Question

调用（user.getlat）和（user.getlng）时出现问题。我已经全局声明了用户对象，并在getUserData（）方法中对其进行了初始化。

但是当我运行该应用程序时，该应用程序崩溃了，并且在logcat中显示：

java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法'double com.example.profile.User.getLat（）'

public class Profile extends FragmentActivity implements OnMapReadyCallback {

private GoogleMap mMap;
private LocationManager locationManager;
private LocationListener locationListener;

private FirebaseAuth mAuth;
DatabaseReference database = FirebaseDatabase.getInstance().getReference();
DatabaseReference myRef = database.child("users");
User user;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_profile);

    SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
            .findFragmentById(R.id.mapFragment);
    mapFragment.getMapAsync(this);

    mAuth = FirebaseAuth.getInstance();
}

public void getUserData(){
    String uID = mAuth.getCurrentUser().getUid();
    myRef.child(uID).addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            dataSnapshot.getChildren();
            user = dataSnapshot.getValue(User.class);

            Toast.makeText(Profile.this, "" + user.lng, Toast.LENGTH_LONG).show(); // Prints the value normally!

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Toast.makeText(Profile.this, "Failed to read from database!", Toast.LENGTH_LONG).show();
        }
    });
}

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

    LatLng userL = new LatLng(user.getLat(),user.getLng()); // The problem occurs here! 
    mMap.clear();
    mMap.addMarker(new MarkerOptions().position(userL).title("Your Location"));
    mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(userL,17));

}

}

Answer 1

首先，您没有在任何地方调用getUserData()方法。我了解到您也尝试在onCreate()中添加该逻辑，但这没有用。

添加侦听器并不意味着您可以立即获得期望的效果。

仅使用onMapReady()方法设置地图，其设置不依赖于User。仅当您检测到User数据中的更改时，才添加/修改地图标记。查看更改：

public void getUserData(){
    String uID = mAuth.getCurrentUser().getUid();
    myRef.child(uID).addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            dataSnapshot.getChildren();
            user = dataSnapshot.getValue(User.class);
            LatLng userL = new LatLng(user.getLat(),user.getLng());
            if(mMap!=null){
               mMap.clear();
               mMap.addMarker(new MarkerOptions().position(userL).title("Your 
               Location"));
             mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(userL,17));
            }
            Toast.makeText(Profile.this, "" + user.lng, Toast.LENGTH_LONG).show(); // Prints the value normally!

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Toast.makeText(Profile.this, "Failed to read from database!", Toast.LENGTH_LONG).show();
        }
    });
}

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

请调用方法getUserData()或将其代码移至onCreate()。

应用程式因空物件参照而当机

1 个答案: