How to use PageRequest to realize Pagination

[Samgin Shen]

I ues Ektorp 1.4.2. I can follow PageRequest's test source to get next page,like : 

PageRequest NextPageRequest = PageRequest.firstPage(Num).nextRequest(nextKey, nextId).page(1).build(); 

But I can't use getPreviousPageRequest() to get previous page. This function return  Id&key same  nextRequest.

In Documentation, It described not clear.

[Samgin Shen]