public static GroupByBuilder<List<?>> groupBy(Expression<?>... keys) {
        return new GroupByBuilder<List<?>>(Projections.list(keys));
    }
public static GroupByBuilder < List < ? > > groupBy ( Expression < ? > ... keys ) { return new GroupByBuilder < List < ? > > ( Projections . list ( keys ) ) ; }
Create a new GroupByBuilder for the given key expressions

@param keys keys for aggregation
@return builder for further specification