Java8: Filter and map on same method output

Question

We are trying to refactor below code to java 8:

List<String> list = new ArrayList<>();
Iterator<Obj> i = x.iterator();
while (i.hasNext()) {
   String y = m(i.next().getKey());
   if (y != null) {
      list.add(y);
   }
}
return list;

So far we have come up with:

return x.stream()
  .filter(s -> m(s.getKey()) != null)
  .map(t -> m(t.getKey()))
  .collect(Collectors.toList());

But the method m() is being invoked twice here. Is there any way around?

Answer 1

Well you can do the filtering after the mapping step:

x.stream()
  .map(s -> m(s.getKey()))
  .filter(Objects::nonNull)
  .collect(Collectors.toList());