我是让一个小圆球从左上角移动到右下角，就出现了这样的情况，小球经过的地方有阴影。
代码：
public class TestThread extends Thread{

        boolean flag = true;
        TestDemo testDemo;
        SurfaceHolder surfaceHolder;
        public TestThread(TestDemo testDemo,SurfaceHolder surfaceHolder){
                this.testDemo = testDemo;
                this.surfaceHolder = surfaceHolder;
        }
        @Override
        public void run(){
                while(flag){
                        Canvas c = null;
                        try {
                                c = surfaceHolder.lockCanvas();
                                synchronized (surfaceHolder) {
                                        testDemo.onDraw(c);
                                }
                        } finally {
                                if(c!=null){
                                        surfaceHolder.unlockCanvasAndPost(c);
                                }
                        }
                        try {
                                Thread.sleep(200);
                        } catch (Exception e) {
                                // TODO: handle exception
                        }
                }
                
        }
}
SurfaceView：
public class TestDemo extends SurfaceView implements SurfaceHolder.Callback{

        Bitmap bg;
        Bitmap bullet;
        SurfaceHolder holder;
        Paint paint;
        float x = 0;
        float y = 0;
        TestThread testThread;
        public TestDemo(Context context) {
                super(context);
                holder = this.getHolder();
                holder.addCallback(this);
        }

        public void onDraw(Canvas c){
                c.drawBitmap(bg, 0, 0, paint);
                c.drawBitmap(bullet, x+=10, y+=10, paint);
        }
        @Override
        public void surfaceCreated(SurfaceHolder holder) {
                // TODO Auto-generated method stub
                
        }

        @Override
        public void surfaceChanged(SurfaceHolder holder, int format, int width,
                        int height) {
                paint = new Paint();
                testThread = new TestThread(this,holder);
                bg = BitmapFactory.decodeResource( getResources(), R.drawable.bg );
                bullet = BitmapFactory.decodeResource( getResources(), R.drawable.bullet);
                testThread.start();
        }

        @Override
        public void surfaceDestroyed(SurfaceHolder holder) {
                // TODO Auto-generated method stub
                
        } 

}

还请大神指点