It's harder than it looks.

The problem is that some implementations of Collection have configuration state
which is independent of the elements they contain.  E.g. a TreeMap is
configured with a Comparator, and HashMaps have a load-factor.  So, although
you could create an empty copy with:

   Collection copy = (Collection)collection.getClass().newInstance();

that would not necessarily have the right Comparator.

Many of the standard Collections implementations do implement Clonable, and
clone() answers a shallow copy (presumably with the correct Comparator, etc).
So you could use that, and then try to remove the entries you don't want -- but
that will fail if the collection is not mutable (as is quite legal).

I don't think that a general, reliable, and easy-to-use solution is possible
given the Collections API.

   -- chris

java.util.Collections contains methods for creating checked (type safe)
or synchronized views of existing collections. Maybe the definitions and
implementations of those contain some useful techniques?

Patricia

   You could cop out and ask the user to provide you with the instance of
collection to populate.

<code tested="false">
public <T extends Collection> T filter(T collectionToReadFrom, T
collectionToPopulate) {
 /*Weird things happen if collectionToReadFrom == collectionToPopulate*/
 for(Object obj : collectionToReadFrom) {
   if(satisfiesSomeCondition(obj)) {
     collectionToPopulate.add(obj);
   }
 }
 return collectionToPopulate;
}

...

ArrayList input = /*whatever*/;
ArrayList output = filter(input, new ArrayList());
</code>

   - Oliver