wildcard generics

[rmacnak]

What is the difference between List<Whatever> and List<? extends
Whatever>?

 

[Oliver Wong]

What is the difference between List<Whatever> and List<? extends
Whatever>?

You can add an instance of Whatever to a List<Whatever>, but you can't
to a List<? extends Whatever>.

- Oliver

 

[grasp06110]

Sounds like it would be useful if Whatever is an abstract class or
interface. If you used ArrayList<Whatever> in this case and added an
instanace of Whatever to the list and then tried to execute an abstract
method you would get a runtime error. By using ArrayList<? extends
Whatever> if you tried to do this you would get a compile time error.
Cool.

 

[Oliver Wong]

Sounds like it would be useful if Whatever is an abstract class or
interface. If you used ArrayList<Whatever> in this case and added an
instanace of Whatever to the list and then tried to execute an abstract
method you would get a runtime error. By using ArrayList<? extends
Whatever> if you tried to do this you would get a compile time error.
Cool.

Note that you can't actually create an instance of an abstract class or
interface; you can only create an instance of one of its subclasses. So
whenever you invoke an abstract method, it would always resolve to some
real, concrete method.

- Oliver

 

[Bart]

What is the difference between List<Whatever> and List<? extends
Whatever>?

AFAIK, ListIterator<Whatever> is not compatible with
ListIterator<SomethingThatExtendsWhatever>.

Regards,
Bart.