[rules-users] Multiple Object Referencing problem

Hi, I am trying out this example from the book : public class Room { private String name // getter and setter methods here } public classs Sprinkler { private Room room; private boolean on; // getter and setter methods here } public class Fire { private Room room; // getter and setter methods here } I load 2 input files: 1. with data of fire and Room { f1, r1;f2, r2} 2. with data of sprinkler and Room { " ", r1; " " , r2} I am writing rule " if fire exists in room start sprinkler" { so if f1 and r1 in this case: I should get s1, r1 } when Fire($room : room) $sprinkler : Sprinkler( room == $room, modified == false ) Then $sprinkler.setmodified(true); modify( $sprinkler ) { setSprinkle( "s1" ) }; end But this gives me error: Exception in thread "main" java.lang.NullPointerException at java.io.Writer.write(Unknown Source) at com.org.RulesTest.main(RulesTest.java:58) I want this rule to be fired on the data in my second input file. The rules for my first input file get executed properly. Can any1 tell me where I am going wrong?? Is there any way in which we can use the modified data { after rules are fired on the first input data and it is modified}to fire another set of rules on the same input depending on the next input file??? Thank you. -M </pre> <P><hr size=1></P> <P><STRONG><font color=green>Please consider the environment before printing this email.</font></STRONG></P> <P><STRONG>Visit our website at <a href="http://www.nyse.com">http://www.nyse.com</a> <br> ***************************************************************************** <br> Note: The information contained in this message and any attachment to it is privileged, confidential and protected from disclosure. If the reader of this message is not the intended recipient, or an employee or agent responsible for delivering this message to the intended recipient, you are hereby notified that any dissemination, distribution or copying of this communication is strictly prohibited. If you have received this communication in error, please notify the sender immediately by replying to the message, and please delete it from your system. Thank you. NYSE Euronext. </STRONG></P><pre>