继承小例子

public class Shape{
   private String name;
   protected double area;
   public Shape(String name){
       this.name=name;

   }
   public String getName(){
     return this.name;
   }
   public double countArea(){
        return 0.0;
   } 
}
class Rectangle extends Shape{
     private double length,width;

     public Rectangle(String name,double length,double width){
       super(name);
       this.length=length;
       this.width=width;
       }
     public double countArea(){
        return this.area=length*width;

     }  

}
class Triangle extends Shape{
     private double length,width;

     public Triangle(String name,double length,double width){
       super(name);
       this.length=length;
       this.width=width;
       }
     public double countArea(){
        return this.area=1/2*length*width;

     }  

}
class Circle extends Shape{
    private double r;
    public Circle (String name,double r){
       super(name);
       this.r= r;
    }
    public double countArea(){
     return this.area=Math.PI*Math.pow(r,2.0);  

    }

}
class TestShape{

     public static void calShapeArea(Shape r){
       System.out.println(r.countArea());
     }  
   public static void main(String[] args){
      Shape r = new Rectangle("矩形",58,76);        
        calShapeArea(r);
       r = new Circle("圆形",12);
        calShapeArea(r);
       r= new Triangle("三角形",378,38);
        calShapeArea(r);
       r= new Circle("圆形",38);
        calShapeArea(r);
     }

}

public interface ShapeInterface{


  public static final double PI=3.1415926;
  public static void countArea();
  public static double getArea();

}
   interface comparableShape extends ShapeInterface{}
class CircleImpl implements ShapeInterface{
     private double r;
     private double area;
     public CircleImpl(double r){
         this.r=r;
     }
     public void countArea(){
      this.area = this.PI * Math.pow(r,2.0);  
     }
     public double getArea(){return area;}
}
class Triangle implements ShapeInterface{
     private double length;
     private double width;
     private double area;
     public void countArea(){
        this.area= 1/2*length*width;
     }
     public double getArea(){return area;}
}
class ShapeInterfaceApp{

   public static void printShapeArea(ShapeInterface shape){

     shape.countArea();
     System.out.println(shape.getArea());
   }
 public static void main(String[] args){

         ShapeInterface circle = new CircleImpl(23.4);
         printShapeArea(circle);
}
}