java <? super Fruit>与<? extends Fruit>

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package Test2016;   import java.util.ArrayList; import java.util.List;   public class Test2016 {       public static void main(String[] args) {                   List<? super Fruit> first = new ArrayList<Fruit>(); //Fruit -> Apple                   first.add(new Apple());         first.add(new Fruit());         first.add(new RedApple());           //      first.add(new Food());  //error                   System.out.println(first.get(0));         System.out.println(first.get(1));                   List<? extends Apple> second = new ArrayList<Apple>();                   second.add(null);     }       }     class Food { }   class Fruit extends Food { }   class Apple extends Fruit { }   class RedApple extends Apple { }