實驗四 （1）：定義一個形狀類（Shape）方法：計算周長，計算面積

（1）定義一個形狀類（Shape）方法：計算周長，計算面積
子類：
矩形類（Rectangle） ：額外的方法：differ（） 計算長寬差
圓形類（Circle）
三角形類（Triangle）
正方形類（Square） 矩形的子類
生成幾個不同的形狀對象，放在一個Shape類型的數組里，分別求每個形狀的周長和面積。如果形狀對象是一個矩形，且不是正方形，則計算長寬差。

package com.tiger.practice;

 class Shape {
    
    public double length(){
        return 0;
    }
    
    public double area() {
        return 0;
    }
}
 
 class Rectangle extends Shape {

        private double width;
        private double height;


        public Rectangle(double height,double width) {
            this.height=height;
            this.width = width;
        }

        @Override
        public double length() {
            // TODO Auto-generated method stub
            return 2*(width+height);
        }

        @Override
        public double area() {
            // TODO Auto-generated method stub
            return width*height;
        }

        public double differ() {
            return Math.abs(height-width);
        }

    }
 
 class Square extends Rectangle {
    
     public double edge;
 
     public Square(double edge) {
         super(edge,edge);
         setEdge(edge);
         // TODO Auto-generated constructor stub
     }

     public double getEdge() {
         return edge;
     }
 
     public void setEdge(double edge) {
         this.edge = edge;
     }  
 }
 
 class Triangle extends Shape {
        private double a;
        private double b;
        private double c;
        
        public Triangle(double a,double b,double c) {
            this.a= a;
            this.b = b;
            this.c = c;
        }

        @Override
        public double length() {
            // TODO Auto-generated method stub
            return a+b+c;
        }

        @Override
        public double area() {
            double p= (a+b+c)/2;
            // TODO Auto-generated method stub
            return Math.sqrt(p*(p-a)*(p-b)*(p-c));
        }
    }
 
 class circle extends Shape{
     private double r;
     public circle(double r){
         this.r = r;
     }
     public double length(){
         return 3.1415*2*r;
     }
     public double area(){
         return 3.1415*r*r;
     }
     
 }

public class shapes {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Shape[] shapes= {
                new Rectangle(5,10),
                new circle(5),
                new Square(3),
                new Triangle(3, 4, 5)
            };
        for(int i=0;i<shapes.length;i++) {
            double length=shapes[i].length();
            double area=shapes[i].area();
            if(shapes[i]instanceof circle) {
                System.out.println("Shape("+(i+1)+"): "+
                        "length"+String.format("%.4f", length)+",area"
                        +String.format("%.4f", area));
            }
            else {
                        System.out.println("Shape("+(i+1)+"): "+
                            "length"+length+",area"+area);
                    if(shapes[i]instanceof Rectangle) {
                        Rectangle r=(Rectangle)shapes[i];
                        System.out.println("長寬差是:   "+r.differ());
                    }
            }
        }
    }

}