public <RES extends Collection<Group>> ResultTransformer<RES> collection(Supplier<RES> resultFactory, Expression<?>... expressions) {
        return new GroupByGenericCollection<K, Group, RES>(resultFactory, key, expressions);
    }
Get the results as a collection.

@param resultFactory The collection factory to use, i.e. {@code HashSet::new}.
@param expressions projection
@return new result transformer
querydsl-core/src/main/java/com/querydsl/core/group/GroupByBuilder.java