9.20 图形结构

定义类Shape，用来表示一般二维图形。Shape具有抽象方法area()和perimeter()，分别用来计算形状的面积和周长。
试定义一些二维形状类（如矩形、三角形、圆形、椭圆形等），这些类均为Shape类的子类。

abstract class AbstractShape {
	public abstract double area() ;// 求面积
	public abstract double perimeter() ;// 求周长
}

class Circular extends AbstractShape {
	private double radius ;
	public Circular(double radius) {
		this.radius = radius ;
	}
	public double area() {
		return 3.1415926 * this.radius * this.radius ;
	}
	public double perimeter() {
		return 2 * 3.1415926 * this.radius ;
	}
}

class Rectangle extends AbstractShape {
	private double length ;
	private double width ;
	public Rectangle(double length,double width) {
		this.length = length ;
		this.width = width ;
	}
	public double area() {
		return this.length * this.width ;
	}
	public double perimeter() {
		return 2 * (this.length + this.width) ;
	}
}

class Factory {
	public static AbstractShape getInstance(String className,double ... args) {
		if ("Circular".equalsIgnoreCase(className)) {
			return new Circular(args[0]) ;
		} else if ("Rectangle".equalsIgnoreCase(className)) {
			return new Rectangle(args[0],args[1]) ;
		} else {
			return null ;
		}
	}
}

public class HelloWorld {
	public static void main(String args[]) {
		AbstractShape asa = Factory.getInstance("Circular",1.1) ;
		AbstractShape asb = Factory.getInstance("Rectangle",1.5,10.2) ;
		System.out.println("圆形面积：" + asa.area() + "、圆形周长：" + asa.perimeter()) ;
		System.out.println("矩形面积：" + asb.area() + "、矩形周长：" + asb.perimeter()) ;
	}
}

• 注意：AbstractShape 在每个出现位置的含义!!