Retrieving all classes

I am reading "A Semantic Web Primer, Second Edition" book. One of the
examples (page 105) is

 

PREFIX rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#> 

PREFIX rdfs:    <http://www.w3.org/2000/01/rdf-schema#> 

 

 SELECT  ?c 

  WHERE {  ?c rdf:type rdfs:Class .  }

 

It says that it will retrieve all classes.

 

But when I executed it on Jena 2.6.0. It returned nothing. What did I do
wrong? I'd appreciate any help/pointer.

 

Thanks in advance for your help.

 

Best,

-- On Lee