更改Firebase数据后，ListView不会更新

Question

I am currently developing an app that displays all orders data from firebase in a listView. I have a problem that when the Firebase data is changed listview will not be updated, when I delete a row in firebase app is crashing ( Error: Failed to convert value of type java.util.HashMap to String ). Can anyone help me where I'm wrong?

MainActivity.java

    public class MainActivity extends AppCompatActivity {

    private DatabaseReference md;
    private ListView maListView;

    private ArrayList<OrdersModel> dataModels = new ArrayList<>();
    private OrderCustomAdapter adapter;
    Context c;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        c = this;

        md = FirebaseDatabase.getInstance().getReference("PERA");
        maListView = (ListView) findViewById(R.id.ma_list_view);

        adapter = new OrderCustomAdapter((ArrayList<OrdersModel>) dataModels, c);
        maListView.setAdapter((ListAdapter) adapter);


        maListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                final OrdersModel dataModel = dataModels.get(position);

                View popupView = getLayoutInflater().inflate(R.layout.popup_layout, null);

                final PopupWindow popupWindow = new PopupWindow(popupView,
                        AbsListView.LayoutParams.WRAP_CONTENT, AbsListView.LayoutParams.WRAP_CONTENT);

                TextView tvpStart = (TextView) popupView.findViewById(R.id.tvpStart);
                TextView tvpZiel = (TextView) popupView.findViewById(R.id.tvpZiel);
                TextView tvpName = (TextView) popupView.findViewById(R.id.tvpName);
                TextView tvpTelefonnummer = (TextView) popupView.findViewById(R.id.tvpTelefonnummer);
                TextView tvpPreis = (TextView) popupView.findViewById(R.id.tvpPreis);
                TextView tvpFahrer = (TextView) popupView.findViewById(R.id.tvpFahrer);
                TextView tvpFahrertelefonnummer = (TextView) popupView.findViewById(R.id.tvpFahrertelefonnummer);
                Button callButton = (Button) popupView.findViewById(R.id.btnCall);


                tvpStart.setText(dataModel.getStart());
                tvpZiel.setText(dataModel.getZiel());
                tvpName.setText(dataModel.getName());
                tvpTelefonnummer.setText(dataModel.getTelefonnummer());
                tvpPreis.setText("€ " + dataModel.getPreis());
                tvpFahrer.setText(dataModel.getFahrer());
                tvpFahrertelefonnummer.setText(dataModel.getFahrerTelefonnummer());


                callButton.setOnClickListener(new View.OnClickListener() {
                    Intent call = new Intent(Intent.ACTION_DIAL);

                    public void onClick(View v) {
//                                  call(dataModel.getFahrerTelefonnummer());

                        call.setData(Uri.parse("tel:" + dataModel.getFahrerTelefonnummer()));
                        startActivity(call);
                    }
                });


                popupWindow.setFocusable(true);

                Display display = getWindowManager().getDefaultDisplay();
                Point size = new Point();
                display.getSize(size);
                int width = size.x;
                int height = size.y;

                popupWindow.setWidth(width - 100);
                popupWindow.setHeight(height - 100);
                popupWindow.setContentView(popupView);

                popupWindow.showAtLocation(view, Gravity.CENTER, 0, 0);

            }
        });
        md.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot postSnapshot, String s) {

                OrdersModel ds = postSnapshot.getValue(OrdersModel.class);
                dataModels.add(ds);
                adapter.notifyDataSetChanged();

            }

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

                adapter.notifyDataSetChanged();
            }

            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {
                Map<String, Object> map = (Map<String, Object>) dataSnapshot.getValue();

                String string = dataSnapshot.getValue(String.class);
                dataModels.remove(map);
                adapter.notifyDataSetChanged();
            }

            @Override
            public void onChildMoved(DataSnapshot dataSnapshot, String s) {
                adapter.notifyDataSetChanged();
            }

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

    public void call(String number) {
        Intent callIntent = new Intent(Intent.ACTION_CALL);
        callIntent.setData(Uri.parse(number));
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.CALL_PHONE) != PackageManager.PERMISSION_GRANTED) {
            return;
        }
        startActivity(callIntent);
    }


