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https://issues.apache.org/jira/browse/PDFBOX-1512?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14151927#comment-14151927
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Tom Crossland commented on PDFBOX-1512:
---------------------------------------
Part of the issue here is that {{TextPositionComparator}} breaks the contract
for {{Comparator}}. Specifically, it doesn't ensure that the ordering is
transitive. The
[Comparator|http://docs.oracle.com/javase/7/docs/api/java/util/Comparator.html]
interface specifies that:
{quote}
The implementor must also ensure that the relation is transitive:
{{((compare(x, y)>0) && (compare(y, z)>0))}} implies {{compare(x, z)>0}}.
{quote}
This is currently not the case due to the vertical position tolerance. So
{{TextPositionComparator}} is not a valid {{Comparator}} and it shouldn't be
used for sorting. Using a custom Quicksort probably won't help much, as the
ordering of the results will depend on the order in which elements are compared.
> TextPositionComparator is not compatible with Java 7
> ----------------------------------------------------
>
> Key: PDFBOX-1512
> URL: https://issues.apache.org/jira/browse/PDFBOX-1512
> Project: PDFBox
> Issue Type: Bug
> Components: Text extraction
> Affects Versions: 1.7.1
> Environment: Java 7
> Reporter: Benjamin Papez
> Assignee: Andreas Lehmkühler
> Attachments: FOP-2252.pdf, TextPositionComparator.java, Topo.pdf,
> Topo.txt, TopoContained.pdf, TopoContained.txt, TopoOverlap.pdf,
> TopoOverlap.txt, WFI_PDFParser_TextPostionComparator.txt,
> illustration-of-inconsistent-sorting.png, immo-kurier_arsenal_93x62.pdf,
> quicksort.patch
>
>
> The TextPostionCompartor causes the following exception running on Java 7:
> Unexpected RuntimeException from
> org.apache.tika.parser.ParserDecorator$1@9007fa2 Original cause: Comparison
> method violates its general contract!
> I think the problem is with this check:
> if ( yDifference < .1 ||
> (pos2YBottom >= pos1YTop && pos2YBottom <= pos1YBottom) ||
> (pos1YBottom >= pos2YTop && pos1YBottom <= pos2YBottom))
> as it violates the contract requirement:
> The implementor must also ensure that the relation is transitive:
> ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0.
> Finally, the implementor must ensure that compare(x, y)==0 implies that
> sgn(compare(x, z))==sgn(compare(y, z)) for all z.
> Java 7 now is strict and throws exceptions when the contract is violated.
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