java - 在 Android 中使用空指针异常强制关闭

Question

我有下面的代码，我首先尝试在模拟器上进行测试，以确保它工作正常，然后我可以开始在真实设备上进行测试。

下面的代码在 Android 屏幕的上半部分和 TextView 的下半部分创建了一个 Google Map。据我所知，无论何时启动具有 Google Map 的应用程序，都需要从 DDMS 的角度传递纬度和经度坐标。

但在我的例子中，我没有传递任何位置坐标，下面的程序在这一行抛出 NULL POINTER EXCEPTION-

mScreenPoints = mapView.getProjection().toPixels(pointToDraw, mScreenPoints);

我不确定为什么会这样，据我所知，Google Map 应该首先加载，然后它应该等待从 DDMS 角度传递位置坐标，但尽快当我启动我的应用程序时，我得到了 NPE 的force closed。

知道为什么会这样吗？

下面是完整的代码-

private MapView mapView;
private ListView listView;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mapView = (MapView) findViewById(R.id.mapView);
    listView = (ListView) findViewById(R.id.mylist);

    locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE);    

    locationListener = new GPSLocationListener(mapView);

    locationManager.requestLocationUpdates(
            LocationManager.GPS_PROVIDER, 
            35000, 
            0, 
            locationListener);


    mapView.setStreetView(true);
    mapView.setBuiltInZoomControls(true);
    mapController = mapView.getController();
    mapController.setZoom(15);
}

位置更新类-

    private class GPSLocationListener implements LocationListener {

    MapOverlay mapOverlay;

    public GPSLocationListener(MapView mapView) {
        mapOverlay = new MapOverlay(this,android.R.drawable.star_on);
        List<Overlay> listOfOverlays = mapView.getOverlays();
        listOfOverlays.add(mapOverlay);
    }

    @Override
    public void onLocationChanged(Location location) {
        if (location != null) {
            GeoPoint point = new GeoPoint(
                    (int) (location.getLatitude() * 1E6), 
                    (int) (location.getLongitude() * 1E6));

            mapController.animateTo(point);
            mapController.setZoom(15);

            mapOverlay.setPointToDraw(point);
            mapView.invalidate();
        }
    }

    @Override
    public void onProviderDisabled(String provider) {
    }

    @Override
    public void onProviderEnabled(String provider) {
    }

    @Override
    public void onStatusChanged(String provider, int status, Bundle extras) {
    }
}

下面是在当前位置的 map 上绘制一个圆的类，NPE 只发生在这个类中-

    class MapOverlay extends Overlay {
    private GeoPoint pointToDraw;
    int[] imageNames=new int[6];
    private Point mScreenPoints;
    private Bitmap mBitmap;
    private Paint mCirclePaint;


    public MapOverlay(GPSLocationListener gpsLocationListener, int currentUser) {
        imageNames[0]=currentUser;
        mCirclePaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        mCirclePaint.setColor(0x30000000);
        mCirclePaint.setStyle(Style.FILL_AND_STROKE);
        mBitmap = BitmapFactory.decodeResource(getResources(),imageNames[0]);
        mScreenPoints = new Point();
    }

    public void setPointToDraw(GeoPoint point) {
        pointToDraw = point;
    }

    public GeoPoint getPointToDraw() {
        return pointToDraw;
    }

    public boolean draw(Canvas canvas, MapView mapView, boolean shadow, long when) {
        super.draw(canvas, mapView, shadow);

                   // NPE happening here
        mScreenPoints = mapView.getProjection().toPixels(pointToDraw, mScreenPoints);

        int totalCircle=4;
        int radius=40;
        int centerimagesize=35;

        for (int i = 1; i <= totalCircle; i ++) { 
            canvas.drawCircle(mScreenPoints.x,mScreenPoints.y, i*radius, mCirclePaint); 
        } 

        canvas.drawBitmap(mBitmap, (mScreenPoints.x-(centerimagesize/2)),(mScreenPoints.y-(centerimagesize/2)), null);
        super.draw(canvas,mapView,shadow);

        return true;
    }
}

更新:-

我发现了问题，我猜当我将叠加层添加到列表中时，它总是会立即开始绘制。我是否得到一个位置并不重要。 我怎样才能有一种安全有效的方式来添加叠加层，直到有一个位置可以设置。？

Answer 1

安全方法:

public boolean draw(Canvas canvas, MapView mapView, boolean shadow, long when) {
    if(pointToDraw == null){ // it isn't found the location yet.
         return super.draw(canvas, mapView, shadow); // do the default
    }        

   // else:
    super.draw(canvas, mapView, shadow);

                   // NPE happening here
        mScreenPoints = mapView.getProjection().toPixels(pointToDraw, mScreenPoints);

        int totalCircle=4;
        int radius=40;
        int centerimagesize=35;

        for (int i = 1; i <= totalCircle; i ++) { 
            canvas.drawCircle(mScreenPoints.x,mScreenPoints.y, i*radius, mCirclePaint); 
        } 

        canvas.drawBitmap(mBitmap, (mScreenPoints.x-(centerimagesize/2)),(mScreenPoints.y-(centerimagesize/2)), null);
        super.draw(canvas,mapView,shadow);

        return true;
    }