Re: Java Generics question

From:
"Oliver Wong" <owong@castortech.com>
Newsgroups:
comp.lang.java.help
Date:
Wed, 11 Oct 2006 18:07:14 GMT
Message-ID:
<mpaXg.12287$P7.3582@edtnps89>
"Nicklas" <nicklas.lundgren@malmo.com> wrote in message
news:1160568089.922735.73620@h48g2000cwc.googlegroups.com...

-------------------------- Code Example --------------------------
import java.util.*;
public class AnimalGenericTester {

 public AnimalGenericTester() {
   ArrayList<Animal> myList = new ArrayList<Animal>();
   myList.add(new Dog());
   myList.add(new Cat());
   printAnimal(myList);
 }

 void printAnimal(ArrayList<? super Cat> list) {
   //What is the purpose of this???
   //The myList now contains a Dog and a Cat but Dog is NOT a super of
cat
   //I really can't see the point of this generic functionality
 }


    Dog might not be a superclass of cat, but Animal is, and if you pass
myList to that method, you're passing a List<Animal>, not a List<Dog>, so
everything checks out okay at compile time.

    A List<? super Cat> doesn't mean that all the elements in the list are a
superclass of cat. It means that you've got a list of something (let's call
it A), and A is a superclass of Cat, and everything in that list is an
instance of A. In our specific case, A is Animal, but your printAnimal
method doesn't (need to) know that.

    A List<? super Cat> is useful in that you know it is always safe to add
an instance of Cat to that list, because that list is a list of something
(again, let's call it A), and A is a superclass of Cat, so any Cat you add
is guaranteed to be an instance of A, and thus can be a member of that List.

    - Oliver

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