How to pass Parameters from main report to sub-report in Jasper reports

 Create Parameter of same name in both Main and Sub report.

1) Open Design tab of Main-report
2) Select Subreport and go to subreport properties
3) click  Edit Parameters 

 4) Add new parameter // Note: Both parameter name should be same in main and subreport 

 5) Done!. you can directly pass parameters in a map like following:

 Map<String, Object> parameters = new HashMap<String, Object>();

        parameters.put("REPORT_TYPE", report_type);
        parameters.put("CMS", cms);
        parameters.put("SITE", site);
        parameters.put("OPPORTUNITY_ID", opportunity_id);