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();

}

}