code: package it.unibz.inf.ontop.rdf4j.repository;
  
  
  import org.eclipse.rdf4j.query.TupleQueryResult;
  import org.junit.AfterClass;
  import org.junit.BeforeClass;
  import org.junit.Test;
  
  import java.io.IOException;
  import java.sql.SQLException;
  
  import static org.junit.Assert.assertFalse;
  import static org.junit.Assert.assertTrue;
  
  public class RDF4JNullMetaMappingTest extends AbstractRDF4JTest {
      private static final String CREATE_DB_FILE = "/meta/null/meta-null-create.sql";
      private static final String MAPPING_FILE = "/meta/null/meta-null.obda";
  
      @BeforeClass
      public static void before() throws IOException, SQLException {
          initOBDA(CREATE_DB_FILE, MAPPING_FILE);
      }
  
      @AfterClass
      public static void after() throws SQLException {
          release();
      }
  
      @Test
      public void test() {
          String query = "" +
                  "PREFIX : <http://example.org/>\n"+
                  "PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>\n"+
                  "SELECT  ?s ?c\n" +
                  "WHERE {\n" +
                  "  ?s a ?c \n" +
                  "}";
  
          TupleQueryResult result = evaluate(query);
          assertTrue(result.hasNext());
          result.next();
          assertFalse(result.hasNext());
          result.close();
      }
  }
  
  