CODEWITHSUNDEEP
javasparqljenadbpedia
user2022416
user2022416

Reputation: 71

How to write Sparql endpoint to DBpedia in java

Below is the sparql query to get places in united state, When I run this code on virtuoaso SPARQL I get the required output. But when I write this code in java its not giving the output.

  select (str(?label) as ?strLabel) where {  
    ?place a dbpedia-owl:PopulatedPlace ;
     dbpedia-owl:country dbpedia:United_States ;
     rdfs:label ?label
 filter langMatches( lang(?label), 'en' )`
 }
 limit 100

To my knowledge I wrote it but its not giving any result.

   public class TestDbpedia {
    public static void main(String[] args) {

        String sparqlQueryString = " select (str(?label) as ?strLabel) "+
                            "where {"+
                            "?place a dbpedia-owl:PopulatedPlace ;"+
                            "dbpedia-owl:country dbpedia:United_States ;"+
                            "rdfs:label ?label"+                                
                            "filter langMatches( lang(?label), 'en' )"+
                            "limit 10"+
                            "}";

                    Query query = QueryFactory.create(sparqlQueryString);

                    QueryExecution qexec =   QueryExecutionFactory.sparqlService("http://dbpedia.org/sparql", query);


                    try {
                        ResultSet results = qexec.execSelect();
                        for ( ; results.hasNext() ; )
                    {
                        QuerySolution soln = results.nextSolution() ;                           
                        System.out.println(soln.get("?strLabel"));                               
                    }
                    }
                    catch(Exception e){

                        e.printStackTrace();
                    }    

                    finally { qexec.close(); }

                    }
}

Upvotes: 0

Views: 1326

Answers (1)

Joshua Taylor
Joshua Taylor

Reputation: 85813

You need to declare the prefixes

One, you need to include the necessary prefixes in your SPARQL query. That is, you'll need to write 

prefix dbpedia-owl: <http://dbpedia.org/ontology/>

and the rest at the beginning of your query. The predefined prefixes only get applied to queries you make with the browser-based web interface, as far as I know.

You need to use some spaces or newlines

When you're constructing queries from strings, you need to be very careful with newlines and spaces and the like. For instance, the Java lines

"rdfs:label ?label"+                                
"filter langMatches( lang(?label), 'en' )"+

end up giving you a query string containing

rdfs:label ?labelfilter langMatches( lang(?label), 'en' )

You end up with an invalid query. Add some spaces or newlines to your strings.

You need to write a proper query

Your query doesn't end correctly. You have

"filter langMatches( lang(?label), 'en' )"+
"limit 10"+
"}";

which will give you (once you add some newlines and spaces):

  filter …
  limit 10
}

That's not the right syntax. You need to have 

  filter …
}
limit 10

Print and check your queries if you don't get useful error messages

I'm surprised that DBpedia didn't give you a more useful error message (or maybe it did; you didn't show us the stacktrace, after all). You can test your queries at sparql.org's query validator, too. Also, if you used one of Jena's ParameterizedSparqlStrings, you could have parsed the query beforehand, which should have caught the syntax error.

Upvotes: 3

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