For some reason cant deserialize object from Firebase response

问题

Everything looks good. Need help in finding a mistake in code. By the logs that is the snapshot from firebase

DataSnapshot
        { key = SecondGame,
                value = {background=https://firebasestorage.googleapis.com/v0/b/fantasygameapp11.appspot.com/o/background_black.jpg?alt=media&token=b3ec1477-6b52-48b4-9296-f57f63f26837, description=SecondGameDescription, tag=https://firebasestorage.googleapis.com/v0/b/fantasygameapp11.appspot.com/o/hot_icon.png?alt=media&token=65516b45-1aca-4cac-9a39-3eddefffe499, 
            title=SecondGame, type=regular} }

That is the model

data class GameUnit (val background: String, val description: String, val tag: String, val title: String,  val type: String)

Thats the code of the response

 mReference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            GameUnit post = dataSnapshot.getValue(GameUnit.class);
        }

I know it might be already asked but I need to find the issue first of all. Is it also possible the problem is that model is in Kotlin but firebase response in Java?

Error

com.google.firebase.database.DatabaseException: Class com.model.GameUnit does not define a no-argument constructor. If you are using ProGuard, make sure these constructors are not stripped.
    at com.google.firebase.database.core.utilities.encoding.CustomClassMapper$BeanMapper.deserialize(com.google.firebase:firebase-database@@17.0.0:552)
    at com.google.firebase.database.core.utilities.encoding.CustomClassMapper$BeanMapper.deserialize(com.google.firebase:firebase-database@@17.0.0:545)
    at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.convertBean(com.google.firebase:firebase-database@@17.0.0:415)
    at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.deserializeToClass(com.google.firebase:firebase-database@@17.0.0:214)
    at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.convertToCustomClass(com.google.firebase:firebase-database@@17.0.0:79)
    at com.google.firebase.database.DataSnapshot.getValue(com.google.firebase:firebase-database@@17.0.0:212)
    at com.adultgaming.fantasygameapp.utils.FirebaseManager$1.onDataChange(FirebaseManager.java:47)
    at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@17.0.0:75)
    at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@17.0.0:63)
    at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@17.0.0:55)

回答1:

The deserializer that comes with the Realtime Database (and also Cloud Firestore) Android SDKs requires that the class you pass to it look like a JavaBean type object. This means that it must have a default no-arg constuructor, as you can tell from the error message, and also setter methods that map to each database field.

Kotlin data classes don't provide a default no-arg constructor in order to ensure that all of its fields have an initial value. You can tell Kotlin that it's OK for all of the fields not to have an initial value by giving null or some other value as a default value:

data class GameUnit (
    var background: String = null,
    var description: String = null,
    var tag: String = null,
    var title: String = null,
    var type: String = null
)

For the above data class, Kotlin will generate a default no-arg constructor for the Firebase SDK to use. It will also generate setter methods for each var. Note that each property is var and provides a default null value.

If this is not what you want your data class to look like, you won't be able to use automatic deserialization. You will have to read each value out of the snapshot, make sure they are each not null, and pass them all to the constructor that Kotlin provides.

来源：https://stackoverflow.com/questions/57278298/for-some-reason-cant-deserialize-object-from-firebase-response