FireBase数据库空对象

Question

我从Firebase数据库读取一些数据，并返回对象为null。

我尝试了很多方法。当我有（如现在在代码中）

marker(mMap.addMarker(new MarkerOptions().position(new LatLng(value, value2)).title("Mehehe"));) 

onDataChange之外的方法发生错误。但是当我把标志物onDataChange方法和设置都：

DatabaseReference myRef2 = database2.getReference("latitude"); 

和

DatabaseReference myRef = database.getReference("latitude"); 

如果有类似getRefrences没有错误，但如果我尝试

DatabaseReference myRef2 = database2.getReference("latitude"); 

和

DatabaseReference myRef = database.getReference("longitude"); 

然后出现类似的错误，例如.addMarker在onDataChange方法之外。

public class MapsActivity extends FragmentActivity implements OnMapReadyCallback, LocationListener, View.OnClickListener { 

    private ImageView imageView; 

    private GoogleMap mMap; 

    private GoogleApiClient client; 

    private DatabaseReference mDatabase; 

    private String email; 

    private double latitude; 
    private double longitude; 

    Double value; 
    Double value2; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_maps); 
     // Obtain the SupportMapFragment and get notified when the map is ready to be used. 
     SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager() 
       .findFragmentById(R.id.map); 
     mapFragment.getMapAsync(this); 

     FirebaseDatabase database = FirebaseDatabase.getInstance(); 
     DatabaseReference myRef = database.getReference("[email protected] - username"); 

     myRef.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. 
       String value = dataSnapshot.getValue(String.class); 
       Log.d("TAG", "Value is: " + value); 
       Toast.makeText(getApplicationContext(), value.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(); 
      } 
     }); 

     client = new GoogleApiClient.Builder(this).addApi(AppIndex.API).build(); 

     imageView = (ImageView) findViewById(R.id.imageView); 
     imageView.setOnClickListener(this); 

    } 

    @Override 
    public void onClick(View view) { 

     if (view == imageView) { 

      Intent intent = new Intent(getApplicationContext(), ProfileSetUp.class); 
      startActivity(intent); 

     } 

    } 

    /** 
    * Manipulates the map once available. 
    * This callback is triggered when the map is ready to be used. 
    * This is where we can add markers or lines, add listeners or move the camera. In this case, 
    * we just add a marker near Sydney, Australia. 
    * If Google Play services is not installed on the device, the user will be prompted to install 
    * it inside the SupportMapFragment. This method will only be triggered once the user has 
    * installed Google Play services and returned to the app. 
    */ 
    @Override 
    public void onMapReady(GoogleMap googleMap) { 
     mMap = googleMap; 

     if (ActivityCompat.checkSelfPermission(this, android.Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, android.Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) { 
      // TODO: Consider calling 
      // ActivityCompat#requestPermissions 
      // here to request the missing permissions, and then overriding 
      // public void onRequestPermissionsResult(int requestCode, String[] permissions, 
      //           int[] grantResults) 
      // to handle the case where the user grants the permission. See the documentation 
      // for ActivityCompat#requestPermissions for more details. 
      return; 
     } 

     mMap.setMyLocationEnabled(true); 

     LocationManager locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE); 

     Criteria criteria = new Criteria(); 

     String provider = locationManager.getBestProvider(criteria, true); 

     Location myLocation = locationManager.getLastKnownLocation(LocationManager.PASSIVE_PROVIDER); 
     //  Location myLocation = locationManager.getLastKnownLocation(provider); 

     mMap.setMapType(GoogleMap.MAP_TYPE_NORMAL); 

     //  double latitude = myLocation.getLatitude(); 
     //  double longitude = myLocation.getLongitude(); 

     latitude = myLocation.getLatitude(); 
     longitude = myLocation.getLongitude(); 

     mDatabase = FirebaseDatabase.getInstance().getReference(); 

     //  mDatabase.child("latitude").setValue(latitude); 
     //  mDatabase.child("longitude").setValue(longitude); 

     LatLng latLng = new LatLng(latitude, longitude); 

     mMap.moveCamera(CameraUpdateFactory.newLatLng(latLng)); 
     mMap.animateCamera(CameraUpdateFactory.zoomTo(14)); 
     mMap.addMarker(new MarkerOptions().position(new LatLng(latitude, longitude)).title("You are here!").snippet("Consider yourself located")); 
     //  mMap.addMarker(new MarkerOptions().position(new LatLng(48.7352022, 19.1187914)).title("You are here!").snippet("Consider yourself located")); 


     FirebaseDatabase database2 = FirebaseDatabase.getInstance(); 
     DatabaseReference myRef2 = database2.getReference("latitude"); 

     myRef2.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. 
       value2 = dataSnapshot.getValue(Double.class); 
       Log.d("TAG", "Value is: " + value2); 
       //    Toast.makeText(getApplicationContext(), value3.toString(), Toast.LENGTH_SHORT).show(); 
       Toast.makeText(getApplicationContext(), value2.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(); 
      } 

     }); 

     FirebaseDatabase database = FirebaseDatabase.getInstance(); 
     DatabaseReference myRef = database.getReference("longitude"); 

     myRef.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. 
       value = dataSnapshot.getValue(Double.class); 
       Log.d("TAG", "Value is: " + value); 

