It may not be a good idea to use a wrapper class from java.lang for collecting results from rule execution. Setting and retrieving a global is simple, as suggested by Frank and Esteban: Double d1 = new java.lang.Double(0); ksession.setGlobal("result", d1 ); // execute the session Double d2 = (Double)ksession.setGlobal("result" ); But how do you add something to your result? You might think that this is it: rule "not a good idea" when ... $x ... then result += $x; end But this isn't going to work, because the Double is made available as a local copy. To update the global, you'll have to use something like kcontext.getKnowledgeRuntime().setGlobal( "result", result+$x ); It would be better to write a simple class, call it Sum, with a field double value and a getter and a setter, and then you can do Sum s = new Sum(); ksession.setGlobal("result", s ); // execute the session double result = s.getValue(); // the reference is still good And the rule uses then result.setValue( result.getValue() + $x ); end or add a convenience method plus(double d) to Sum then result.plus( $x ); end -W On 15/01/2013, Esteban Aliverti <esteban.aliverti(a)gmail.com&gt; wrote: