How to get filtered list of files from SFTP server using SSHJ

I am trying to get a filtered list of files from a SFTP server using SSHJ. It works fine without filter but I am not getting any example of filter.

I am using list files like

            Map<String, String> map = new HashMap();
            map.put("ext", "*.txt");
            RemoteResourceFilter filter = null;
            PathComponents path = new PathComponents(null, null, null).;
            FileAttributes attr = new FileAttributes.Builder().withExtended(map).build();
            
            RemoteResourceInfo rri = new RemoteResourceInfo(path, attr);
            filter.accept(rri);

            List<RemoteResourceInfo> rs =  sftp.ls("/sftpuser1", filter);
            System.out.println(rs.size());
            for(int i=0; i<rs.size(); i++) {
                System.out.println(rs.get(i).getName().toString());
            }

Please could you help me with an example of retrieving files with filter?