我正在研究一个试图同时在mapView上显示多个不同叠加层的应用程序。不同叠加的原因需要是不同的颜色。我有一个允许滚动和缩放的工作地图，但是在添加多个叠加后，每当我尝试滚动时，地图现在都会给出一个空指针异常，并且缩放按钮已经消失。
这是我的地图活动：
package com.example.UVAMapsProject;
import java.util.ArrayList;
import java.util.List;
import com.example.UVAMapsProject.R;
import com.example.UVAMapsProject.R.drawable;
import com.example.UVAMapsProject.R.id;
import com.example.UVAMapsProject.R.layout;
import com.google.android.maps.GeoPoint;
import com.google.android.maps.MapActivity;
import com.google.android.maps.MapController;
import com.google.android.maps.MapView;
import com.google.android.maps.MyLocationOverlay;
import com.google.android.maps.Overlay;
import com.google.android.maps.OverlayItem;
import android.app.Activity;
import android.content.Context;
import android.graphics.drawable.Drawable;
import android.os.Bundle;
public class MapsActivity extends MapActivity {
/** Called when the activity is first created. */
Context mContext;
MapView mapView;
private MapController mapController;
List<Overlay> mapOverlays;
Drawable drawableStandard;
Drawable drawableComplete;
Drawable drawableNext;
HelloItemizedOverlay itemizedOverlayStandard;
HelloItemizedOverlay itemizedOverlayNext;
HelloItemizedOverlay itemizedOverlayComplete;
ArrayList<GeoPoint> overlayPoints = new ArrayList<GeoPoint>();
ArrayList<Destination> tourListTemp = SplashActivity.tourApp1.getTheTour().getTourList();
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.maps);
//get current center, create map at center
int lat = (int)NavigatorActivity.currentLocation.getLatitude();
int lon = (int)NavigatorActivity.currentLocation.getLongitude();
GeoPoint point = new GeoPoint((int)(lat*1e6), (int)(lon*1e6) );
mapView = (MapView) findViewById(R.id.mapview);
mapView.setBuiltInZoomControls(false);
mapController = mapView.getController();
mapController.setZoom(10);
mapController.setCenter(point);
// Initializes three different images and itemized overlays for each type of overlay
drawableStandard = this.getResources().getDrawable(R.drawable.androidmarker);
itemizedOverlayStandard = new HelloItemizedOverlay(drawableStandard, mapView);
drawableNext = this.getResources().getDrawable(R.drawable.androidmarkernext);
itemizedOverlayNext = new HelloItemizedOverlay(drawableNext, mapView);
drawableComplete = this.getResources().getDrawable(R.drawable.androidmarkercomplete);
itemizedOverlayComplete = new HelloItemizedOverlay(drawableStandard, mapView);
// get points of Destinations & make overlays
addOverlayPoints();
mapController.zoomToSpan(itemizedOverlayStandard.getLatSpanE6(), itemizedOverlayStandard.getLonSpanE6());
final MyLocationOverlay myPosition = new MyLocationOverlay(this,mapView);
mapOverlays = mapView.getOverlays();
mapOverlays.add(myPosition);
mapOverlays.add(itemizedOverlayStandard);
mapOverlays.add(itemizedOverlayNext);
mapOverlays.add(itemizedOverlayComplete);
myPosition.enableMyLocation();
}
public void addOverlayPoints(){
for (Destination d: tourListTemp) {
GeoPoint temp = new GeoPoint((int)(tourListTemp.get(tourListTemp.indexOf(d)).getLatitude()*1e6),
(int)(tourListTemp.get(tourListTemp.indexOf(d)).getLongitude()*1e6));
overlayPoints.add(temp);
OverlayItem overlayitem = new OverlayItem(temp, d.getName(), d.getInfo());
itemizedOverlayStandard.addOverlay(overlayitem);
}
itemizedOverlayNext.addOverlay(itemizedOverlayStandard.removeOverlay(tourListTemp.get(0)));
}
这是我的叠加课程：
package com.example.UVAMapsProject;
import java.util.ArrayList;
import android.app.AlertDialog;
import android.content.Context;
import android.graphics.drawable.Drawable;
import com.google.android.maps.ItemizedOverlay;
import com.google.android.maps.MapView;
import com.google.android.maps.OverlayItem;
public class HelloItemizedOverlay extends ItemizedOverlay {
