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Question

如何正确保护GoogleMap免于空？

目前在drawOnMap()方法中说明了mMap null，但在将其添加到onMapReady()方法时有效。

我想知道，如果我在drawOnMap()内onMapReady()请求public class DetailActivity extends AppCompatActivity implements OnMapReadyCallback { private GoogleMap mMap; private String locationA; private String locationB; private ArrayList<LatLng> latLngList; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_detail); // Check if Google Maps have been activated, else starting it again. if (mMap == null) { SupportMapFragment smf = (SupportMapFragment) this.getSupportFragmentManager().findFragmentById(R.id.map); smf.getMapAsync(this); } // Settings up the views TextView mDistance = (TextView) findViewById(R.id.distanceDetail); TextView mDuration = (TextView) findViewById(R.id.durationDetail); TextView mLocationA = (TextView) findViewById(R.id.locationA); TextView mLocationB = (TextView) findViewById(R.id.locationB); // Getting the values Intent i = getIntent(); locationA = i.getStringExtra("locA"); locationB = i.getStringExtra("locB"); String distance = i.getStringExtra("distance"); String duration = i.getStringExtra("duration"); // initiate the values. mDistance.setText(distance); mDuration.setText(duration); mLocationA.setText(locationA); mLocationB.setText(locationB); drawOnMap(); } public void drawOnMap(){ latLngList = getIntent().getParcelableArrayListExtra("points"); // Används för att rita ut marksen på kartan sedan, start och stop. // Get the first and last postion to write out the marks on map LatLng startLatLng = latLngList.get(0); LatLng endLatLng = latLngList.get(latLngList.size() - 1); // Zooms into the start position; mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(startLatLng, 10)); // Adding mark to start location mMap.addMarker(new MarkerOptions() .position(startLatLng) .icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_GREEN)) .title(locationA)); // Adding mark to end location mMap.addMarker(new MarkerOptions() .position(endLatLng) .icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_RED)) .title(locationB)); if (latLngList.size() > 0) { // Draw the route on map mMap.addPolyline(new PolylineOptions() .addAll(latLngList) .color(Color.argb(255, 55, 160, 255)) .width(20)); } } @Override public void onMapReady(GoogleMap googleMap) { mMap = googleMap; UiSettings mapSettings = mMap.getUiSettings(); mapSettings.setZoomControlsEnabled(true); } 之前，我可能会在从我的意图中获取值之前启动地图吗？

  

mMap.moveCamera（CameraUpdateFactory.newLatLngZoom（startLatLng，   10））;

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目录下载

  

java.lang.NullPointerException：尝试调用虚方法'void   com.google.android.gms.maps.GoogleMap.moveCamera（com.google.android.gms.maps.CameraUpdate）”   在null对象引用上                                                                                          在   android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2416）                                                                                          在   android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2476）                                                                                          在android.app.ActivityThread.-wrap11（ActivityThread.java）                                                                                          在   android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1344）                                                                                          在android.os.Handler.dispatchMessage（Handler.java:102）                                                                                          在android.os.Looper.loop（Looper.java:148）                                                                                          在android.app.ActivityThread.main（ActivityThread.java:5417）                                                                                          at java.lang.reflect.Method.invoke（Native Method）                                                                                          在   com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:726）                                                                                          在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:616）                                                                                       引起：java.lang.NullPointerException：尝试调用虚拟   方法'无效   com.google.android.gms.maps.GoogleMap.moveCamera（com.google.android.gms.maps.CameraUpdate）”   在null对象引用上                                                                                          在   com.example.rasmusjosefsson.rjcar.DetailActivity.drawOnMap（DetailActivity.java:113）                                                                                          在   com.example.rasmusjosefsson.rjcar.DetailActivity.onCreate（DetailActivity.java:96）                                                                                          在android.app.Activity.performCreate（Activity.java:6237）                                                                                          在   android.app.Instrumentation.callActivityOnCreate（Instrumentation.java:1107）                                                                                          在   android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2369）                                                                                          在   android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2476）                                                                                          在android.app.ActivityThread.-wrap11（ActivityThread.java）                                                                                          在   android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1344）                                                                                          在android.os.Handler.dispatchMessage（Handler.java:102）                                                                                          在android.os.Looper.loop（Looper.java:148）                                                                                          在android.app.ActivityThread.main（ActivityThread.java:5417）                                                                                          at java.lang.reflect.Method.invoke（Native Method）                                                                                          在   com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:726）                                                                                          在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:616）

Answer 1

在地图准备好后调用drawOnMap方法。

@Override
public void onMapReady(GoogleMap googleMap) {
    mMap = googleMap;
    UiSettings mapSettings = mMap.getUiSettings();
    mapSettings.setZoomControlsEnabled(true);

    drawOnMap();
}

Answer 2

您将使用onMapReady()方法获取Google地图对象，以便从此处调用drawOnMap()

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

   // Call drawOnMap() from here
}