Image explanation of super and extends in generics

1. // Apple orange inherits from the fruit class and Jonathan is a subclass of apple.
2. List <?ExtendsFruit> List =NewArraylist <Apple> ();
3. // List. Add (new Apple (); Error
4. // List. Add (new fruit (); Error
6. List <?SuperFruit> List =NewArraylist <Apple> ();
7. List. Add (NewApple (); // Yes
8. List. Add (NewFruit (); // Yes
10. List <?ExtendsFruit> List =NewArraylist <Apple> ();
11. Why can't fruit and fruit classes be added to the list?
12. List <?ExtendsFruit> indicates that the upper limit is fruit. The following values are valid.
13. List <?ExtendsFruit> list1 =NewArraylist <fruit> ();
14. List <?ExtendsFruit> list2 =NewArraylist <Apple> ();
15. List <?ExtendsFruit> list3 =NewArraylist <Jonathan> ();
16. If list <?ExtendsFruit> If the add method is supported
17. List1 can add fruit and all fruit subclasses.
18. List2 can add apple and all apple subclasses.
19. List3 can add Jonathan and all the child classes of Jonathan.
20. In this case, the problem arises.
21. List <?ExtendsFruit> the object to be held is a subclass of fruit.
22. The subclass is still unknown, so adding any fruit subclass will be problematic, because if you add apple
23. It is possible to list <?ExtendsThe object held by fruit> isNewArraylist <Jonathan> ()
24. Apple cannot join. If you add Jonathan
25. It is possible to list <?ExtendsThe object held by fruit> isNewArraylist <subclass of Jonathan> ()
26. The addition of Jonathan is invalid, so list <?ExtendsFruit> list cannot be added
29. And list <?SuperFruit> list indicates that the object held by the list is the parent class of fruit, and the lower limit is fruit. Therefore, it is no problem to add the fruit or fruit subclass to the list.