how to pass a child list to a parentlist

[chaohua wang]

hi Folks,

I have a
cat class , its super class is animal class.

cat extends animal{

}
i got a cat list:

List<cat> catlist= task.getCatList();

I need to pass this catlist to a method:

public takeCareOfAllanimals (List<animal> animals).


I am not sure how to do this? I tied to cast catlist to (animal)catlist, but it doesn't allow me.

thank you

Chwang

[Paul Clapham]

You can't do that. Here's why:

List<Cat> cats = getAListOfCats(); // Here we have a list of animals which are guaranteed to be Cats.
List<Animal> animals = cats; // As you know this is not legal, but let's suppose it was.
animals.add(0, new Rabbit()); // Here we added a Rabbit to the beginning of the  list. That's okay because it's just a list of Animals.
Cat notACat = cats.get(0); // Here we get the first entry from the list.
// But it's a Rabbit. Then we assign it to a Cat? That's a problem because we guaranteed the list only contains Cats.

So, no. List<Cat> is not a subtype of List<Animal>. You should be doing this:

public takeCareOfAllanimals (List<? extends Animal> animals)

[chaohua wang]

Paul,
I see, Thank you very much for the excellent explanation.

Best Regards,
Chwnag

[Campbell Ritchie]

Look through the Java™ Tutorials; there are two sections on generics, which should help you. Also Google for Angelika Langer Java Generics FAQ.