定义一个形状类（Shape）方法：计算周长，计算面积。子类： 矩形类、圆形类、三角形类

定义一个形状类（Shape）方法：计算周长，计算面积

子类：
矩形类（Rectangle） ：额外的方法：differ（） 计算长宽差

圆形类（Circle）

三角形类（Triangle）
正方形类（Square） 矩形的子类
生成几个不同的形状对象，放在一个Shape 类型的数组里，分别求每个形状的周长和面积。
如果形状对象是一个矩形，且不是正方形，则计算长宽差。

Circle.java

package ex8;

import java.util.List;

public class SongDao {
    
    private List<Song> songList;
    
    public SongDao() {
        
    }

    public SongDao(List<Song> songList) {
        super();
        this.songList = songList;
    }

//    新增歌曲 
    public void add(Song song) {
        songList.add(song);
    }
    
//    查询歌曲
//    按id查找
    public Song search(int sid) {
        for(Song song : songList) {
            if(song.getSid()==sid){
                return song;
            }
        }
        return null;
    }
//    按对象查找
    public Song search(Song song) {
        if(song==null) {
            return null;
        }
        return search(song.getSid());
    }
    
//    删除歌曲
//按id删除歌曲
    public void delete(int sid) {
        Song song=search(sid);
        if(song!=null) {
            songList.remove(song);
        }
    }
//按对象删除歌曲
    public void delete(Song song) {
        if(song!=null) {
            delete(song.getSid());
        }
    }
    
    public List<Song> getSongList() {
        return songList;
    }

    public void setSongList(List<Song> songList) {
        this.songList = songList;
    }


    
    
}

Rectangle.java

package ex4_1;

public class Rectangle extends Shape {

    private double width;
    private double height;


    public Rectangle(double height,double width) {
        super();
        // TODO Auto-generated constructor stub
        setHeight(height);
        setWidth(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);
    }
    
    public double getWidth() {
        return width;
    }

    public void setWidth(double width) {
        this.width = width;
    }

    public double getHeight() {
        return height;
    }

    public void setHeight(double height) {
        this.height = height;
    }

}

Shape.java

package ex4_1;

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

Square.java

package ex4_1;

public 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;
    }

    
}

Tester.java

package ex4_1;

public class Tester {

    public static void main (String[] args) {
        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("%.2f", length)+",area"
                        +String.format("%.2f", 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("长宽差是"+r.differ());
                    }
            }
        }
    }
}

Triangle.java

package ex4_1;

public class Triangle extends Shape {
    private double a;
    private double b;
    private double c;
    
    public Triangle(double a,double b,double c) {
        super();
        // TODO Auto-generated constructor stub
        setA(a);
        setB(b);
        setC(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));
    }

    public double getA() {
        return a;
    }

    public void setA(double a) {
        this.a = a;
    }

    public double getB() {
        return b;
    }

    public void setB(double b) {
        this.b = b;
    }

    public double getC() {
        return c;
    }

    public void setC(double c) {
        this.c = c;
    }
}