OverlayItem's werden nicht dargestellt

package de.unicate.android.maptest;

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.OverlayItem;

public class MyOverlayCollection extends ItemizedOverlay<OverlayItem> {

    private ArrayList<OverlayItem> _overlays = new ArrayList<OverlayItem>();
    private Context _context;

    public MyOverlayCollection(Drawable defaultMarker, Context context) {
        super(defaultMarker);
        _context = context;
    }

    @Override
    protected OverlayItem createItem(int i) {
        return _overlays.get(i);
    }

    @Override
    public int size() {
        return _overlays.size();
    }

    public void addOverlay(OverlayItem item) {
        _overlays.add(item);
        populate();
    }

    @Override
    protected boolean onTap(int index) {
        OverlayItem item = _overlays.get(index);
        AlertDialog.Builder dialog = new AlertDialog.Builder(_context);
        dialog.setTitle(item.getTitle());
        dialog.setMessage(item.getSnippet());
        dialog.show();
        return true;
    }
    
}

package de.unicate.android.maptest;

import java.util.List;

import android.os.Bundle;

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.Overlay;
import com.google.android.maps.OverlayItem;

public class MapTestActivity extends MapActivity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        MapView mapView = (MapView) findViewById(R.id.mapview);
        mapView.setBuiltInZoomControls(true);
        GeoPoint geoPoint = new GeoPoint(51081597,13748922);
        mapView.setSatellite(false);
        MapController myMC = mapView.getController();
        myMC.setCenter(geoPoint);
        myMC.setZoom(10);

        
        List<Overlay> items = mapView.getOverlays();
        MyOverlayCollection stations = new MyOverlayCollection(getResources().getDrawable(R.drawable.androidmarker), this);
        OverlayItem overlayitem = new OverlayItem(geoPoint, "Hallo, Dresden!", "Hier ist Dresden");
        stations.addOverlay(overlayitem);        
        items.add(stations);        
    }
    @Override
    protected boolean isRouteDisplayed() {
        return false;
    }
}