How extract sorted list of distinct integers from a Set of objects using Java streams?

Question

I have a set of value objects:

Set<EntityKey> clientAssignedPlaceholderEntityKeys

Where the EntityKey class has the following properties:

private Integer objectRef;
private String entityType;

What is the most efficient way to extract the distinct objectRef values into a sorted list using streams?

I have the following but the fact that it calls stream() twice seems like a bad smell:

  // Extract into a sorted list all the distinct placeholder objectRefs (regardless of type).
  List<Integer> sortedPlaceholderObjectRefs = clientAssignedPlaceholderEntityKeys.stream()
          .map(entityKey -> entityKey.getObjectRef())
          .collect(Collectors.toSet())
          .stream()  // having to call stream() a 2nd time here feels sub-optimal
          .sorted()
          .collect(Collectors.toList());

@TimBiegeleisen I'm sure It's possible with single stream. – SamHoque Oct 26 '18 at 08:49 — SamHoque, Oct 26 '18 at 08:49

Mershel · Answer 1 · 2018-10-26T08:54:23.500

4

Maybe:

sortedPlaceholderObjectRefs = clientAssignedPlaceholderEntityKeys.stream()
                      .map(entityKey -> entityKey.getObjectRef())
                      .sorted()
                      .distinct()
                      .collect(Collectors.toList());

EDIT:

calling .distinct() before .sorted() might be more optimal

edited Oct 26 '18 at 08:54

answered Oct 26 '18 at 08:49

Mershel

542
1
9
17

I like the look of that but wonder if it might be more efficient if distinct() were done before sorted()? – David Easley Oct 26 '18 at 08:52
Yes, i was just editing my comment to change that. It should be more optimal – Mershel Oct 26 '18 at 08:53

score 2 · Answer 2 · answered Oct 26 '18 at 08:56

List<Integer> sortedRefs = clientAssignedPlaceholderEntityKeys
                      .stream()
                      .map(EntityKey::getObjectRef)
                      .distinct()
                      .sorted()
                      .collect(Collectors.toList());

redshift · Accepted Answer · 2018-10-27T04:37:57.990

2

      clientAssignedPlaceholderEntityKeys.stream()
                                         .map(ek -> ek.getObjectRef())
                                         .sorted()
                                         .distinct()
                                         .collect(Collectors.toList());

Regarding your doubt about the order of sorted() and distinct()

Quoting the very first answer from here: Chaining distinct() operation after sorted(), the implementation will utilize the sorted nature of the data and avoid building an internal HashSet.

edited Oct 27 '18 at 04:37

answered Oct 26 '18 at 09:09

redshift

91
5

That's good to know @redshift. In case future readers might get confused, could you please edit your answer to show distinct() operation after sorted()? I will then accept this answer. – David Easley Oct 26 '18 at 17:08

How extract sorted list of distinct integers from a Set of objects using Java streams?

3 Answers3