说明：  一个多线程Applet，实现简单的动画，具体实现：自动出现由小到大变换的圆，位置和颜色随机，变到直径为150时擦除，重新再出现圆；变换速度可以由控制sleep时间的方式来完成。
(a multithreaded Applet achieve simple animation, concrete realization : automatically transform the small to large, round random location and color, change to the diameter of 150 erasure, re-emerged circle; changing the speed can be controlled from the sleep time to complete.)