我目前正在构建一个 Android 应用程序并使用 Firebase 作为其后端数据库,但是我很难检索数据并将它们显示在ListView
. 我尝试了一些代码,但是一旦我显示列表活动,应用程序就会崩溃。
下面是我的数据库的屏幕截图,还有我的代码和我尝试运行时遇到的错误:
Firebase 数据库
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;
}
}
你得到那个错误,因为你想念一个孩子。要解决此问题,请更改以下代码行:
String value = dataSnapshot.getValue(String.class);
至
String value = dataSnapshot.child("Name").getValue(String.class);
logat 中的输出将是所有客户端的名称。
PS还要注意,你会在你的项目中遇到其他错误,因为你的Client
类中的字段名称都是小写的,而在你的数据库中都是以大写字母开头。名称应该相同。也请看这里。
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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