相关mongodb java group by的问题

// 查询总的销量和销售额

                StringBuilder sb = new StringBuilder(100); 
                sb.append("function(doc, out){out.") 
                        .append(ProductItem.FIELD_SALES).append("+=doc.value.") 
                        .append(ProductItem.FIELD_SALES).append(";out.") 
                        .append(ProductItem.FIELD_MONEY).append("+=doc.value.") 
                        .append(ProductItem.FIELD_MONEY).append('}'); 
                String reduce = sb.toString(); 
                obj = new BasicDBObject(); 
                obj.put(ProductItem.FIELD_SALES, 0); 
                obj.put(ProductItem.FIELD_MONEY, (Double) 0.0); 
                BasicDBList groups = (BasicDBList) collection.group( 
                        new BasicDBObject(), new BasicDBObject(), obj, reduce);