java接口相关例题

 interface Graphics
{  //接口里面只能用抽象方法
  public abstract double area();
    }
//设置 平面类
class PlaneGraphics1
{
    private String shape; //形状
    //构造方法,有参数
    public PlaneGraphics1(String shape)
    {
       this.shape=shape;
    }
    //无参数
    public PlaneGraphics1()
    {
       this('未知图形1');
    }
    //打印
    public void print()
    {
      
    }
   
    }
//长方形 ，继承接口必要实现接口的抽象方法，不然会报错的。
class Rectangle extends PlaneGraphics1 implements Graphics
{
    protected  double length;
    protected double width;
    //构造方法
    public Rectangle(double length,double width)
    {
       this.length=length;
       this.width=width;
    }
    //正方形构造方法
    public Rectangle(double width)
    {
       this.length=width;
       this.width=width;
    }
    //无参数构造方法
    public Rectangle()
    {
      
    }
    public   double area()
    {
       return width*length;
    }
    public void print()
    {
       System.out.println('长方形面积为：'+this.area());
    }
   
    }
//椭圆
class Eclipse extends PlaneGraphics1 implements Graphics
{
    protected  double radius_a;
    protected double radius_b;
    //椭圆构造方法
    public Eclipse(double radius_a,double radius_b)
    { 
       super('椭圆');
       this.radius_a=radius_a;
       this.radius_b=radius_b;
      
    }
    //圆
    public Eclipse(double radius_a)
    {
       super('圆');
       this.radius_a=radius_a;
       this.radius_b=radius_a;
    }
    public Eclipse()
    {
      
    }
     public double area()
     {
       return Math.PI*radius_a*radius_b;
     }
     public void print()
     {
       System.out.println('圆的面积为'+this.area());
     }
    }

class Triangle extends PlaneGraphics1 implements Graphics
{
    private double bottom ;
    private double height;
    public Triangle(double bottom ,double height)
    {
       this.bottom=bottom;
       this.height=height;
    }
    public double area()
    {
       return bottom*height/2;
    }
    public void print()
    {
       System.out.println('三角形面积为：'+this.area());
    }
    }
public class PlaneGraphics_ex1 {
    public static void main(String[]args)
    {    
       PlaneGraphics1 g=new Rectangle(10.0,20.0);
       g.print();
       g=new Eclipse(10.0,12.0);
       g.print();
       g=new Triangle(10.0,6.0);
       g.print();
    }

}