？ Extends E;? Super E

？ Extends e: Can receive subtypes of type E or E. Ceiling.
？ Super E: Can receive the Type E or the parent of E. Lower

　　　　

<? Extends class> is a type of restriction wildcard that can accept all <Class> and Class subtypes. However, the cost of the call is read-only access

2. list<?    Super fruit> list = new arraylist<apple> ();
4. List.add (new Apple ()); Can be
6. List.add (new Fruit ()); Can be
10. list<?    extends fruit> list = new arraylist<apple> ();
12. Why can't I join the subclasses of the fruit class and the fruit class in the list because of this?
14. list<? extends fruit> indicates that the upper limit is Fruit, and the following assignments are legal
16. list<?    extends fruit> list1 = new arraylist<fruit> ();
18. list<?    extends fruit> list2 = new arraylist<apple> ();
20. list<?    extends fruit> list3 = new arraylist<jonathan> ();
22. If list<? extends fruit> Support the method of the Add method is valid
24. List1 can add fruit and all fruit subclasses
26. List2 can add apple and all Apple sub-classes
28. List3 can add Jonathan and all of Jonathan's subclasses
30. In that case, the problem will arise.
32. list<? extends fruit> should hold the object is Fruit subclass, and specifically which one
34. Subclasses are still unknown, so adding any subclass of fruit will be problematic, because if the add Apple
36. words, may list<? extends fruit> holds the object is new arraylist<jonathan> ()
38. Apple's accession is certainly not going to work if if add Jonathan
40. words, may list<? extends Fruit> holds a subclass of new Arraylist<jonathan > ()
42. Jonathan's accession is not legal, so list< extends fruit> list cannot be add
48. and list<? The Super fruit> list indicates that the list holds objects that are Fruit's parent class, and the lower bound is Fruit, so the list of add Fruit or Fruit subclasses is not a problem

？ Extends E;? Super E