抽象类Shape及其子类

定义抽象类Shape，圆形Circle、长方形Square、Rectangle为子类。

Shape类有一个数据成员（double型），定义带参构造方法、抽象方法calArea()(返回值类型：double)在Circle、Square两个类中，实现calArea()方法。在Rectangle类中，增加一个数据成员（double型），实现calArea()方法。编写测试类：定义一个Shape类引用shape，分别指向一个Circle类、Square类、Rectangle类对象，利用多态，计算各平面图形的面积并输出。（结果保留三位小数）要求：1.数据成员全部为私有成员；2.数据从键盘输入，调用带参构造方法创建子类对象。提示：使用Math类的PI

import java.util.Scanner;
public class ShapeDemo {
     
	public static void main(String[] args) {
     
		Scanner in=new Scanner(System.in);
		double a,b,c,d;
		a=in.nextDouble();
		b=in.nextDouble();
		c=in.nextDouble();
		d=in.nextDouble();
		Shape sh=new Circle(a);
		System.out.printf("圆的面积：%.3f",sh.calArea());
		System.out.println();
		sh=new Square(b);
		System.out.printf("正方形的面积：%.3f",sh.calArea());
		System.out.println();
		sh=new Rectangle(c,d);
		System.out.printf("矩形的面积：%.3f",sh.calArea());
		in.close();
	}

}
abstract class Shape
{
     
	private double a;
	public Shape()
	{
     
		a=0;
	}
	public Shape(double a)
	{
     
		this.a=a;
	}
	abstract double calArea();
	public double getA() {
     
		return a;
	}
	public void setA(double a) {
     
		this.a = a;
	}
}
class Circle extends Shape
{
     
	public Circle(double a)
	{
     
		super(a);
	}
	double calArea() {
     
		return getA()*getA()*Math.PI;
	}
	
}
class Square extends Shape
{
     
	public Square(double a)
	{
     
		super(a);
	}
	double calArea() {
     
		return getA()*getA();
	}
	
}
class Rectangle extends Shape
{
     
	private double b;
	public Rectangle(double a,double b)
	{
     
		super(a);
		this.b=b;
	}
	double calArea() {
     
		return getA()*b;
	}
	
}