Question

我正在从Firebase数据库中读取一些数据，并且返回的对象为null。

我尝试了很多方法。当我有（像现在的代码）

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

在onDataChange方法之外发生错误。但是当我在onDataChange方法中放置标记并设置两者时：

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

和

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

如果有类似的getRefrences则没有错误但是如果我尝试

 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("samo@gmailcom - 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) {



    }
}

并且有错误消息： -

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firebase数据库代码段。

Answer 1

以下是可用于获取纬度和经度的代码

   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

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) {
            });
    } 
}

<强>解释建议使用模型类，其中包含将初始化其字段的默认构造函数，语句Item myItem = dataSnapshot.getValue(Item.class);将自动调用构造函数并初始化相应的值。现在，此语句如何知道Latitude的哪个值以及Longitude是哪个值。请注意，在我们的Item课程中，我们将字段命名为与Firebase数据库中显示的字段相同。这非常重要，它们的名称应该完全相同。现在我们已经设置了对象初始化和值，我们只需使用getter从数据库中获取值 你的错误。
您撰写的声明DatabaseReference myRef = database.getReference("longitude");表示＆＃34;获取Firebase数据库基准参考，然后获取孩子的经度＆＃39;参考＆＃34 ;.所以基本上它正在寻找longitude孩子，而不是获得longitude值。

FireBase数据库null对象

1 个答案: