter<ChatMessage, ChatHolder>(options) {

        override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ChatHolder {

            val rootView = LayoutInflater.from(parent.context).inflate(R.layout.chat_message_item, parent, false)

            return ChatHolder(rootView)
        }

        override fun onBindViewHolder(holder: ChatHolder, position: Int, model: ChatMessage) {

            holder.populateItem(model)
        }
    }

if u are using map object in your database should use get method directly with your object

doc: DocumentSnapshot val data = doc.get("location", SaleLocations::class.java)

I faced a similar issue where I could not retrieve the data from the database; having a DatabaseException error with a message

can't convert from this type to that type.

The mistake I was making was I was writing incorrectly to the database. I wrote something like ...

mdatabaseReference.setValue(myObject)

instead of

mdatabaseReference.child(myObject.getId()).setValue(myObject)

So essentially the answer I am suggesting is: you should ensure your data is written correctly to the database at the right node in the JSON tree if not you would likely get this kind of error when you are reading from the database.

you can use constructor oop normally like this

it's the same and no Error anymore

CommandObject command = new CommandObject(ds.child("").getValue()....

fill it and make sure your reference in firebase data should not be empty

In my case the problem was, that I using Kotlin class, and there were no default constructor such as

 constructor() : this(0, "")