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https://issues.apache.org/jira/browse/COLLECTIONS-424?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13459830#comment-13459830
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Thomas Neidhart commented on COLLECTIONS-424:
---------------------------------------------
Hi,
thanks for providing the patch, but unfortunately I am not sure if it goes into
the right direction. Constraining a CompositeCollection to just one specific
collection type is against the idea of the class (to provide a composite
interface for a set of collections). Also the type safety for the element type
is lost due to the fact that CompositeCollections extends Collection<Object>.
Tbh, I am more in favor of rejecting this issue or removing the inheritance to
CompositeCollection in CompositeSet as there is no real need for it apart from
code re-use.
> Surprising exception by CompositeSet in a situation where CompositeCollection
> works fine
> ----------------------------------------------------------------------------------------
>
> Key: COLLECTIONS-424
> URL: https://issues.apache.org/jira/browse/COLLECTIONS-424
> Project: Commons Collections
> Issue Type: Bug
> Components: Set
> Affects Versions: 3.2.1
> Environment: All environments
> Reporter: Michael Pradel
> Attachments: collections424.patch
>
>
> We have a method that uses a CompositeCollection. Here's a simplified version
> of it:
> void m(CompositeCollection coll) {
> coll.addComposited(new TreeBag());
> }
> It works fine when the argument is a CompositeCollection, but it throws an
> exception when the argument is a CompositeSet. E.g.:
> m(new CompositeCollection()); // OK
> m(new CompositeSet()); // IllegalArgumentException
> Although the exception is documented in CompositeSet, this behavior is very
> surprising. Is there a way to have m() accept CompositeCollections without
> running into this exception? The only solution that comes to my mind is to
> dynamically check the type of 'coll' in m(), but this is a rather nasty
> work-around.
> A better solution may be to make the genericity of CompositeCollection
> explicit by adding a type parameter:
> class CompositeCollection<T extends Collection> {
> void addComposited(T c) { /* .. */ }
> }
>
> class CompositeSet extends CompositeCollection<Set> {
> @Override void addComposited(Set c) { /* .. */ }
> }
> This way, users of CompositeCollection must choose the kind of collections
> that can be composed and will not encounter surprises, such as the above.
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