用Java定义Shape接口实现多态。 interface Shape{ void draw(); void erase(); } Circle类、Square类、Triangle类实现Shape，编写程序NewShapes.java，利用Random类实现随机9次调用不同图形类绘图。

下面是实现代码：

import java.util.Random;

interface Shape{
   void draw();
   void erase();
}

class Circle implements Shape{
   public void draw(){
      System.out.println("Circle.draw()");
   }
   public void erase(){
      System.out.println("Circle.erase()");
   }
}

class Square implements Shape{
   public void draw(){
      System.out.println("Square.draw()");
   }
   public void erase(){
      System.out.println("Square.erase()");
   }
}

class Triangle implements Shape{
   public void draw(){
      System.out.println("Triangle.draw()");
   }
   public void erase(){
      System.out.println("Triangle.erase()");
   }
}

public class NewShapes{
   public static void main(String[] args){
      Shape[] shapes = new Shape[9];
      Random rand = new Random();
      for(int i=0;i<9;i++){
         int j = rand.nextInt(3);
         switch(j){
            case 0:
               shapes[i] = new Circle();
               break;
            case 1:
               shapes[i] = new Square();
               break;
            case 2:
               shapes[i] = new Triangle();
               break;
            default:
               break;
         }
         shapes[i].draw();
         shapes[i].erase();
      }
   }
}

运行结果：

Circle.draw()
Circle.erase()
Square.draw()
Square.erase()
Triangle.draw()
Triangle.erase()
Circle.draw()
Circle.erase()
Square.draw()
Square.erase()
Circle.draw()
Circle.erase()
Square.draw()
Square.erase()
Triangle.draw()
Triangle.erase()
Circle.draw()
Circle.erase()
Square.draw()
Square.erase()