}

OrderCustomAdapter.java

public class OrderCustomAdapter extends ArrayAdapter<OrdersModel> implements View.OnClickListener{


    private ArrayList<OrdersModel> dataSet;
    Context mContext;

    // View lookup cache
    private static class ViewHolder {
        TextView txtDatumZeit;
        TextView txtFahrer;
        TextView txtPreis;
    }



    public OrderCustomAdapter(ArrayList<OrdersModel> data, Context context) {
        super(context, R.layout.order_item, data);
        this.dataSet = data;
        this.mContext=context;

    }


    @Override
    public void onClick(View v) {


        int position=(Integer) v.getTag();
        Object object= getItem(position);
        OrdersModel dataModel=(OrdersModel)object;

    }

    private int lastPosition = -1;
    @RequiresApi(api = Build.VERSION_CODES.N)
    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        OrdersModel dataModel = getItem(position);
        ViewHolder viewHolder;
        final View result;
        final OrdersModel s = this.getItem(position);
        String dwa = s.getDriverConfirm();

        if (convertView == null) {


            viewHolder = new ViewHolder();
            LayoutInflater inflater = LayoutInflater.from(getContext());
            convertView = inflater.inflate(R.layout.order_item, parent, false);
            viewHolder.txtDatumZeit = (TextView) convertView.findViewById(R.id.tvDatumZeit);
            viewHolder.txtFahrer = (TextView) convertView.findViewById(R.id.tvFahrer);
            viewHolder.txtPreis = (TextView) convertView.findViewById(R.id.tvPreis);

            result=convertView;

            convertView.setTag(viewHolder);
        } else {
            viewHolder = (ViewHolder) convertView.getTag();
            result=convertView;
        }
        SimpleDateFormat df = new SimpleDateFormat("dd.MM.yyyy - HH:mm");
        String hg = df.format(dataModel.getDatumZeit());
        viewHolder.txtDatumZeit.setText(hg);
        viewHolder.txtFahrer.setText(dataModel.getFahrer());
        viewHolder.txtPreis.setText("€ "+dataModel.getPreis());
        if (dwa.contentEquals("ja")) {
            convertView.setBackgroundColor(Color.parseColor("#1FBB5E"));
        }else{
            convertView.setBackgroundColor(Color.parseColor("#FFFFFF"));
        }
        return convertView;
    }
}

OrdersModel.java

public class OrdersModel {

    public OrdersModel() {}

    public String telefonnummer;
    public String start;
    public String adminConfirm;
    public String url;
    public String orderId;
    public String fahrerTelefonnummer;
    public String preis;
    public String name;
    public String fahrer;
    public long datumZeit;
    public String ziel;
    public String driverConfirm;

    public String getTelefonnummer() {
        return telefonnummer;
    }

    public void setTelefonnummer(String telefonnummer) {
        this.telefonnummer = telefonnummer;
    }

    public String getStart() {
        return start;
    }

    public void setStart(String start) {
        this.start = start;
    }

    public String getAdminConfirm() {
        return adminConfirm;
    }

    public void setAdminConfirm(String adminConfirm) {
        this.adminConfirm = adminConfirm;
    }

    public String getUrl() {
        return url;
    }

    public void setUrl(String url) {
        this.url = url;
    }

    public String getOrderId() {
        return orderId;
    }

    public void setOrderId(String orderId) {
        this.orderId = orderId;
    }

    public String getFahrerTelefonnummer() {
        return fahrerTelefonnummer;
    }

    public void setFahrerTelefonnummer(String fahrerTelefonnummer) {
        this.fahrerTelefonnummer = fahrerTelefonnummer;
    }

    public String getPreis() {
        return preis;
    }
    public void setPreis(String preis) {
        this.preis = preis;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getFahrer() {
        return fahrer;
    }

    public void setFahrer(String fahrer) {
        this.fahrer = fahrer;
    }

    public long getDatumZeit() {
        return datumZeit;
    }
    public void setDatumZeit(long datumZeit) {
        this.datumZeit = datumZeit;
    }

    public String getZiel() {
        return ziel;
    }

    public void setZiel(String ziel) {
        this.ziel = ziel;
    }

    public String getDriverConfirm() {
        return driverConfirm;
    }

    public void setDriverConfirm(String driverConfirm) {
        this.driverConfirm = driverConfirm;
    }
}

order_item.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="100dp"
    android:orientation="vertical">

