Draw a transparent circle onto a filled android canvas

Try this:

public class TransparentCircle extends View {

    Bitmap bm;
    Canvas cv;
    Paint eraser;

    public TransparentCircle(Context context) {
        super(context);
        Init();
    }

    public TransparentCircle(Context context, AttributeSet attrs) {
        super(context, attrs);
        Init();
    }

    public TransparentCircle(Context context, AttributeSet attrs,
                             int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        Init();
    }

    private void Init(){

        eraser = new Paint();
        eraser.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
        eraser.setAntiAlias(true);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {

        if (w != oldw || h != oldh) {
            bm = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
            cv = new Canvas(bm);