       //    mMap.addMarker(new MarkerOptions().position(new LatLng(value, value2)).title("Mehehe")); 
       Toast.makeText(getApplicationContext(), value.toString(), Toast.LENGTH_SHORT).show(); 
       Toast.makeText(getApplicationContext(), value2.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(); 
      } 
     }); 

     mMap.addMarker(new MarkerOptions().position(new LatLng(value, value2)).title("Mehehe")); 

    } 

    @Override 
    public void onLocationChanged(Location location) { 



    } 
} 

而且有错误信息： -

08-15 10:47:53.591 4280-4280/com.samo.facedatefb E/AndroidRuntime: FATAL EXCEPTION: main 
Process: com.samo.facedatefb, PID: 4280 
java.lang.NullPointerException: Attempt to invoke virtual method 'double java.lang.Double.doubleValue()' on a null object reference 
    at com.samo.facedatefb.MapsActivity.onMapReady(MapsActivity.java:268) 
    at com.google.android.gms.maps.SupportMapFragment$zza$1.zza(Unknown Source) 
    at com.google.android.gms.maps.internal.zzt$zza.onTransact(Unknown Source) 
    at android.os.Binder.transact(Binder.java:385) 
    at xz.a(:com.google.android.gms.DynamiteModulesB:82) 
    at maps.ad.u$5.run(Unknown Source) 
    at android.os.Handler.handleCallback(Handler.java:815) 
    at android.os.Handler.dispatchMessage(Handler.java:104) 
    at android.os.Looper.loop(Looper.java:194) 
    at android.app.ActivityThread.main(ActivityThread.java:5549) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at java.lang.reflect.Method.invoke(Method.java:372) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:964) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:759) 

我的火力数据库片段。

Answer 1

这里是你可以用它来获得你的纬度和经度

class Item { 
    private Double longitude; 
    private Double latitude; 

    public Item(Double longitude, Double latitude) { 
     this.latitude = latitude; 
     this.longitude = longitude; 
    } 

    public Double getLatitude() { 
     return latitude; 
    } 

    public Double getLongitude() { 
     return longitude; 
    } 
} 

您的活动代码

class MapsActivity extends FragmentActivity implements OnMapReadyCallback, LocationListener, View.OnClickListener { 

    //... 

    @Override 
    public void onMapReady(GoogleMap googleMap) { 
     mMap = googleMap; 

     if (ActivityCompat.checkSelfPermission(this, android.Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, android.Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) { 
      return; 
     } 

     mMap.setMyLocationEnabled(true); 

     LocationManager locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE); 

     Criteria criteria = new Criteria(); 

     String provider = locationManager.getBestProvider(criteria, true); 

     Location myLocation = locationManager.getLastKnownLocation(LocationManager.PASSIVE_PROVIDER); 
//  Location myLocation = locationManager.getLastKnownLocation(provider); 

     mMap.setMapType(GoogleMap.MAP_TYPE_NORMAL); 

//  double latitude = myLocation.getLatitude(); 
//  double longitude = myLocation.getLongitude(); 

     latitude = myLocation.getLatitude(); 
     longitude = myLocation.getLongitude(); 

     mDatabase = FirebaseDatabase.getInstance().getReference(); 

//  mDatabase.child("latitude").setValue(latitude); 
//  mDatabase.child("longitude").setValue(longitude); 

     LatLng latLng = new LatLng(latitude, longitude); 

     mMap.moveCamera(CameraUpdateFactory.newLatLng(latLng)); 
     mMap.animateCamera(CameraUpdateFactory.zoomTo(14)); 
     mMap.addMarker(new MarkerOptions().position(new LatLng(latitude, longitude)).title("You are here!").snippet("Consider yourself located")); 
//  mMap.addMarker(new MarkerOptions().position(new LatLng(48.7352022, 19.1187914)).title("You are here!").snippet("Consider yourself located")); 

     DatabaseReference mDatabase = FirebaseDatabase.getInstance().getReference(); 

     mDatabase.addValueEventListener(new ValueEventListener() { 
      @Override 
      public void onDataChange(DataSnapshot dataSnapshot) { 

       Item myItem = dataSnapshot.getValue(Item.class); 
       mMap.addMarker(new MarkerOptions().position(new LatLng(myItem.getLat(), myItem.getLng())).title("Buhaahaha")); 
     // ... 
      } 

      @Override 
      public void onCancelled(DatabaseError databaseError) { 
      }); 
    } 
} 

说明 建议使用模型类，包含一个默认构造函数，将会初始化它的字段，语句为Item myItem = dataSnapshot.getValue(Item.class);会自动调用构造函数并初始化相应的值。现在，该声明如何知道哪个值为Latitude，哪个值为Longitude。请注意，在我们的Item类中，我们已将我们的字段命名为与他们在Firebase数据库中显示的字段相同。这是非常重要的，他们应该是完全一样的名字。现在我们已经设置了对象的初始值和值，我们只需用getters就可以从数据库中获取值。
你的错误。
您写入的语句DatabaseReference myRef = database.getReference("longitude");的意思是“获取Firebase数据库基础参考，然后获取孩子的”经度“参考”。所以基本上它寻找longitude孩子，而不是获得longitude价值。