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Re: RDFSchema in end-user application: Which Java-API to use?

From: Dave Reynolds <der@hplb.hpl.hp.com>
Date: Fri, 22 Apr 2005 13:13:08 +0100
Message-ID: <4268EA54.3080801@hplb.hpl.hp.com>
To: Max Voelkel <max@xam.de>
CC: semantic-web@w3.org

Max Voelkel wrote:

> 1. Is there a nicer way (shorter, more readdable) to do this in Jena?

I realize your overall msg is directed to the wider community but 
jena-dev@yahoogroups.com is a good place to get support for use of jena.

> [3] Source code in Jena
> -------------------------------------------------
> jenaDataModel = ModelFactory.createDefaultModel();
> 
> // create RDF [1] according to Schema [2]
> Property name = jenaDataModel.createProperty("schema://hasName");
> Property knows = jenaDataModel.createProperty("schema://hasFriend");

You can use schemagen[a] to convert your schema to a set of predefined 
constants. This both shortens the code slightly and avoids typing mistakes.

> Resource p1 = jenaDataModel.createResource("data://p1");
> Resource p2 = jenaDataModel.createResource("data://p2");
> p1.addProperty( RDF.type, "schema://Person");
> p2.addProperty( RDF.type, "schema://Person");

Some people find the cascading style more readable:
    model.createResource(..).addProperty(..).addProperty(..);
a matter of taste.

>    System.out.println(s.getSubject().getProperty(name).getLiteral()
>       .getLexicalForm()

For plain literals you don't need getLexicalForm.

Personally I generally have a getPropertyValue(resource, property) 
operation lying around for dereferencing statements and coping with nulls. 
OntResource has that convenience method built in.

>       + jenaDataModel.createResource(
>           s.getObject().asNode().getURI()).getProperty(name)
>           .getLiteral().getLexicalForm());

The object is already a resource you don't need to create a new one.

With those minor tweaks (and assuming you call the schemagen'ed class FOAF) 
the code would look like:
[[[
         Model jenaDataModel = ModelFactory.createDefaultModel();

         Resource p1 = jenaDataModel.createResource("data://p1")
                         .addProperty( RDF.type, FOAF.Person)
                         .addProperty( FOAF.hasName, "Rudi Studer");
         Resource p2 = jenaDataModel.createResource("data://p2")
                         .addProperty( RDF.type, FOAF.Person)
                         .addProperty( FOAF.hasName, "York sure");
         p1.addProperty(FOAF.knows, p2);

//      query model and render output
         StmtIterator it = p1.listProperties(FOAF.knows);
         while (it.hasNext()) {
            Statement s = it.nextStatement();
            System.out.println(
                getPropertyValue(s.getSubject(), FOAF.hasName)
                    + " knows " +
                getPropertyValue((Resource)s.getObject(), FOAF.hasName) );
         }
]]]
where getPropertyValue [b] is
[[[
     public static RDFNode getPropertyValue(Resource r, Property p) {
         Statement s = r.getProperty(p);
         return (s == null) ? null : s.getObject();
     }
]]]

We should probably do any follow ups off-list or, better, over on jena-dev, 
to avoid cluttering this list with API specific details.

Dave

[a] http://jena.sourceforge.net/how-to/schemagen.html

[b] Alterantively use getRequiredPropertyValue and let any exceptions 
propagate. Use of nulls for empty returns is controversial.
Received on Friday, 22 April 2005 12:13:32 UTC

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