多形練習

package 多形;

abstract class Shape //這裡加abstract 是因為還沒打完完整的code 先給個雛形?
{
    public double x;
    public double y;
    abstract void area();
}

class Circle extends Shape
{
    private double r;

    public Circle(double x,double y , double r) //這種算副程式嗎
    {
        this.x=x;
        this.y=y;
        this.r=r;
    }

    public void area() //這個是副程式我知道
    {
        System.out.println("圓面積: "+3.1416*r*r);
    }
}

class Rectangle extends Shape
{
    private double x1;
    private double y1;

    public Rectangle(double x , double y , double x1 ,double y1)
    {
        this.x=x;
        this.y=y;
        this.x1=x1;
        this.y1=y1;
    }

    public void area()
    {
        System.out.println("長方形面積: "+(y1-y)*(x1-x));
    }

}

class Triangle extends Shape
{
    private double x1;
    private double y1;
    private double l;

    public Triangle(double x , double y , double x1 ,double y1,double l)
    {
        this.x=x;
        this.y=y;
        this.x1=x1;
        this.y1=y1;
        this.l=l;
    }

    public void area()
    {
        System.out.println("三角形面積: "+(y1-y)*l/2.0);
    }

}

public class 多形
{
    public static void main(String[] args)
    {
        Shape s;

        Circle c = new Circle(5.0 , 10.0 , 4.0);
    //這樣Circle不會誤判嗎? class circle 跟 class中的public Circle(double x,double y , double r)
        Rectangle r = new Rectangle(10.0 , 10.0 , 20.0 , 20.0);
        Triangle t = new Triangle(10.0 , 10.0 , 5.0 , 25.0 , 5.0);

        for(int i=1;i<=3;i++)
        {
            if(i==1) s=c;
            else if (i==2) s=r;
            else s=t;
            s.area();
        }
    }
}