java编程。类。 常见平面图形（如三角形、圆、矩形和正方形等）的面积。利用抽象类，编写程序实现该

abstract class Shape{ abstract double area();}class Circle extends Shape { private double r; private final double PI=3.14; public Circle(double r) { this.r = r; } @Override double area() { return PI*r*r; }} class Rectangle extends Shape { private double