IllegalArgumentException thrown when serializing to RDF/XML using JDK 7

[ignazio1977]

With JDK 7 an exception is thrown when serializing to RDF/XML.

java.lang.IllegalArgumentException: Comparison method violates its general contract!
at java.util.TimSort.mergeHi(TimSort.java:868)
at java.util.TimSort.mergeAt(TimSort.java:485)
at java.util.TimSort.mergeForceCollapse(TimSort.java:426)
at java.util.TimSort.sort(TimSort.java:223)
at java.util.TimSort.sort(TimSort.java:173)
at java.util.Arrays.sort(Arrays.java:659)
at java.util.Collections.sort(Collections.java:217)
at org.coode.owlapi.rdf.model.RDFGraph.getSortedTriplesForSubject(RDFGraph.java:123)
at org.coode.owlapi.rdf.rdfxml.RDFXMLRenderer.render(RDFXMLRenderer.java:193)
at org.coode.owlapi.rdf.renderer.RDFRendererBase.renderEntity(RDFRendererBase.java:302)
at org.coode.owlapi.rdf.renderer.RDFRendererBase.renderEntities(RDFRendererBase.java:294)
at org.coode.owlapi.rdf.renderer.RDFRendererBase.renderClasses(RDFRendererBase.java:258)
at org.coode.owlapi.rdf.renderer.RDFRendererBase.renderInOntologySignatureEntities(RDFRendererBase.java:242)
at org.coode.owlapi.rdf.renderer.RDFRendererBase.renderOntologyComponents(RDFRendererBase.java:230)
at org.coode.owlapi.rdf.renderer.RDFRendererBase.render(RDFRendererBase.java:215)
at org.coode.owlapi.rdf.rdfxml.RDFXMLOntologyStorer.storeOntology(RDFXMLOntologyStorer.java:86)
at org.semanticweb.owlapi.util.AbstractOWLOntologyStorer.storeOntology(AbstractOWLOntologyStorer.java:94)
at uk.ac.manchester.cs.owl.owlapi.OWLOntologyManagerImpl.saveOntology(

It occurred with a version of CHEBI.

I think the issue is at the end of the compareTo() method:

if (diff == 0) {
diff = 1;
}
return diff;

This means that 0 will never be returned, so suppose there are two elements which are equal, i.e., in a distance you'd expect to get 0: a.compareTo(b) returns 1, and b.compareTo(a) returns 1 as well. If this is extended to three elements, then it's clear this is not transitive if the elements are close enough that diff is 0 at the end of the method, because it'd look like:
a<b b<c c<a.

The exception depends on JDK 7, as in this version a check for this condition was introduced, while the previous JDKs would ignore the condition and accept the wrong sorting obtained as a result.