private ArrayList<OverlayItem> mOverlays = new ArrayList<OverlayItem>();
Context mContext;
MapView mapView;
public HelloItemizedOverlay(Drawable defaultMarker) {
super(boundCenterBottom(defaultMarker));
// TODO Auto-generated constructor stub
}
public HelloItemizedOverlay(Drawable defaultMarker, MapView mv) {
super(boundCenterBottom(defaultMarker));
mContext = mv.getContext();
}
@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;
}
@Override
protected OverlayItem createItem(int i) {
return mOverlays.get(i);
}
public int getOverlayIndex(Destination d){
int out = SplashActivity.tourApp1.getTheTour().getTourList().indexOf(SplashActivity.tourApp1.getNextDestination());
return out;
}
public OverlayItem removeOverlay(Destination d) {
int index = getOverlayIndex(d);
OverlayItem temp = mOverlays.get(index);
mOverlays.remove(index);
return temp;
}
public void addOverlay(OverlayItem overlay) {
mOverlays.add(overlay);
populate();
}
@Override
public int size() {
return mOverlays.size();
}
}
以下是点击地图时收到的错误：
04-18 15:56:46.980: E/AndroidRuntime(3262): FATAL EXCEPTION: main
04-18 15:56:46.980: E/AndroidRuntime(3262): java.lang.NullPointerException
04-18 15:56:46.980: E/AndroidRuntime(3262): at com.google.android.maps.ItemizedOverlay.getItemsAtLocation(ItemizedOverlay.java:617)
04-18 15:56:46.980: E/AndroidRuntime(3262): at com.google.android.maps.ItemizedOverlay.getItemAtLocation(ItemizedOverlay.java:586)
04-18 15:56:46.980: E/AndroidRuntime(3262): at com.google.android.maps.ItemizedOverlay.handleMotionEvent(ItemizedOverlay.java:498)
04-18 15:56:46.980: E/AndroidRuntime(3262): at com.google.android.maps.ItemizedOverlay.onTouchEvent(ItemizedOverlay.java:572)
04-18 15:56:46.980: E/AndroidRuntime(3262): at com.google.android.maps.OverlayBundle.onTouchEvent(OverlayBundle.java:63)
04-18 15:56:46.980: E/AndroidRuntime(3262): at com.google.android.maps.MapView.onTouchEvent(MapView.java:643)
04-18 15:56:46.980: E/AndroidRuntime(3262): at android.view.View.dispatchTouchEvent(View.java:3766)
04-18 15:56:46.980: E/AndroidRuntime(3262): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:897)
04-18 15:56:46.980: E/AndroidRuntime(3262): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:863)
04-18 15:56:46.980: E/AndroidRuntime(3262): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:863)
04-18 15:56:46.980: E/AndroidRuntime(3262): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:863)
04-18 15:56:46.980: E/AndroidRuntime(3262): at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1671)
04-18 15:56:46.980: E/AndroidRuntime(3262): at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1107)
04-18 15:56:46.980: E/AndroidRuntime(3262): at android.app.Activity.dispatchTouchEvent(Activity.java:2086)
04-18 15:56:46.980: E/AndroidRuntime(3262): at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1655)
04-18 15:56:46.980: E/AndroidRuntime(3262): at android.view.ViewRoot.handleMessage(ViewRoot.java:1785)
04-18 15:56:46.980: E/AndroidRuntime(3262): at android.os.Handler.dispatchMessage(Handler.java:99)
04-18 15:56:46.980: E/AndroidRuntime(3262): at android.os.Looper.loop(Looper.java:123)
04-18 15:56:46.980: E/AndroidRuntime(3262): at android.app.ActivityThread.main(ActivityThread.java:4627)
04-18 15:56:46.980: E/AndroidRuntime(3262): at java.lang.reflect.Method.invokeNative(Native Method)
04-18 15:56:46.980: E/AndroidRuntime(3262): at java.lang.reflect.Method.invoke(Method.java:521)
04-18 15:56:46.980: E/AndroidRuntime(3262): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
04-18 15:56:46.980: E/AndroidRuntime(3262): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
04-18 15:56:46.980: E/AndroidRuntime(3262): at dalvik.system.NativeStart.main(Native Method)
答案 0 :(得分：0)
在HelloItemizedOverlay构造函数中，您需要调用populate()