java - 应用程序因空对象引用而崩溃

Question

我在调用 (user.getlat) 和 (user.getlng) 时遇到问题。我已经全局声明了用户对象，并在 getUserData() 方法中初始化了它。

但是当我运行应用程序时，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。我读到您也尝试在 onCreate() 中添加该逻辑，但它不起作用。

添加监听器并不意味着您会立即获得期望的结果。

仅使用onMapReady()方法来设置 map ，其设置不依赖于User。仅当您检测到用户数据发生更改时才添加/修改 map 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()中。