Is object property functional? - owl

Question

I am using the OWL-API 5 to load all the object property axioms in my ontology as follows:
File ontology = new File("examples/ontology.owl");
File individual = new File("examples/individuals.owl");
OWLOntologyManager manager = OWLManager.createOWLOntologyManager();
IRI documentIRI = IRI.create(ontology);
IRI ontologyIRI = IRI.create("http://www.semanticweb.org/2020/0/test");
SimpleIRIMapper mapper = new SimpleIRIMapper(ontologyIRI, documentIRI);
manager.getIRIMappers().add(mapper);
OWLOntology kb = manager.loadOntologyFromOntologyDocument(individual);
Stream<OWLObjectPropertyAssertionAxiom> objectPropertyAxioms = kb.axioms(AxiomType.OBJECT_PROPERTY_ASSERTION);
objectPropertyAxioms.forEach(axiom -> {
System.out.println("Found object property axiom " + axiom);
OWLIndividual object = axiom.getObject();
OWLIndividual subject = axiom.getSubject();
OWLObjectPropertyExpression property = axiom.getProperty();
});
Returns:
Found object property axiom ObjectPropertyAssertion(<http://www.semanticweb.org/2020/0/test#Q> <http://www.semanticweb.org/2020/0/test#x> <http://www.semanticweb.org/2020/0/test#y>)
Now, I'd like to determine if the property is functional. This is what I tried so far:
if (EntitySearcher.isFunctional(property, kb)) {
LOGGER.debug("Property " + property + " is declared as functional");
} else {
LOGGER.debug("Property " + property + " is NOT declared as functional");
}
Returns:
Property <http://www.semanticweb.org/2020/0/test#Q> is NOT declared as functional
I think that EntitySearcher.isFunctional(p,o) is looking for functional object property axioms that make the specified object property functional, which doesn't seems to exist in my ontology (i.e., ontology.axioms(AxiomType.FUNCTIONAL_OBJECT_PROPERTY) returns nothing).
This is what I have in my ontology:
<?xml version="1.0"?>
<rdf:RDF xmlns="http://www.semanticweb.org/2020/0/test#" xml:base="http://www.semanticweb.org/2020/0/test" xmlns:owl="http://www.w3.org/2002/07/owl#" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:xml="http://www.w3.org/XML/1998/namespace" xmlns:xsd="http://www.w3.org/2001/XMLSchema#" xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">
<owl:Ontology rdf:about="http://www.semanticweb.org/2020/0/test"/>
<owl:ObjectProperty rdf:about="http://www.semanticweb.org/2020/0/test#Q">
<rdf:type rdf:resource="http://www.w3.org/2002/07/owl#FunctionalProperty"/>
</owl:ObjectProperty>
</rdf:RDF>
And individuals:
<?xml version="1.0"?>
<rdf:RDF xmlns="http://www.semanticweb.org/2020/0/test#" xmlns:owl="http://www.w3.org/2002/07/owl#" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:xml="http://www.w3.org/XML/1998/namespace" xmlns:xsd="http://www.w3.org/2001/XMLSchema#" xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">
<owl:Ontology rdf:about="">
<owl:imports rdf:resource="http://www.semanticweb.org/2020/0/test"/>
</owl:Ontology>
<owl:NamedIndividual rdf:about="http://www.semanticweb.org/2020/0/test#x">
<Q rdf:resource="http://www.semanticweb.org/2020/0/test#y"/>
</owl:NamedIndividual>
<owl:NamedIndividual rdf:about="http://www.semanticweb.org/2020/0/test#y"/>
</rdf:RDF>
(Both files were created using Prótegé 5.5.0). Any suggestions? Thank you.

Answer 1

To answer the implicit question about EntitySearcher::isFunctional, yes, it checks if there are functional property axioms for the property received in input.
I believe your ontology has enough information for that.
To provide a complete example:
<?xml version="1.0"?>
<rdf:RDF xmlns="http://www.semanticweb.org/2020/0/test#" xml:base="http://www.semanticweb.org/2020/0/test" xmlns:owl="http://www.w3.org/2002/07/owl#" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:xml="http://www.w3.org/XML/1998/namespace" xmlns:xsd="http://www.w3.org/2001/XMLSchema#" xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#" xmlns:test="http://www.semanticweb.org/owlapi/test#">
<owl:Ontology rdf:about="http://www.semanticweb.org/2020/0/test"/>
<owl:ObjectProperty rdf:about="http://www.semanticweb.org/owlapi/test#Q">
<rdf:type rdf:resource="http://www.w3.org/2002/07/owl#FunctionalProperty"/>
</owl:ObjectProperty>
<owl:NamedIndividual rdf:about="http://www.semanticweb.org/owlapi/test#x">
<test:Q rdf:resource="http://www.semanticweb.org/owlapi/test#y"/>
</owl:NamedIndividual>
<owl:NamedIndividual rdf:about="http://www.semanticweb.org/owlapi/test#y"/>
</rdf:RDF>
With this ontology, EntitySearcher returns true when checking for functional properties.
Edit: in your updated question, you're using kb.axioms(AxiomType), you need to tell it to include imports.