schema.org contains a bunch of anonymous classes like the union of other classes which are used as domain or range of a property, that's why you get null values because they do not have a URI. If you'd just call
|System.out.println(clazz);| you'd see the blank node Ids. Among all those blank nodes, there is indeed the one top level class http://schema.org/Thing . I thought just adding a filter on the iterator is enough, i.e. |topClazzez = topClazzez.filterDrop(OntResource::isAnon);| but while it doesn't return the blank nodes anymore, it leads to an exception: |Exception in thread "main" org.apache.jena.ontology.ConversionException: Cannot convert node http://www.w3.org/2000/01/rdf-schema#Class to OntClass: it does not have rdf:type owl:Class or equivalent|| || at org.apache.jena.ontology.impl.OntClassImpl$1.wrap(OntClassImpl.java:82)|| || at org.apache.jena.enhanced.EnhNode.convertTo(EnhNode.java:152)|| || at org.apache.jena.enhanced.EnhNode.convertTo(EnhNode.java:31)|| || at org.apache.jena.enhanced.Polymorphic.asInternal(Polymorphic.java:62)|| || at org.apache.jena.enhanced.EnhNode.as(EnhNode.java:107)|| || at org.apache.jena.ontology.impl.OntResourceImpl.lambda$listDirectPropertyValues$8(OntResourceImpl.java:1536)|| || at org.apache.jena.util.iterator.Map1Iterator.next(Map1Iterator.java:46)|| || at org.apache.jena.ontology.impl.OntResourceImpl.computeDirectValues(OntResourceImpl.java:1580)|| || at org.apache.jena.ontology.impl.OntResourceImpl.listDirectPropertyValues(OntResourceImpl.java:1553)|| || at org.apache.jena.ontology.impl.OntClassImpl.listSuperClasses(OntClassImpl.java:180)|| || at org.apache.jena.ontology.impl.OntClassImpl.isHierarchyRoot(OntClassImpl.java:739)|| || at org.apache.jena.util.iterator.FilterIterator.hasNext(FilterIterator.java:56)|| || at org.apache.jena.util.iterator.WrappedIterator.hasNext(WrappedIterator.java:90)|| || at org.apache.jena.util.iterator.FilterIterator.hasNext(FilterIterator.java:55)|| || at com.company.Main.main(Main.java:25)| On 05.09.19 03:11, Alex To wrote: > Hi I am trying to load schema.org ontology and get all top classes > using ontModel.listHierarchyRootClasses() but can't get the expected > results with different OntModelSpec. > > If I use OWL_MEM, it lists 2500+ records with all the records have "null" > URI. > If I use OWL_DL_MEM, it lists 0 records > > The code is very simple as follows > > Model model = ModelFactory.createDefaultModel(); > model.read("https://schema.org/docs/schemaorg.owl";); > OntModel ontModel = > ModelFactory.createOntologyModel(OntModelSpec.OWL_MEM, model); > ExtendedIterator<OntClass> topClazzez = ontModel.listHierarchyRootClasses(); > while (topClazzez.hasNext()) { > OntClass clazz = topClazzez.next(); > System.out.println(clazz.getURI()); > } > > A minimal Maven project ready to run to demonstrate my problem is here > https://github.com/AlexTo/jena-lab (have a look at the Main.java) > > Thanks a lot
