public SolrQuery addSort(SortClause sortClause){
  if (sortClauses == null)   sortClauses=new ArrayList<>();
  sortClauses.add(sortClause);
  serializeSorts();
  return this;
}

public SolrQuery addSort ( SortClause sortClause ) { if ( sortClauses == null ) sortClauses = new ArrayList < > ( ) ; sortClauses . add ( sortClause ) ; serializeSorts ( ) ; return this ; }
adds a single sort clause to the end of the query .
