public static <K, V, T, U> AbstractGroupExpression<Pair<K, V>, Map<T, U>> map(GroupExpression<K, T> key,
                                                                                  GroupExpression<V, U> value) {
        return new GMap.Mixin<K, V, T, U, Map<T, U>>(key, value, GMap.createLinked(QPair.create(key, value)));
    }
Create a new aggregating map expression using a backing LinkedHashMap

@param key key for the map entries
@param value value for the map entries
@return wrapper expression
querydsl-core/src/main/java/com/querydsl/core/group/GroupBy.java