Question

我已经按照一些在线教程向android中的google maps api添加标记。代码的结构与我的不同，但总的来说，我看到它们在onCreate方法中执行。下面我有一个非常基本的代码尝试在地图中间获取一个标记，但是，我得到一个空指针异常。有谁知道这个简单的解决方案？

这是错误的细节，下面是我的方法。我将地图声明为全局变量。

java.lang.NullPointerException：尝试调用虚拟方法'com.google.android.gms.maps.model.Marker com.google.android.gms.maps.GoogleMap.addMarker（com.google.android.gms。 maps.model.MarkerOptions）'对空对象引用

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    serviceManager = ServiceManager.getInstance(getActivity());
    userID = getString(R.string.mobile_health_client_user_id);
    client = MobileIOClient.getInstance(getContext(), userID);
    //client = MobileIOClient.getInstance(userID);
    map.addMarker(new MarkerOptions()
            .position(new LatLng(0, 0))
            .title("Hello world"));
}

我还浏览了覆盖onMapReady（）的google教程，但该方法对我来说也不起作用。我不知道如何让它在我的代码中工作，并且无法在线找到足够的资源来帮助我。任何帮助，将不胜感激。

谢谢

Answer 1

以下是我使用的代码：

public class Map extends AppCompatActivity implements OnMapReadyCallback, GoogleApiClient.ConnectionCallbacks,GoogleApiClient.OnConnectionFailedListener, LocationListener {

private GoogleMap mMap;
public GoogleApiClient mGoogleApiClient;
public static final String TAG = Map.class.getSimpleName();
public LocationRequest mLocationRequest;
double latitude;
double longitude;

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

    // 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);

    mGoogleApiClient = new GoogleApiClient.Builder(this)
            .addConnectionCallbacks(this)
            .addOnConnectionFailedListener(this)
            .addApi(LocationServices.API)
            .build();

    // Create the LocationRequest object
    mLocationRequest = LocationRequest.create()
            .setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY)
            .setInterval(10 * 1000)        // 10 seconds, in milliseconds
            .setFastestInterval(1 * 1000); // 1 second, in milliseconds

}


/**
 * 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. 
 * 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;


    LatLng here = new LatLng(latitude,longitude);
    mMap.addMarker(new MarkerOptions().position(here).title("Here!"));
    mMap.moveCamera(CameraUpdateFactory.newLatLng(here));
}

@Override
public void onConnected(@Nullable Bundle bundle) {
    Log.i(TAG, "Location services connected.");
    Location location = null;
    if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION)
            == PackageManager.PERMISSION_GRANTED) {
        location = LocationServices.FusedLocationApi.getLastLocation(mGoogleApiClient);
    }

    if (location == null) {
        LocationServices.FusedLocationApi.requestLocationUpdates(mGoogleApiClient, mLocationRequest, this);
    }
    else {
        handleNewLocation(location);
    };
}

@Override
public void onConnectionSuspended(int i) {
    Log.i(TAG, "Location services suspended. Please reconnect.");
}

@Override
public void onConnectionFailed(@NonNull ConnectionResult connectionResult) {

}

@Override
protected void onResume(){
    super.onResume();
    //setUpMapIfNeeded();
    mGoogleApiClient.connect();
}

@Override
protected void onPause() {
    super.onPause();
    if (mGoogleApiClient.isConnected()) {
        LocationServices.FusedLocationApi.removeLocationUpdates(mGoogleApiClient, this);
        mGoogleApiClient.disconnect();
    }
}

private void handleNewLocation(Location location) {
    Log.d(TAG, location.toString());
    latitude = location.getLatitude();
    longitude = location.getLongitude();
    LatLng latLng = new LatLng(latitude,longitude);

    MarkerOptions options = new MarkerOptions()
            .position(latLng)
            .title("I am here!");
    mMap.addMarker(options);
    mMap.moveCamera(CameraUpdateFactory.newLatLng(latLng));
}

@Override
public void onLocationChanged(Location location) {
    handleNewLocation(location);
    }    
}

Answer 2

您不应该在活动或片段的onCreate（）方法中与地图对象进行交互。简单的推理是地图可能尚未准备好。处理此问题的正确方法是实施OnMapReadyCallback界面，并在onMapReady(GoogleMap googleMap)函数的实现中添加标记。

如果您使用MapFragment与MapView，则您的解决方案需要略有不同，但总体思路保持不变。

示例：

public class MyActivity extends Activity implements OnMapReadyCallback { . . @Override public void onMapReady(GoogleMap googleMap) { map = googleMap; // Set your local instance of GoogleMap for future use map.addMarker(new MarkerOptions() .position(new LatLng(0, 0)) .title("Hello world")); } . . }

如果您使用的是MapView，则需要获取布局中的视图句柄，并明确调用map.getMapAsync(this)以附加onMapReady()侦听器。

我希望这有帮助！

Answer 3

只需在 onMapReady
googleMap.addMarker(new MarkerOptions().position(new LatLng(0, 0));

中撰写

添加标记到谷歌api

3 个答案: