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From Rodney Waldhoff <rwaldh...@apache.org>
Subject Re: [collections] Last call before release 3.0
Date Thu, 04 Sep 2003 17:27:21 GMT
On Wed, 3 Sep 2003, Phil Steitz wrote:

> Here is one code change that I would recommend:
>
> Currently, CollectionUtils.isProperSubCollection(a,b) returns
>
> CollectionUtils.isSubCollection(a,b) &&
> (!(CollectionUtils.isEqualCollection(a,b)));
>
> This is very inefficient, since it is equivalent to
>
> (a.size() < b.size()) && CollectionUtils.isSubCollection(a,b);
>
> There are currently no tests for this method, but the latter works for
> the ones that I have coded.
>

The latter approach assumes that the size() function returns the total
number of elements in the collection (i.e., the "length" of the Iterator
returned by collection.iterator()), whereas the former approach does not.
For example, a Bag-ish implementation that returns the number of distinct
elements for size(), but the duplicate elements in the iterator, would
break the latter implementation.

This assumption (that size() will return the number elements available in
iterator()) is probably quite reasonable, and the latter implementation
will be much more efficient when isProperSubCollection is going to return
true, so I'd say go for it.  (Though we may want to document the
assumption.)

Also, the latter approach doesn't quite work for extremely large
collections.  For example, if both a and b contain more than
Integer.MAX_VALUE elements, this method may fail (since size() will still
return Integer.MAX_VALUE in that case).  This is probably more of an
academic concern than an practical one, but again, may be a reasonable
limitation to note.

> Phil

- Rod <http://radio.weblogs.com/0122027/>

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