绘制Canvas的使用二

public class MainActivity extends Activity {/** Called when the activity is first created. */@Overridepublic void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    Panel v = new Panel(this);    setContentView(v);}class Panel extends View{ float x;float y;    public Panel(Context context)     {          super(context);     }        @Override    public void onDraw(Canvas canvas){      super.onDraw(canvas);        Paint paint = new Paint();        paint.setColor(Color.BLUE);                /* 注意这边是大学的Color，而不是color */    canvas.drawColor(Color.BLACK);      canvas.drawCircle(x, y, 50, paint);        }  @Overridepublic boolean onTouchEvent (MotionEvent event) {x = event.getX();y = event.getY();/* invalidate是为了让程序重新调用onDraw进行绘制 */    invalidate();            return true;}    }     }