Question

我对android和firebase很新。我正在创建一个通过蓝牙连接设备的应用程序，然后在配对后，用户可以共享设备。现在一切顺利，直到我尝试从Firebase检索数据。这是我的数据库结构： Database Structure

共享设备是用户可以与他人共享的设备。用户设备是用户保存的设备。 “meh”是设备的名称。以下是我尝试访问数据的方法：

public class RecievedKeys extends AppCompatActivity {
    String key;
    TextView mName;
    TextView mDevice;
    String name;
    String MAC;
    User user = new User();
    DatabaseReference mDatabaseRef;
    String decPass;
    String password;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_recieved_keys);
        mDatabaseRef = FirebaseDatabase.getInstance().getReference().child("Shared_With").child(User.getUid());
        mName = (TextView)findViewById(R.id.name);
        mDevice = (TextView)findViewById(R.id.MAC);
        mDatabaseRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                name = dataSnapshot.child("Name").getValue().toString();
                MAC = dataSnapshot.child("MAC").getValue().toString();
                password = dataSnapshot.child("password").getValue().toString();
                System.out.println(name);
                mName.setText(name);
                mDevice.setText(MAC);
                decPassword(password);
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });
    }

我无法得到任何东西，相反，我得到一个异常抛出。任何指导都将不胜感激。

这是抛出的异常，NullPointer：

   05-12 06:10:25.608 5268-5268/habibqureshi.simplegui.simplegui E/UncaughtException: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
                                                                                       at habibqureshi.simplegui.SmartLock.RecievedKeys$1.onDataChange(RecievedKeys.java:38)
                                                                                       at com.google.android.gms.internal.zzbmz.zza(Unknown Source)
                                                                                       at com.google.android.gms.internal.zzbnz.zzYj(Unknown Source)
                                                                                       at com.google.android.gms.internal.zzboc$1.run(Unknown Source)
                                                                                       at android.os.Handler.handleCallback(Handler.java:739)
                                                                                       at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                       at android.os.Looper.loop(Looper.java:135)
                                                                                       at android.app.ActivityThread.main(ActivityThread.java:5910)
                                                                                       at java.lang.reflect.Method.invoke(Native Method)
                                                                                       at java.lang.reflect.Method.invoke(Method.java:372)
                                                                                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1405)
                                                                                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1200)
05-12 06:10:25.608 5268-5426/habibqureshi.simplegui.simplegui D/FA: Logging event (FE): _ae, Bundle[{_o=crash, _sc=RecievedKeys, _si=4592305897079779979, timestamp=1494551425619, fatal=1}]
05-12 06:10:25.939 5268-5268/habibqureshi.simplegui.simplegui E/AndroidRuntime: FATAL EXCEPTION: main
                                                                                Process: habibqureshi.simplegui.simplegui, PID: 5268
                                                                                java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
                                                                                    at habibqureshi.simplegui.SmartLock.RecievedKeys$1.onDataChange(RecievedKeys.java:38)
                                                                                    at com.google.android.gms.internal.zzbmz.zza(Unknown Source)
                                                                                    at com.google.android.gms.internal.zzbnz.zzYj(Unknown Source)
                                                                                    at com.google.android.gms.internal.zzboc$1.run(Unknown Source)
                                                                                    at android.os.Handler.handleCallback(Handler.java:739)
                                                                                    at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                    at android.os.Looper.loop(Looper.java:135)
                                                                                    at android.app.ActivityThread.main(ActivityThread.java:5910)
                                                                                    at java.lang.reflect.Method.invoke(Native Method)
                                                                                    at java.lang.reflect.Method.invoke(Method.java:372)
                                                                                    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1405)
                                                                                    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1200)

我需要做的是访问Shared_With-＆gt; UID-＆gt; Email-＆gt; name-＆gt; children中的值。

Answer 1

也许您需要以不同的方式构建数据。你可以这样做：

{ 
    shared {
        receiver_id {
            user_id: true,
            user_id: true,
            user_id: true
        }
    }

    devices {
        user_id {

            meh: {
                attribute: 'lorem ipsum'
            }
        }
    }
}

Answer 2

要实现此目的，请使用以下代码：

mDatabaseRef = FirebaseDatabase.getInstance().getReference().child("Shared_With").child(User.getUid()).child(User.getEmail()).child("meh");
    mName = (TextView)findViewById(R.id.name);
    mDevice = (TextView)findViewById(R.id.MAC);
    mDatabaseRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            name = (String) dataSnapshot.getKey();
            MAC = (String) dataSnapshot.child("MAC").getValue();
            password = (String) dataSnapshot.child("password").getValue();
            System.out.println(name);
            mName.setText(name);
            mDevice.setText(MAC);
            decPassword(password);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

问题是你遗失了reference中的一些孩子。

希望它有所帮助。

如何访问没有值

2 个答案: