DatabaseException：无法将类型java.lang.String的对象转换为type

Question

对于类似的问题，我已经查看了其他一些答案，但不明白为什么我的系统无法正常工作。

我正在尝试让我的应用从Firebase读取命令并移动无人机。firebase中的命令来自单独的软件。该应用程序是基于Parrot Drone
SDK示例代码构建的。

它似乎能够从命令对象中获取文本并将其附加到textview中，但是添加新的子项只会使其崩溃。添加新孩子时出现此错误。

E/AndroidRuntime: FATAL EXCEPTION: main
                Process: com.parrot.sdksample, PID: 10592
                com.google.firebase.database.DatabaseException: Can't convert object of type java.lang.String to type com.parrot.sdksample.activity.CommandObject
                    at com.google.android.gms.internal.zg.zzb(Unknown Source)
                    at com.google.android.gms.internal.zg.zza(Unknown Source)
                    at com.google.firebase.database.DataSnapshot.getValue(Unknown Source)
                    at com.parrot.sdksample.activity.MiniDroneActivity$13.onChildAdded(MiniDroneActivity.java:383)
                    at com.google.android.gms.internal.px.zza(Unknown Source)
                    at com.google.android.gms.internal.vj.zzHX(Unknown Source)
                    at com.google.android.gms.internal.vp.run(Unknown Source)
                    at android.os.Handler.handleCallback(Handler.java:739)
                    at android.os.Handler.dispatchMessage(Handler.java:95)
                    at android.os.Looper.loop(Looper.java:148)
                    at android.app.ActivityThread.main(ActivityThread.java:5417)
                    at java.lang.reflect.Method.invoke(Native Method)
                    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

下面是我在firebase中的数据结构示例。

{
"drones" : {
    "commands" : {
    "-L-n9HwaQktOdI2VEVlH" : {
        "executed" : false,
        "text" : "TAKE_OFF",
        "timestamp" : 1.512686825309134E9
    },
    "-L-nAuK5Ifde7Cdnan8K" : {
        "executed" : false,
        "text" : "LAND",
        "timestamp" : 1.512687248764272E9
    }
    }
}
}

我的活动中从Firebase获取数据的功能如下所示。

private void initFirebase(){
        mCommandTextView = (TextView) findViewById(R.id.commandTextView);

        FirebaseDatabase database = FirebaseDatabase.getInstance();
        DatabaseReference commandsRef = database.getReference("drones/commands");

        ChildEventListener childEventListener = new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String previousChildName) {
                Log.d(TAG, "onChildAdded:" + dataSnapshot.getKey());
                CommandObject command = dataSnapshot.getValue(CommandObject.class);
                mCommandTextView.append(command.text + "\n");
// I've tried commenting out the if statements below this, but it doesn't seem to make a difference.
                if ("TAKE_OFF".equals(command.text)) {
                    mMiniDrone.takeOff();
                } else if ("LAND".equals(command.text)) {
                    mMiniDrone.land();
                }
            }

            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String previousChildName){

            }

            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {

            }

            public void onChildMoved(DataSnapshot dataSnapshot, String previousChildName){

            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        };
        commandsRef.addChildEventListener(childEventListener);
    }

我的CommandObject类看起来像这样。

public class CommandObject {
    public String text;
    public float timestamp;
    public boolean executed;

    public CommandObject() {
    }

    public CommandObject(boolean executed, String text, float timestamp){
        this.executed = executed;
        this.text = text;
        this.timestamp = timestamp;
    }
}

我也尝试过使用值事件侦听器，但是发生了相同的问题。

Answer 1

您收到此错误：

Can't convert object of type java.lang.String to type com.parrot.sdksample.activity.CommandObject

因为您正在尝试读取类型String为type 的数据，CommandObject以及为什么收到此错误的原因。

获取这些值的一种更简单的方法是使用如下所示的String类：

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference commandsRef = rootRef.child("drones").child("commands");
ValueEventListener eventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            boolean executed = ds.child("executed").getValue(Boolean.class);
            String text = ds.child("text").getValue(String.class);
            double timestamp = ds.child("timestamp").getValue(Double.class);
            Log.d("TAG", executed + " / " + text + " / " + timestamp);
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
commandsRef.addListenerForSingleValueEvent(eventListener);

而这种使用CommandObject类对象的方法：

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference commandsRef = rootRef.child("drones").child("commands");
ValueEventListener eventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            CommandObject commandObject = ds.getValue(CommandObject.class);
            Log.d("TAG", commandObject.getExecuted() + " / " + 
                commandObject.getText() + " / " + 
                commandObject.getTimestamp());
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
commandsRef.addListenerForSingleValueEvent(eventListener);

在这两种情况下，您的输出均为：

false / TAKE_OFF / 1.512686825309134E9
false / LAND / 1.512687248764272E9