java上机。。。抽象类。

public class abstract_test {    public static void main(String[] args) {        Square s = new Square();        s.area();        Cricle c = new Cricle();        c.PI=4;        c.area();    }}abstract class Shape{    public double PI;     void area(){}    public double getPI() {        return PI;    }    public void setPI(double pI) {        PI = pI;    }}class Cricle extends Shape{    int r=4;    void area(){        System.out.println( PI*r*r);    }}class Square extends Shape{     int a = 2, b = 4;     void area(){         System.out.println(a*b);     }}