定义抽象类Shape，抽象方法为showArea()，求出面积并显示，定义矩形类Rectangle,正方形类Square,圆类 Circle，根据各自的属性，用showArea方法求出各自的面积，在main方法中构造3个对象，调用showArea方法-Define abstract class Shape, abstract methods showArea (), find the area and showed that define the rectangle class Rectangle, square type Square, round classes Circle, according to their properties, obtained by the respective area showArea method, constructed in the main method three objects, method calls showArea