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

问题描述

我目前正在构建一个Android应用，并将Firebase用作其后端数据库，但是我很难检索数据并将其显示在ListView中.我尝试了一些代码，但显示列表活动后，该应用程序便崩溃了.

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.

下面是我的数据库的屏幕快照，还有我的代码和尝试运行时遇到的错误:

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

Firebase数据库

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) {

        }
    });
}


}

错误日志:

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)

客户端对象类:

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;
}
}

推荐答案

由于缺少孩子，您遇到了该错误.要解决此问题，请更改以下代码行:

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);

到

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

您的登录名中的输出将是所有客户端的名称.

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

PS .还要注意，在项目中还会遇到其他错误，因为Client类中的字段名称全部为小写，而数据库中的名称均以大写字母开头.名称应相同.另请参阅 .

P.S. 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; }
}

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