java泛型不止可以放元素还可以放元素的子类型

class GrannySmith extends Apple{}
class Gala extends Apple{}
class Fuji extends Apple{}
class Braeburn extends Apple{}


public class GenericsAndUpcasting{
   public static void main(String[] args){
      ArrayList<Apple> apples = new ArrayList<Apple>();
      apples.add(new GrannySmith () );
      apples.add(new Gala );
      apples.add(new Fuji );
      apples.add(new Braeburn );
   for (Apple c: apples){
      System.out.println(c);
   }
   }
}