private static byte[] groupKey(List<Object> groups){
  StringBuilder builder=new StringBuilder();
  groups.forEach(null);
  return Bytes.toBytes(builder.toString());
}

private static byte [ ] groupKey ( List < Object > groups ) { StringBuilder builder = new StringBuilder ( ) ; groups . forEach ( null ) ; return Bytes . toBytes ( builder . toString ( ) ) ; }
builds the ' group ' component of the row key .
