abstract class Shape{
    public abstract void draw();
}
class Rectangle extends Shape{
    public void draw(){
        System.out.println("画一个矩形");
    }
}
class Circle extends Shape{
    public void draw(){
        System.out.println("画一个圆");
    }
}
public class Main{
    public static void drawShape(Shape s){
        s.draw();
    }
    public static void main(String[] args){
        Shape shape = new Rectangle();
        drawShape(shape);
        Shape shape2 = new Circle();
        drawShape(shape2);
    }
}