I'm currently writing an app displaying a MapView showing a google map. i used the "hello map view" tutorial from http://developer.android.com/resources/tutorials/views/hello-mapview.html and this works so far: the map is displayed, centered and zoomed correctly, even the OverlayItems i added are displayed correctly.
But when i click on one of those overlay items i added, the application crashes with a null pointer exception:
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): java.lang.NullPointerException
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at com.android.internal.app.AlertController$AlertParams.<init>(AlertController.java:753)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at android.app.AlertDialog$Builder.<init>(AlertDialog.java:273)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at net.xenonite.wifiloc.act_map$MapItemizedOverlay.onTap(act_map.java:182)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at com.google.android.maps.ItemizedOverlay.onTap(ItemizedOverlay.java:453)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at com.google.android.maps.OverlayBundle.onTap(OverlayBundle.java:83)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at com.google.android.maps.MapView$1.onSingleTapUp(MapView.java:347)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at com.google.android.maps.GestureDetector.onTouchEvent(GestureDetector.java:533)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at com.google.android.maps.MapView.onTouchEvent(MapView.java:647)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at android.view.View.dispatchTouchEvent(View.java:3709)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:874)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:924)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:924)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:924)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:924)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:924)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:924)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1695)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1116)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at android.app.Activity.dispatchTouchEvent(Activity.java:2068)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1679)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at android.view.ViewRoot.handleMessage(ViewRoot.java:1708)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at android.os.Handler.dispatchMessage(Handler.java:99)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at android.os.Looper.loop(Looper.java:123)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at android.app.ActivityThread.main(ActivityThread.java:4595)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at java.lang.reflect.Method.invokeNative(Native Method)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at java.lang.reflect.Method.invoke(Method.java:521)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
07-23 16:24:48.167: ERROR/AndroidRuntime(2530): at dalvik.system.NativeStart.main(Native Method)
my code is mostly copied from the tutorial:
public class act_map extends MapActivity
{
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.map);
intent = getIntent();
lat = intent.getDoubleExtra("lat", 0.0);
lng = intent.getDoubleExtra("lng", 0.0);
MapView = (MapView) findViewById(R.id.act_map_mapview);
List<Overlay> mapOverlays = MapView.getOverlays();
Drawable positionMarker
= getResources().getDrawable(R.drawable.act_map_androidmarker);
MapItemizedOverlay itemizedOverlay
= new MapItemizedOverlay(positionMarker);
geoPoint = new GeoPoint((int) (lat * 1000000), (int) (lng * 1000000));
Pointer = new OverlayItem(geoPoint, "Thats me!", "wohooo");
itemizedOverlay.addOverlay(Pointer);
mapOverlays.add(itemizedOverlay);
MapView.setSatellite(false);
MapController = MapView.getController();
MapController.setCenter(geoPoint);
MapController.setZoom(18);
MapView.setBuiltInZoomControls(true);
MapView.displayZoomControls(true);
}
@Override
protected boolean isRouteDisplayed()
{
return false;
}
public boolean onKeyDown(int keyCode, KeyEvent event)
{
if (keyCode == KeyEvent.KEYCODE_I)
{
MapView.getController().setZoom(MapView.getZoomLevel() + 1);
return true;
}
else if (keyCode == KeyEvent.KEYCODE_O)
{
MapView.getController().setZoom(MapView.getZoomLevel() - 1);
return true;
}
else if (keyCode == KeyEvent.KEYCODE_S)
{
MapView.setSatellite(true);
return true;
}
else if (keyCode == KeyEvent.KEYCODE_M)
{
MapView.setSatellite(false);
return true;
}
return false;
}
public class MapItemizedOverlay extends ItemizedOverlay<OverlayItem>
{
private ArrayList<OverlayItem> mOverlays = new ArrayList<OverlayItem>();
private Context mContext;
public MapItemizedOverlay(Drawable defaultMarker)
{
super(boundCenterBottom(defaultMarker));
}
public MapItemizedOverlay(Drawable defaultMarker, Context context)
{
super(defaultMarker);
mContext = context;
}
public void addOverlay(OverlayItem overlay)
{
mOverlays.add(overlay);
populate();
}
@Override
protected OverlayItem createItem(int i)
{
return mOverlays.get(i);
}
@Override
public int size()
{
return mOverlays.size();
}
@Override
protected boolean onTap(int index)
{
OverlayItem item = mOverlays.get(index);
AlertDialog.Builder dialog = new AlertDialog.Builder(mContext);
dialog.setTitle(item.getTitle());
dialog.setMessage(item.getSnippet());
dialog.show();
return true;
}
}
How do I fix this?
mContext is null. Since your overlay is an inner class of the activity, replace mContext with act_map.this in your AlertDialog constructor call.
hasanzuav
I also had a problem with the Android MapView tutorial. When I clicked on the icon, the screen would crash (a null mContext would be passed):
The above fix did not work for me. Instead, I changed the following line of code from the MapActivity class from:
HelloItemizedOverlay itemizedoverlay = new HelloItemizedOverlay(drawable);
to this:
HelloItemizedOverlay itemizedoverlay = new HelloItemizedOverlay(drawable, this);
You also need to fix constructor by adding boundCenterBottom which is missing:
public HelloItemizedOverlay(Drawable defaultMarker, Context context) {
super(boundCenterBottom(defaultMarker));
mContext = context;
}
(Had beautiful screenshots but can't post because I am new :P)
Hope that helps others who are still having a problem with the tutorial and fixes their apps!
Use this instead, it is a far better tutorial than the one provided by google.
also call populate() in mapoverlay constructor
来源：https://stackoverflow.com/questions/3319158/googlemaps-mapview-crashes-ontap-in-android