Map set of indices to set of Objects, using Java streams

Question

Trying to map a set of indices to their corresponding object in a list, then return as a set.

List aList = Arrays.asList(new Object(), new Object(), new Object(), new Object(), new Object(), new Object());

Set.of(0, 2, 3)
     .stream()
     .flatMap(index -> aList.get(index))     // error on this line
     .collect(Collectors.toSet());

Error message: no instance(s) of type variable(s) R exist so that Object conforms to Stream<? extends R>

Answer 1

You need map in this example, not flatMap . flatMap is used when each element of the original Stream is mapped to multiple elements.

Set.of(0, 2, 3)
  .stream()
  .map(index -> aList.get(index))
  .collect(Collectors.toSet());

EDIT: your updated question makes my alternative suggestions obsolete, but the remaining snippet still works.