Question

我想从我的数据库中获取所有孩子。我试过像Google开发者发布的YouTube上几乎类似的代码，但从那时起就有了改变。我只想获取用户数据以下的所有信息（有用户的配置文件）。我的数据库看起来像：

我希望得到 userdata 下的所有信息这是我的代码。

FirebaseDatabase database5 = FirebaseDatabase.getInstance();
    DatabaseReference myRef5 = database5.getReference("userdata");

    myRef5.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            // This method is called once with the initial value and again
            // whenever data at this location is updated.
            Map<String, String> map = dataSnapshot.getValue(Map.class);
            map.getClass();
            Log.i("Mufz", "Value is: " + map.getClass());


 //                Toast.makeText(getApplicationContext(), `value3.toString(), Toast.LENGTH_SHORT).show();`

        }

        @Override
        public void onCancelled(DatabaseError error) {
            // Failed to read value
            Log.w("TAG", "Failed to read value.", error.toException());
//                Toast.makeText(getApplicationContext(), error.toException().toString(), Toast.LENGTH_SHORT).show();
        }

    });

运行之后没有任何反应，它会运行prevoroius活动或出现错误

08-16 19:29:47.225 32507-32507/com.samo.facedatefb E/AndroidRuntime: FATAL EXCEPTION: main
                                                                 Process: com.samo.facedatefb, PID: 32507
                                                                 com.google.firebase.database.DatabaseException: Class java.util.Map has generic type parameters, please use GenericTypeIndicator instead
                                                                     at com.google.android.gms.internal.zzaln.zzb(Unknown Source)
                                                                     at com.google.android.gms.internal.zzaln.zza(Unknown Source)
                                                                     at com.google.firebase.database.DataSnapshot.getValue(Unknown Source)
                                                                     at com.samo.facedatefb.MapsActivity$3.onDataChange(MapsActivity.java:306)
                                                                     at com.google.android.gms.internal.zzaie.zza(Unknown Source)
                                                                     at com.google.android.gms.internal.zzaje.zzcta(Unknown Source)
                                                                     at com.google.android.gms.internal.zzajh$1.run(Unknown Source)
                                                                     at android.os.Handler.handleCallback(Handler.java:815)
                                                                     at android.os.Handler.dispatchMessage(Handler.java:104)
                                                                     at android.os.Looper.loop(Looper.java:194)
                                                                     at android.app.ActivityThread.main(ActivityThread.java:5549)
                                                                     at java.lang.reflect.Method.invoke(Native Method)
                                                                     at java.lang.reflect.Method.invoke(Method.java:372)
                                                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:964)
                                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:759)

哪里可能有问题？感谢。

Answer 1

更改

Map<String, String> map = dataSnapshot.getValue(Map.class);

要

Map<String, Object> map = dataSnapshot.getValue(Map.class);

Answer 2

使用GenericTypeIndicator类来封送数据。根据文件：

由于Java实现泛型（类型擦除）的方式，它是必须使用稍微复杂的方法来正确使用在运行时解析泛型集合的类型。

而不是：

Map<String, String> map = dataSnapshot.getValue(Map.class);

使用GenericTypeIndicator：

@Override
public void onDataChange(DataSnapshot dataSnapshot) {
    // This method is called once with the initial value and again
    // whenever data at this location is updated.
    for(DataSnapshot snapshot : dataSnapshot.getChildren()){
        GenericTypeIndicator<Map<String, Object>> m = new GenericTypeIndicator<Map<String, Object>>(){};
        Map<String, Object> map = snapshot.getValue(m);
        long age = (long)map.getValue("age");

        Log.i("Mufz", "User Data Value is: " + snapshot.getKey());
        Log.i("Mufz", "User Name Value is: " + map.getValue("username"));
        Log.i("Mufz", "User Age Value is: " + age);
    }
}

Answer 3

据我所知，DataSnapshot会为您提供array而不是object。那么也许你可以尝试这样的事情：

@Override
public void onDataChange(DataSnapshot dataSnapshot) {
    for(DataSnapshot snapshot : dataSnapshot.getChildren()){
        YourObject obj = snapshot.getValue(YourObject.class);
    }
}

Firebase数据库获取所有子项

3 个答案: