Java例题

面向抽象编程

public abstract class Geometry  {

   public abstract double getArea(); 

}

public class Pillar{

 Geometry bottom; //bottom是抽象类Geometry声明变量

 double height;

 public Pillar(Geometry bottom,double height) {

	this.bottom=bottom;

	this.height=height;

}

 public double getVolume() {

	 if(bottom==null){

		 System.out.println("没有底，无法计算体积");

		 return -1;

	 }

	return bottom.getArea()*height;  //bottom可以调用子类重写的getArea方法

}

}

 

public class Circle extends Geometry{

   double r;

    Circle(double r) {

		this.r=r;

	}

	public double getArea() {

		return (3.14*r*r);

	}

}

 

public class Rectangle extends Geometry {

    double a,b;

    public Rectangle(double a,double b) {

	 this.a=a;

	 this.b=b;

    }

    public double getArea(){

    	return a*b;

    }

}

 

public class Application {

 

	public static void main(String[] args) {

        Pillar pillar;

        Geometry bottom=null;

        pillar=new Pillar(bottom,10);  //null底的柱体

        System.out.println("体积"+pillar.getVolume());

        bottom=new Rectangle(12,22);

        pillar =new Pillar(bottom, 58);  //pillar三具有矩形底的柱体

        System.out.println("体积"+pillar.getVolume());

        bottom =new Circle(10);

        pillar=new Pillar(bottom, 58);  //pillar是具有圆形底的柱体

        System.out.println("体积"+pillar.getVolume());

	}

 

}