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

Question

My Problem

      08-10 04:23:28.820 21501-21501/my.org.medicare.medicareapp E/AndroidRuntime: FATAL EXCEPTION: main                                              Process: my.org.medicare.medicareapp, PID: 21501                                             com.google.firebase.database.DatabaseException: Can't convert object of type java.lang.String to type my.org.medicare.medicareapp.protest
at com.google.android.gms.internal.zzaln.zzd(Unknown Source)
at com.google.android.gms.internal.zzaln.zzb(Unknown Source)
at com.google.android.gms.internal.zzaln.zza(Unknown Source)
at com.google.firebase.database.DataSnapshot.getValue(Unknown Source)
at my.org.medicare.medicareapp.FirebaseHelper.fetchData(FirebaseHelper.java:50)
at my.org.medicare.medicareapp.FirebaseHelper.access$000(FirebaseHelper.java:14)            atmy.org.medicare.medicareapp.FirebaseHelper$1.onChildAdded(FirebaseHelper.java60)at com.google.android.gms.internal.zzahe.zza(Unknown Source)                                      at com.google.android.gms.internal.zzaje.zzcta(Unknown Source)                          at com.google.android.gms.internal.zzajh$1.run(Unknown Source)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
atcom.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

My Main Activity Code:

 rv = (RecyclerView) findViewById(R.id.rv);
        rv.setLayoutManager(new LinearLayoutManager(this));
        //INITIALIZE FIREBASE DB
        db = FirebaseDatabase.getInstance().getReference().child("Pro");;

        helper = new FirebaseHelper(db);
        //ADAPTER
        adapter = new MyAdapter(this, helper.retrieve());
        rv.setAdapter(adapter);

And my FirebaseHelper:

public class FirebaseHelper {
    DatabaseReference db;
    Boolean saved=null;
    ArrayList<protest> spacecrafts=new ArrayList<>();

    public FirebaseHelper(DatabaseReference db) {
        this.db = db;
    }
    //WRITE IF NOT NULL
    public Boolean save(protest spacecraft)
    {
        if(spacecraft==null)
        {
            saved=false;
        }else
        {
            try
            {
                db.child("Pro").push().setValue(spacecraft);
                saved=true;
            }catch (DatabaseException e)
            {
                e.printStackTrace();
                saved=false;
            }
        }
        return saved;
    }

    //IMPLEMENT FETCH DATA AND FILL ARRAYLIST
    private void fetchData(DataSnapshot dataSnapshot)
    {
        spacecrafts.clear();
        //dataSnapshot.getChildren()
        for (DataSnapshot ds : dataSnapshot.getChildren())
        {
            protest spacecraft=ds.getValue(protest.class);
            spacecrafts.add(spacecraft);
        }
    }

    //READ THEN RETURN ARRAYLIST
    public ArrayList<protest> retrieve() {
        db.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                fetchData(dataSnapshot);
            }
            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String s) {
                fetchData(dataSnapshot);
            }
            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {
            }
            @Override
            public void onChildMoved(DataSnapshot dataSnapshot, String s) {
            }
            @Override
            public void onCancelled(DatabaseError databaseError) {
            }
        });
        return spacecrafts;
    }
}

My view Holder

public class MyViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
    TextView nameTxt,propTxt,descTxt;
    ItemClickListener itemClickListener;

    public MyViewHolder(View itemView) {
        super(itemView);
        nameTxt= (TextView) itemView.findViewById(R.id.nameTxt);
        propTxt= (TextView) itemView.findViewById(R.id.propellantTxt);
        descTxt= (TextView) itemView.findViewById(R.id.descTxt);
        itemView.setOnClickListener(this);
    }

    public void setItemClickListener(ItemClickListener itemClickListener)
    {
        this.itemClickListener=itemClickListener;
    }
    @Override
    public void onClick(View view) {
        this.itemClickListener.onItemClick(this.getLayoutPosition());
    }

Below is my data structure, I try to read "Pro"

Data Structure

My Protest structure:

public class protest {
    private String name;
    private String contact;
    private String centre;

    public protest(){};

    public String getName(){return name;}
    public String getContact(){return contact;}
    public String getCentre(){return centre;}
    public void setName(String name) {
        this.name = name;
    }
    public void setContact(String contact) {
        this.contact = contact;
    }
    public void setCentre(String centre) {
        this.centre = centre;
    }
}

May i know what is the problem? I am new to firebase and i need your help. I read some similar question here but i still can't solve my problem. Can any expert here help me?

Update: Tested again with print.. when Datasnapshot go to

  public ArrayList<protest> retrieve() {
        db.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                System.out.println(dataSnapshot.getValue());
                fetchData(dataSnapshot);
            }

08-10 06:26:47.548 26700-26700/my.org.medicare.medicareapp I/System.out: {name=lee, contact=123456, centre=jalan ipoh}

But when Datasnapshot go to

 private void fetchData(DataSnapshot dataSnapshot)
    {
        spacecrafts.clear();
        //dataSnapshot.getChildren()
        for (DataSnapshot ds : dataSnapshot.getChildren())
        {
            System.out.println(ds.getValue());
            protest spacecraft=ds.getValue(protest.class);
            spacecrafts.add(spacecraft);
        }
    }

08-10 06:26:47.548 26700-26700/my.org.medicare.medicareapp I/System.out: jalan ipoh

What is the problem?

Answer 1

Your fetchData function already receives DataSnapshot as a child-node, but even here you still loop through the children which will effectively be looping through individual keys (name, contact, centre). You should simply call getValue on DataSnaphot without looping:

private void fetchData(DataSnapshot dataSnapshot)
    {
        spacecrafts.clear();        
        protest spacecraft=dataSnapshot.getValue(protest.class);
        spacecrafts.add(spacecraft);        
    }

I hope this helps.

Answer 2

Most likely this is your problem, in your fetchData method:

private void fetchData(DataSnapshot dataSnapshot)
{
    spacecrafts.clear();
    //dataSnapshot.getChildren()
    for (DataSnapshot ds : dataSnapshot.getChildren()) //--> At this point, ds is an iterator of dataSnapshot; it will iterate the dataSnapshot's children. In this case, the first child's type is String, thus the first iteration of ds will have a type of String. 
    {
        System.out.println(ds.getValue());
        protest spacecraft=ds.getValue(protest.class); //--> at this point, you tried to assign a String to an Object with type "protest" by conversion. This is illegal, so it will throw an exception instead.
        spacecrafts.add(spacecraft);
    }
}

I hope this helps.