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Beginning Java
how to pass a child list to a parentlist
chaohua wang
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Posts: 62
posted 13 years ago
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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
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posted 13 years ago
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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
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posted 13 years ago
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Paul,
I see, Thank you very much for the excellent explanation.
Best Regards,
Chwnag
Campbell Ritchie
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posted 13 years ago
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Look through the
Java™ Tutorials
; there are two sections on generics, which should help you. Also Google for
Angelika Langer Java Generics FAQ
.
With a little knowledge, a
cast iron skillet
is non-stick and lasts a lifetime.
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