lambda - How to check if Collection is not empty using java Stream -

i new java 8. not able understand wrong in following piece of code. idea sent collection<user> if not empty. if collection empty sent httpstatus.not_found entity response.

@requestmapping(value = "/find/pks",                  method = requestmethod.get,                  produces = mediatype.application_json_value) public responseentity<collection<user>> getusers(@requestbody final collection<string> pks) {     return streamsupport.stream(userrepository.findall(pks).spliterator(), false)          .map(list -> new responseentity<>(list , httpstatus.ok))          .orelse(new responseentity<>(httpstatus.not_found)); } 

eclipse shows me error in following point .orelse

the method orelse(new responseentity<>(httpstatus.not_found)) undefined type stream<responseentity<user>>

my base interface method looks following

iterable<t> findall(iterable<pk> pks); 

you mixing 2 things up. first task convert iterable collection can indeed solve using stream api:

collection<user> list=     streamsupport.stream(userrepository.findall(pks).spliterator(), false)    .collect(collectors.tolist()); 

note stream stream of users, not stream of lists. therefore can’t map list else stream. map operation map each element of stream new element.

then can use list create responseentity

return list.isempty()? new responseentity<>(httpstatus.not_found):                        new responseentity<>(list, httpstatus.ok); 

you can combine these steps creating collector performing steps though not provide advantage, it’s matter of style:

responseentity<user> responseentity=     streamsupport.stream(userrepository.findall(pks).spliterator(), false)    .collect(collectors.collectingandthen(collectors.tolist(),       list -> list.isempty()? new responseentity<>(httpstatus.not_found):                               new responseentity<>(list, httpstatus.ok) ));