How to check if Collection is empty using Java Stream

You are confusing two things. The first task is to convert Iterableit to Collectionsomething you can indeed solve using the StreamAPI:

Collection<User> list=
    StreamSupport.stream(userRepository.findAll(pks).spliterator(), false)
   .collect(Collectors.toList());

Note that this stream is stream Users , not a stream of lists. So you cannot listuse this stream to map a to other objects. This mapoperation maps each element in the stream to a new element.

Then you can use this list to createResponseEntity

return list.isEmpty()? new ResponseEntity<>(HttpStatus.NOT_FOUND):
                       new ResponseEntity<>(list, HttpStatus.OK);

You can combine these steps by creating a single Collectorstep that does this, although this doesn't offer any advantage, it's just a 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) ));