由于空对象引用，应用程序崩溃

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。

但是当我运行应用程序时，应用程序崩溃并且在 logcat 中说：

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getUserData()在任何地方调用方法。我读到您也尝试添加该逻辑onCreate()，但是没有用。添加侦听器并不意味着您会立即获得预期的结果。仅使用该onMapReady()方法设置地图，设置不依赖于User. 仅在检测到User数据更改时添加/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();        }    });}@Overridepublic void onMapReady(GoogleMap googleMap) {    mMap = googleMap;}请调用该方法getUserData()或将其代码移入onCreate().