Crystal Reports Main report link to subreport

Message

     I am having developer license of Crystal Report 2008. I am able to 
generate report using Java JDK 1.5 , JSP,Oracle10g , deployed on 
WebSphere Application Server 6.1.0.29. I am not able to generate Sub 
reports. Can you please help us with sample code to how to call and 
configure subreport and please send configuration CRConfig.xml.

The below Error is Occurred on generating report.
On condition when we link the main report to subreport through parameter.

com.crystaldecisions.sdk.occa.report.lib.ReportSDKException:
 java.lang.NullPointerException---- Error code:-2147467259 Error code 
name:failed
                at com.businessobjects.reports.sdk.JRCCommunicationAdapter.if(SourceFile:741)
                at com.businessobjects.reports.sdk.JRCCommunicationAdapter.a(SourceFile:167)
                at com.businessobjects.reports.sdk.JRCCommunicationAdapter$2.a(SourceFile:529)
                at com.businessobjects.reports.sdk.JRCCommunicationAdapter$2.call(SourceFile:527)
                at com.crystaldecisions.reports.common.ThreadGuard.syncExecute(SourceFile:102)
                at com.businessobjects.reports.sdk.JRCCommunicationAdapter.for(SourceFile:525)
                at com.businessobjects.reports.sdk.JRCCommunicationAdapter.int(SourceFile:424)
                at com.businessobjects.reports.sdk.JRCCommunicationAdapter.request(SourceFile:352)
                at com.businessobjects.sdk.erom.jrc.a.a(SourceFile:54)