将Firebase数据检索到Android Studio中的ListView

Question

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 。 我尝试了一些代码，但显示列表活动后，应用程序立即崩溃。

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：

Firebase数据库图片

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);


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

    clientlistView = (ListView) findViewById(R.id.clientListView);
    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)

Answer 1

要解决此问题，只需移动以下代码行：

clientlistView = (ListView) findViewById(R.id.clientListView);

就在这行之前：

clientlistView.setAdapter(adapter);

在实际使用ListView对象之前，您首先需要实例化它。

Answer 2

您错过了创建列表视图的实例的步骤clientListView = findViewById（R.id.your_listView_id）;

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

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

Answer 3

首先，您需要初始化clientlistView，并且必须将listview设置为适配器，

在您的程序中，您正在设置适配器视图而不初始化clientlistview

您需要这样编写代码

clientListView = findViewById(R.id.your_listView_id);
adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1,list);
clientlistView.setAdapter(adapter);