WeakReference to bitmap null

Question

我正在使用自定义PaintSurface类：

public class PaintSurface extends SurfaceView
        implements SurfaceHolder.Callback {


    private Bitmap mapa;

    private WeakReference<Bitmap> timon;

    int rotacion=0;
    public static int parado=0;

    public PaintSurface(Context context) {
        this(context, null);
    }

    public PaintSurface(Context context, AttributeSet attrs) {
        super(context, attrs);
        getHolder().addCallback(this);

    }


    protected void onElMeuDraw(Canvas canvas,PointF nuevo,PointF viejo) {

        double angle1 =  calcRotationAngleInDegrees(viejo,nuevo);
        Log.d("myTag", "OnElmeuDraw llamado con angulo: "+ angle1 );
       float mCoordX = (float)viejo.x;
        float mCoordY = (float)viejo.y;

        Log.d("myTag2", "Voy a en paintsurface drawbitmap con timom:"+ timon.get().toString() + " width: "+timon.get().getWidth()+ "height: " + timon.get().getHeight()  );
        int mitadPantallaX = MainActivity.screenWidth/2;
        int mitadPantallaY = MainActivity.screenHeight/2;
        Rect corte =new Rect((int)mCoordX-mitadPantallaX,(int)mCoordY-mitadPantallaY,((int)mCoordX+mitadPantallaX),((int)mCoordY+mitadPantallaY));
        Rect pantalla = new Rect(0,0,MainActivity.screenWidth,MainActivity.screenHeight);
        canvas.drawBitmap(mapa,corte,pantalla,null);
        Bitmap plane = getResizedBitmap(BitmapFactory.decodeResource(getContext().getResources(),R.drawable.plane),200,200);
        plane = RotateBitmap(plane,(float)angle1);
        canvas.drawBitmap(plane,mitadPantallaX-plane.getWidth()/2,mitadPantallaY-plane.getHeight()/2,null);
        rotacion=rotacion +3;

       if(parado ==0 ){

           RotateBitmap(timon.get(), rotacion);
       }

        canvas.drawBitmap(timon.get(), ((float) (mitadPantallaX - timon.get().getWidth()/2)) - 250, ((float) (mitadPantallaY -timon.get().getHeight()/2)) , null);
    }

    @Override
    public void surfaceCreated(SurfaceHolder surfaceHolder) {

        timon =    new WeakReference<Bitmap>(BitmapFactory.decodeResource(getContext().getResources(), R.drawable.timo));
        Log.d("myTag", "paintSurface surface created acabo de asignar timon" + timon.toString() + "con timog.get() " + timon.get().toString());
    }

    public static Bitmap RotateBitmap(Bitmap source, float angle)
    {
        Matrix matrix = new Matrix();
        matrix.postRotate(angle);
        return Bitmap.createBitmap(source, 0, 0, source.getWidth(), source.getHeight(), matrix, true);
    }

    public static double calcRotationAngleInDegrees(PointF centerPt, PointF targetPt) {

        double theta = Math.atan2(targetPt.y - centerPt.y, targetPt.x - centerPt.x);

        theta += Math.PI / 2.0;

        double angle = Math.toDegrees(theta);

        if (angle < 0) {
            angle += 360;
        }

        return angle;
    }
}

调用surfaceCreated后，我将新WeakReference分配给本地变量，下一个log为non-null和timon.toString()字符串提供timon.get().toString()字符串{1}}：

  

paintSurface表面创建了acabo de asignar   timonjava.lang.ref.WeakReference@8f80da0con timog.get（）   android.graphics.Bitmap@f84bd59

但是当onElMeuDraw被调用时，logs会产生错误：

  

W / System.err：java.lang.NullPointerException：尝试在空对象引用上调用虚方法'java.lang.String java.lang.Object.toString（）'   04-13 03：34：21.727 16639-16967 / com.example.ivan.prac2 W / System.err：at com.example.ivan.prac2.PaintSurface.onElMeuDraw（PaintSurface.java:65）   04-13 03：34：21.728 16639-16967 / com.example.ivan.prac2 W / System.err：at com.example.ivan.prac2.FilPaint.run（FilPaint.java:75）

我不明白局部变量如何获得null值以及何时发生这种情况......