abstract class Shape{
    abstract void draw();
    abstract void erase();
    }
class Circle extends Shape{
    void draw(){
        System.out.println("I'm a circle");
        }
    void erase(){
        System.out.println("I'm circle s");
        }
    }
class Square extends Shape{
    void draw(){
        System.out.println("I'm a square");
        }
    void erase(){
        System.out.println("I'm square s");
        }
    }
class Triangle extends Shape{
    void draw(){
        System.out.println("I'm a Triangle");
        }
    void erase(){
        System.out.println("I'm Triangle s");
        }
    }
class huxiang{
    public static Shape randShape(){      //能帮我解释下这个方法吗？
        switch((int)(Math.random()*3)){
            default:System.out.println("hello");
            case 0:return new Circle();
            case 1:return new Square();
            case 2:return new Triangle();
            
            }
        }
     public static void main(String args[]){
        Shape[] s=new Shape[9];
        for(int i=0;i<s.length;i++){
            s[i]=randShape();
            s[i].draw();
            s[i].erase();
            }
        }
    }

解释的非常清楚了，谢谢。