how to get the files from merge changesets using client libraries ?

Hello

currently i get the error like illegalargumentexception : while fetching the files from merged changeset ,Is there any API to get that ?

Following is the code where I use to get the files from the changesets :

              Change changedFile = (Change)changedFileObj;

                versionableHandle = changedFile.afterState();

                if(null != versionableHandle){

                    Object file = SCMPlatform.getWorkspaceManager(currentWIRepo).versionableManager().fetchCompleteState(versionableHandle, monitor);

currentWIRepo.itemManager().fetchCompleteItem(changeSet.getComponent(), IItemManager.DEFAULT, null)).getName());

                    if(file instanceof IFileItem){

                          filesList = filesList+((IFileItem) file).getName()+",";

                   }

                } else{

                     versionableHandle = changedFile.beforeState();        

                    Object file = SCMPlatform.getWorkspaceManager(currentWIRepo).versionableManager().fetchCompleteState(versionableHandle, monitor);

                    if(file instanceof IFileItem){

                           filesList = filesList+((IFileItem) file).getName()+"(-),";

                    }

               }