Java-8 – Java 8: filtering and mapping on the same method output

We try to refactor the following code into 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 proposed:

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

But the method m () is called twice here Is there any way?

Solution

Then you can filter after the mapping step:

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