"Failed to bounce to type" error with adding firebase database to android app

Question

I'm trying to add firebase database to my android studio project. when I'm running my app (on emulator) I'm getting the following error:

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.inbar.amit.ksharet, PID: 2778
              com.firebase.client.FirebaseException: Failed to bounce to type
                  at com.firebase.client.DataSnapshot.getValue(DataSnapshot.java:183)
                  at com.inbar.amit.ksharet.updates$1.onChildAdded(updates.java:52)
                  at com.firebase.client.core.ChildEventRegistration.fireEvent(ChildEventRegistration.java:48)
                  at com.firebase.client.core.view.DataEvent.fire(DataEvent.java:45)
                  at com.firebase.client.core.view.EventRaiser$1.run(EventRaiser.java:38)
                  at android.os.Handler.handleCallback(Handler.java:751)
                  at android.os.Handler.dispatchMessage(Handler.java:95)
                  at android.os.Looper.loop(Looper.java:154)
                  at android.app.ActivityThread.main(ActivityThread.java:6077)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
               Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.lang.String out of START_OBJECT token
               at [Source: java.io.StringReader@2729553; line: 1, column: 1]
                  at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:575)
                  at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:46)
                  at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:11)
                  at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2888)
                  at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2034)
                  at com.firebase.client.DataSnapshot.getValue(DataSnapshot.java:181)
                    ... 11 more

that's my code(for reading data from the database):

public class updates extends AppCompatActivity {

    private Firebase mRef;
    private ArrayList<String> messages = new ArrayList<>();
    Random rnd = new Random();

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

        mRef = new Firebase("https://ksharet-d6a24.firebaseio.com/message");

        // Get ListView object from xml
        final ListView listView = (ListView) findViewById(R.id.ListView);

        // Create a new Adapter
        final ArrayAdapter<String> adapter = new ArrayAdapter<>(this,
                android.R.layout.simple_list_item_1, messages);

        // Assign adapter to ListView
        listView.setAdapter(adapter);

        mRef.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                String message = dataSnapshot.getValue(String.class);
                Log.v("E_CHILD_ADDED", message);
                messages.add(0, message);
                adapter.notifyDataSetChanged();
            }

            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String s) {
            }

            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {
            }

            @Override
            public void onChildMoved(DataSnapshot dataSnapshot, String s) {
            }

            @Override
            public void onCancelled(FirebaseError firebaseError) {
            }
        });
    }
}

what should I do to fix it?

Answer 1

I believe if you use String message = dataSnapshot.child("message").getValue(String.class); it should work. Why? Well the structure of your data is like this:

 { "message" : // Listener attached here @ https://ksharet-d6a24.firebaseio.com/message
    { "-KUTSl1Z-UPLM0lst2OL" : // child here
        { 
            "message" : "שלום וברוכים הבאים לאפליקציה החדשה של בית הספר ״יגאל אלון״", 
            "title" : "שלום" 
        } 
    } 
 }

When you call dataSnapshot.getValue(String.class) it's trying to turn this:

    { 
        "message" : "שלום וברוכים הבאים לאפליקציה החדשה של בית הספר ״יגאל אלון״", 
        "title" : "שלום" 
    } 

into a String, which it is not (it's an object with two values, a message and a title). If you use the child method, you can get the message child of that object which is a String.

Note that you can also have the object deserialize into a POJO, if you make a class like this:

public class Message {

    public String message;
    public String title;

    public Message() {
        // Default constructor required for calls to DataSnapshot.getValue(Message.class)
    }

    public User(String message, String title) {
        this.message = message;
        this.title = title;
    }

}

And then have your ArrayList store message objects and use getValue(Message.class)