Java 5 -Collection

[Pradeep bhatt]

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?
Thanks

[Barry Gaunt]

This must be advanced stuff because this code:

import java.util.*;
 
public class SuperT
{
    public static void main(String[] args) {
        HashSet<String> ss = new HashSet<String>();
        ss.add(new Integer(42).toString());
        ss.add(new Integer(0).toString());
        System.out.println(ss.size());
        List<Integer> is = new LinkedList<Integer>();
        is.add(42);
        is.add(0);
        ss.removeAll(is);
        System.out.println(ss.size());
    }
    
}
  

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 ]

[Jeanne Boyarsky]

Pradeep,
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 ]

[Pradeep bhatt]

Thanks Barry and Jeanne. Jeanne, your explanation was quite useful.