大家好，我正在尝试在我的谷歌地图上实现一个覆盖层。然而，在运行时我收到了一个致命错误，似乎是某种NullPointerException。当我删除代码中的“overlayList = mapView.getOverlays();”和“overlayList.add(t);”这两行时，应用程序可以正常运行。非常感谢任何帮助！
主活动：
public class CSActivity extends MapActivity implements LocationListener {
MapController mapController;
MapView mapView;
LocationManager locationManager;
MyLocationOverlay myLocationOverlay;
MyLocationOverlay compass;
private Button click_but;
private Button exit_but;
long start;
long stop;
int x,y;
GeoPoint touchedPoint;
Drawable d;
List<Overlay> overlayList;
String towers;
int lat;
int lng;
public void onCreate(Bundle bundle) {
super.onCreate(bundle);
setContentView(R.layout.main); // bind the layout to the activity
click_but = new Button(this);
click_but = (Button)findViewById(R.id.clickBtn);
exit_but = new Button(this);
exit_but = (Button)findViewById(R.id.exitBtn);
d = getResources().getDrawable(R.drawable.markerblue);
Touchy t = new Touchy();
overlayList = mapView.getOverlays();
overlayList.add(t);
/*compass = new MyLocationOverlay(CSActivity.this, mapView);
overlayList.add(compass);*/
// Configure the Map
mapView = (MapView) findViewById(R.id.mapView);
mapView.setBuiltInZoomControls(true);
mapView.setSatellite(true);
mapController = mapView.getController();
mapController.setZoom(20); // Zoom 1 is world view
locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE);
myLocationOverlay = new MyLocationOverlay(this, mapView);
mapView.getOverlays().add(myLocationOverlay);
Criteria crit = new Criteria();
towers = locationManager.getBestProvider(crit, false);
Location L = locationManager.getLastKnownLocation(towers);
if (L != null){
lat = (int) (L.getLatitude()*1E6);
lng = (int) (L.getLongitude()*1E6);
GeoPoint ourLocation = new GeoPoint(lat, lng);
OverlayItem overlayItem = new OverlayItem(ourLocation, "What's up", "2nd String");
MyItemizedOverlay custom = new MyItemizedOverlay(d, CSActivity.this);
custom.insertPinpoint(overlayItem);
overlayList.add(custom);
}else{
Toast.makeText(CSActivity.this, "Couldn't get provider", Toast.LENGTH_SHORT).show();
}
final CSActivity home = this;
exit_but.setOnClickListener(new OnClickListener(){
@Override
public void onClick(View v)
{
Intent intent = new Intent();
intent.setClass(home, homeActivity.class);
startActivity(intent);
}
});
click_but.setOnClickListener(new OnClickListener(){
@Override
public void onClick(View v)
{
}
});
}
@Override
protected boolean isRouteDisplayed() {
return false;
}
@Override
protected void onResume() {
super.onResume();
myLocationOverlay.enableMyLocation();
myLocationOverlay.enableCompass();
locationManager.requestLocationUpdates(towers, 500, 1, this);
}
@Override
protected void onPause() {
super.onResume();
myLocationOverlay.disableMyLocation();
myLocationOverlay.disableCompass();
locationManager.removeUpdates(this);
}
class Touchy extends Overlay {
public boolean onTouchEvent(MotionEvent e, MapView m){
if (e.getAction() == MotionEvent.ACTION_DOWN){
start = e.getEventTime();
x = (int) e.getX();
y = (int) e.getY();
touchedPoint = mapView.getProjection().fromPixels(x, y);
}
if (e.getAction() == MotionEvent.ACTION_UP){
stop = e.getEventTime();
}
if (stop - start > 1000){
//Perform action
AlertDialog alert = new AlertDialog.Builder(CSActivity.this).create();
alert.setTitle("Pick an Option");
alert.setMessage("I said pick!");
alert.setButton("place a pinpoint", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
// TODO Auto-generated method stub
OverlayItem overlayItem = new OverlayItem(touchedPoint, "What's up", "2nd String");
MyItemizedOverlay custom = new MyItemizedOverlay(d, CSActivity.this);
custom.insertPinpoint(overlayItem);
overlayList.add(custom);
}
});
alert.setButton2("get address", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
// TODO Auto-generated method stub
Geocoder geocoder = new Geocoder(getBaseContext(), Locale.getDefault());
try{
List<Address> address = geocoder.getFromLocation(touchedPoint.getLatitudeE6() / 1E6, touchedPoint.getLongitudeE6() / 1E6, 1);
if (address.size() > 0){
String display = "";
for (int i = 0; i<address.get(0).getMaxAddressLineIndex(); i++){
display += address.get(0).getAddressLine(i) + "\n";
}
Toast t = Toast.makeText(getBaseContext(), display, Toast.LENGTH_LONG);
t.show();
}
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally{
}
}
});
alert.show();
return true;
}
return false;
}
}
@Override
public void onLocationChanged(Location location) {
// TODO Auto-generated method stub
lat = (int) (location.getLatitude() *1E6);
lng = (int) (location.getLongitude()*1E6);
}
@Override
public void onProviderDisabled(String provider) {
// TODO Auto-generated method stub
}
@Override
public void onProviderEnabled(String provider) {
// TODO Auto-generated method stub
}
@Override
public void onStatusChanged(String provider, int status, Bundle extras) {
// TODO Auto-generated method stub
}
}
项目化覆盖活动：
public class MyItemizedOverlay extends ItemizedOverlay<OverlayItem> {
private ArrayList<OverlayItem> pinpoints = new ArrayList<OverlayItem>();
private Context c;
public MyItemizedOverlay(Drawable defaultMarker) {
super(boundCenter(defaultMarker));
// TODO Auto-generated constructor stub
}
public MyItemizedOverlay(Drawable m, Context context) {
// TODO Auto-generated constructor stub
this(m);
c = context;
}
@Override
protected OverlayItem createItem(int i) {
// TODO Auto-generated method stub
return pinpoints.get(i);
}
@Override
public int size() {
// TODO Auto-generated method stub
return pinpoints.size();
}
public void insertPinpoint(OverlayItem item) {
pinpoints.add(item);
this.populate();
}
}
主XML：
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:gravity="center" >
<com.google.android.maps.MapView
android:id="@+id/mapView"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:layout_centerInParent="true"
android:apiKey="(key here)"
android:clickable="true"
android:enabled="true" />
<Button
android:id="@+id/clickBtn"
style="@style/ButtonText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_alignParentRight="true"
android:background="@drawable/blue"
android:text="Click" />
<Button
android:id="@+id/exitBtn"
style="@style/ButtonText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_alignParentLeft="true"
android:background="@drawable/red"
android:text="Home" />
</RelativeLayout>