为什么要定义两个一样的变量

previous是之前的没有运行的class Shape对象的状态，current是当前要准备运行的状态 。在没有运行时current = previous = s;

当运行if (growing) {

current += growthFactor;

} else {

current -= growthFactor;

}

if (current == size || current == 0) {

growing = !growing;

}

两个就不同了，设置current = previous = s;是为了初始化程序