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Java 5 -Collection

Pradeep bhatt
Ranch Hand

Joined: Feb 27, 2002
Posts: 8927

I want to know why the following Collection<E> interface methods, accept Collection<?> as the parameter and not Collection<? super E>

boolean containsAll(Collection<?> c)
boolean removeAll(Collection<?> c)
boolean retainAll(Collection<?> c)

If I create a Collection<String>, does invoking c.containsAll(m) (where m is Collection<Map> make sense? :roll: I dont think so. Could someone explain?
Barry Gaunt
Ranch Hand

Joined: Aug 03, 2002
Posts: 7729
This must be advanced stuff because this code:

compiles and runs without error. Mind you it doesn't do anything either.

It is rather all confusing
[ December 30, 2004: Message edited by: Barry Gaunt ]

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Jeanne Boyarsky
author & internet detective

Joined: May 26, 2003
Posts: 33102

Suppose I have a the Dog-Animal-Object hierarchy. Obviously it makes sense to call collectionOfAnimals.containsAll(collectionOfDogs).

However, it also makes sense to call collectionOfDogs.containsAll(collectionOfAnimals). I may not know what is in my collectionOfAnimals. It is entirely possible that it contains only dogs (or is empty) and then the call makes sense.

Another reason is consistency. It must be valid to call collectionOfDogs.containsAll(collectionOfObjects) and collectionOfObjects.containsAll(collectionOfDogs) for the same reasons explained above. collectionOfObjects is equivalent to the pre-java 1.5 Collection. So I can get this functionality by using a Collection without specifying a type because java 1.5 is backward compatible.
[ December 30, 2004: Message edited by: Jeanne Boyarsky ]

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Pradeep bhatt
Ranch Hand

Joined: Feb 27, 2002
Posts: 8927

Thanks Barry and Jeanne. Jeanne, your explanation was quite useful.
I agree. Here's the link:
subject: Java 5 -Collection
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