Java中的抽象类，为什么我的程序输出的结果都是0.0？

abstract class Shape{
abstract double ShowArea();
}
class Rectangle extends Shape{
double Area;
double i,j;
Rectangle(double x,double y){
i=x;
j=y;
}
double ShowArea(){
Area=i*j;
return Area;
}
}
class Square extends Shape{
double Area;
double i;
Square(double x){
i=x;
}
double ShowArea(){
Area=i*i;
return Area;
}
}
class Circle extends Shape{
double Area;
double i;
Circle(double x){
i=x;
}
double ShowArea(){
Area=i*i;
return Area;
}
}
public class Proprotion{
public static void main(String args[]){
Rectangle R=new Rectangle(2.1,3.4);
Square S=new Square(3.4);
Circle C=new Circle(6.7);
System.out.println("矩形的面积为："+R.Area);
System.out.println("正方形的面积为："+S.Area);
System.out.println("圆的面积为："+C.Area);
}
}