在Android Studio中检索Firebase数据并在ListView中显示

Question

I am currently building an Android app and using Firebase as its backend database, however i'm having difficulties to retrieve the data and display them in a ListView . I tried few codes but the app is crashing as soon as i display the list activity.

Below is a screenshot of my database and also my code and the error I get when I try to run:

Firebase Database

public class ClientLIstActivity extends AppCompatActivity {

private Button clientSelect;
private ListView clientlistView;
private FirebaseDatabase database;
private DatabaseReference databaseReference;

private ArrayList<String> list = new ArrayList<String>();
private ArrayAdapter<String> adapter;

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

    clientlistView = (ListView) findViewById(R.id.clientListView);
    adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1,list);
    clientlistView.setAdapter(adapter);
    adapter.notifyDataSetChanged();
    databaseReference = FirebaseDatabase.getInstance().getReference().child("Clients");


    clientSelect = (Button) findViewById(R.id.selectClient);

    clientSelect.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            onBackPressed();
        }
    });

    databaseReference.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
           String value = dataSnapshot.getValue(String.class);
           list.add(value);
        }

        @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(DatabaseError databaseError) {

        }
    });
}


}

Error Log:

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.javytharanee.quicksolattendance, PID: 6602
              com.google.firebase.database.DatabaseException: Failed to convert value of type java.util.HashMap to String
                  at com.google.android.gms.internal.zzepg.zzb(Unknown Source:93)
                  at com.google.android.gms.internal.zzepg.zza(Unknown Source:0)
                  at com.google.firebase.database.DataSnapshot.getValue(Unknown Source:10)
                  at com.javytharanee.quicksolattendance.ClientLIstActivity$2.onChildAdded(ClientLIstActivity.java:53)
                  at com.google.android.gms.internal.zzegg.zza(Unknown Source:71)
                  at com.google.android.gms.internal.zzelk.zzcal(Unknown Source:2)
                  at com.google.android.gms.internal.zzelq.run(Unknown Source:71)
                  at android.os.Handler.handleCallback(Handler.java:873)
                  at android.os.Handler.dispatchMessage(Handler.java:99)
                  at android.os.Looper.loop(Looper.java:193)
                  at android.app.ActivityThread.main(ActivityThread.java:6669)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

Client Object Class:

public class Client {
String name;
String location;
String latitude;
String longitude;

public Client(String location, String latitude, String longitude) {
    this.location = location;
    this.latitude = latitude;
    this.longitude = longitude;
}

public Map toMap() {
    Map client = new HashMap();

    client.put("Name", this.name);
    client.put("Location", this.location);
    client.put("Latitude", this.latitude);
    client.put("Longitude", this.longitude);

    return client;
}

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

Answer 1

You are getting that error, because you are missing a child. To solve this, please change the following line of code:

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

to

String value = dataSnapshot.child("Name").getValue(String.class);

The output in your logat will be the name of all clients.

PS Also be aware that you'll encounter other errors in your project because the name of your fields inside your Client class are all lowercase while in your database all start with a capital letter. The name should be the same. Please also take a look here .

public class Client {
    private String name, location, latitude, longitude;

    public Client() {}

    public Client(String name, String location, String latitude, String longitude) {
        this.name = name;
        this.location = location;
        this.latitude = latitude;
        this.longitude = longitude;
    }

    public String getName() { return name; }
    public String getLocation() { return location; }
    public String getLatitude() { return latitude; }
    public String getLongitude() { return longitude; }
}

Answer 2

create POJO class ArrayList and add this in your list check below code

    public class ClientLIstActivity extends AppCompatActivity {

private Button clientSelect;
private ListView clientlistView;
private FirebaseDatabase database;
private DatabaseReference databaseReference;

private ArrayList<Client> list = new ArrayList>(); // 1=change here
private ArrayAdapter<String> adapter;

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

    clientlistView = (ListView) findViewById(R.id.clientListView);
    adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1,list);
    clientlistView.setAdapter(adapter);
    adapter.notifyDataSetChanged();
    databaseReference = FirebaseDatabase.getInstance().getReference().child("Clients");


    clientSelect = (Button) findViewById(R.id.selectClient);

    clientSelect.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            onBackPressed();
        }
    });

    databaseReference.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
           Client value = dataSnapshot.getValue(Client.class);  // 2=change here
           list.add(value);
        }

        @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(DatabaseError databaseError) {

        }
    });
}


}