    <LinearLayout
        android:id="@+id/rowOrders"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_alignParentStart="true"
        android:layout_alignParentTop="true"
        android:orientation="horizontal">

        <TextView
            android:id="@+id/tvDatumZeit"
            android:layout_width="420dp"
            android:layout_height="wrap_content"
            android:layout_alignParentStart="true"
            android:layout_weight="1"
            android:text="" />

        <TextView
            android:id="@+id/tvPreis"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:gravity="center"
            android:text="" />

        <TextView
            android:id="@+id/tvFahrer"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:gravity="right"
            android:paddingRight="20dp"
            android:text="" />
    </LinearLayout>


</RelativeLayout>

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <ListView
        android:id="@+id/ma_list_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_alignParentBottom="true"
        android:layout_alignParentStart="true" />


</RelativeLayout>

popup_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#E6044A7F">

    <TextView
        android:id="@+id/tvpName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentStart="true"
        android:layout_alignParentTop="true"
        android:layout_marginLeft="30dp"
        android:layout_marginRight="30dp"
        android:layout_marginTop="30dp"
        android:text="Name"
        android:textColor="#FFFFFF" />

    <TextView
        android:id="@+id/tvpTelefonnummer"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignStart="@+id/tvpName"
        android:layout_below="@+id/tvpName"
        android:text="Telefonnummer"
        android:textColor="#FFFFFF" />

    <TextView
        android:id="@+id/tvpStart"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignStart="@+id/tvpName"
        android:layout_below="@+id/tvpTelefonnummer"
        android:layout_marginRight="30dp"
        android:layout_marginTop="5dp"
        android:text="Start"
        android:textColor="#ffffff" />

    <TextView
        android:id="@+id/tvpZiel"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignStart="@+id/tvpName"
        android:layout_below="@+id/tvpStart"
        android:layout_marginRight="30dp"
        android:layout_marginTop="15dp"
        android:text="Ziel"
        android:textColor="#ffffff" />

    <TextView
        android:id="@+id/tvpPreis"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignStart="@+id/tvpName"
        android:layout_below="@+id/tvpZiel"
        android:layout_marginTop="15dp"
        android:text="Preis"
        android:textColor="#ffffff" />

    <TextView
        android:id="@+id/tvpFahrer"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignStart="@+id/tvpName"
        android:layout_below="@+id/tvpPreis"
        android:layout_marginTop="30dp"
        android:text="Fahrer"
        android:textColor="#ffffff" />

    <TextView
        android:id="@+id/tvpFahrertelefonnummer"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignStart="@+id/tvpName"
        android:layout_below="@+id/tvpFahrer"
        android:text="Fahrer Telefonnummer"
        android:textColor="#ffffff" />

    <Button
        android:id="@+id/btnCall"
        style="@android:style/Widget.DeviceDefault.Light.Button.Inset"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_alignStart="@+id/tvpName"
        android:layout_marginBottom="30dp"
        android:layout_marginRight="30dp"
        android:text="Fahrer anrufen" />

    <Button
        android:id="@+id/btnUberCOnfirm"
        style="@android:style/Widget.DeviceDefault.Light.Button.Inset"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_alignStart="@+id/tvpName"
        android:layout_marginBottom="109dp"
        android:layout_marginRight="30dp"
        android:text="UBER BESTÄTIGEN" />
</RelativeLayout>

Can somebody help me?

Answer 1

1. Your list is not updating because the adapter holds a reference to old items. Call

    adapter.clear(); adapter.addAll(dataModels); 

   instead of notifyDataSetChanged() . Similar question

2. It looks like this line

    String string = dataSnapshot.getValue(String.class); 

   is causing crash. string is never user, it can be removed.