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https://issues.apache.org/jira/browse/LUCENE-2000?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12844130#action_12844130
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Earwin Burrfoot commented on LUCENE-2000:
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I believe we should do this at our next "we're breaking backcompat" release.
Any compile errors that could bring to clients are fixed like, exceptionally
easy. And the code comes out cleaner -> we have way more callees of clone()
than overrides.
> Use covariant clone() return types
> ----------------------------------
>
> Key: LUCENE-2000
> URL: https://issues.apache.org/jira/browse/LUCENE-2000
> Project: Lucene - Java
> Issue Type: Task
> Affects Versions: 3.0
> Reporter: Uwe Schindler
> Priority: Minor
> Attachments: LUCENE-2000-clone_covariance.patch
>
>
> *Paul Cowan wrote in LUCENE-1257:*
> OK, thought I'd jump in and help out here with one of my Java 5 favourites.
> Haven't seen anyone discuss this, and don't believe any of the patches
> address this, so thought I'd throw a patch out there (against SVN HEAD @
> revision 827821) which uses Java 5 covariant return types for (almost) all of
> the Object#clone() implementations in core.
> i.e. this:
> public Object clone() {
> changes to:
> public SpanNotQuery clone() {
> which lets us get rid of a whole bunch of now-unnecessary casts, so e.g.
> if (clone == null) clone = (SpanNotQuery) this.clone();
> becomes
> if (clone == null) clone = this.clone();
> Almost everything has been done and all downcasts removed, in core, with the
> exception of
> Some SpanQuery stuff, where it's assumed that it's safe to cast the clone()
> of a SpanQuery to a SpanQuery - this can't be made covariant without
> declaring "abstract SpanQuery clone()" in SpanQuery itself, which breaks
> those SpanQuerys that don't declare their own clone()
> Some IndexReaders, e.g. DirectoryReader - we can't be more specific than
> changing .clone() to return IndexReader, because it returns the result of
> IndexReader.clone(boolean). We could use covariant types for THAT, which
> would work fine, but that didn't follow the pattern of the others so that
> could be a later commit.
> Two changes were also made in contrib/, where not making the changes would
> have broken code by trying to widen IndexInput#clone() back out to returning
> Object, which is not permitted. contrib/ was otherwise left untouched.
> Let me know what you think, or if you have any other questions.
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