The query not working

[Sidra shah]

Hello

I have a Book class having Java and Semantic Web sub classes.
Java class have some instances: JSP, Java by Dietel, JDBC
Semantic Web has also some instances: SPARQL, Semantic Programming, etc.

I have a data property BookCategory for each instance which will be either
Java or Semantic Web

I have a combo box where users if select Java, all java books will be
displayed and similarly for semantic web.

The following query does not work. (Display nothing)

 String str=jComboBox1.getSelectedItem().toString();

SELECT  *  WHERE { ?s rdf:type mo:Book.?s mo:BookCategory ?book FILTER (
?book ='str' ). }