import com.hp.hpl.jena.ontology.*;
import com.hp.hpl.jena.rdf.model.*;
import com.hp.hpl.jena.util.FileManager;

public class owl {

 public static void main(String[] args) throws IOException
 {  
  OntModel m = ModelFactory.createOntologyModel();
  /*OntDocumentManager dm = m.getDocumentManager();
  dm.addAltEntry( "http://www.xfront.com/owl/ontologies/camera/",
                  "file:examples/data/camera.owl"    );
  m.read( "http://www.xfront.com/owl/ontologies/camera/" );*/
  InputStream in ;
  try{
         in = FileManager.get().open( "camera.owl" );
         m.read(in, "");
         //System.out.println(schema.getProperty(null).toString());
     }
     catch (Exception e ){}
     String NS = "http://www.w3.org/2002/07/owl#";
     Resource r = m.getResource( NS + "Camera" );
     OntClass camera = m.getOntClass( NS + "Camera" );
     try{
      for( Iterator i = camera.listSubClasses(); i.hasNext(); )
      {
       OntClass c = ( OntClass ) i.next();
       System.out.print( c.getLocalName() + " " );
      }
     }catch( NullPointerException e ){}
     
 }
}

为什么总抛出java.lang.NullPointerException
 at owl.owl.main(owl.java:48)
                Exception in thread "main"
大侠们帮忙看看哦！谢谢！