Java8 filter and change properties simultaneously

Question

Reading through this article, I can filter the collection like this:

Set<Status> statusClone = cloner.deepClone(statusList).stream().filter(s -> !(s instanceof FileMetadata)).collect(Collectors.toSet());

However, I will also need to set the properties simultaneously filtering them. After filtering them, I currently iterate through each one and then set properties:

for (Iterator<Status> iterator = statusClone.iterator(); iterator.hasNext();)
        {
            Status s = iterator.next();
//          if (s instanceof FileMetadata)
//          {
//              iterator.remove();
//              continue;
//          }
            s.setStatus(JobStatus.UNINITIATED);
            s.setLastAccessedTime(0);
            s.setOffSet(null);
            s.setStreamNo(null);
        }
        statusList.addAll(statusClone);

Is this possible in Java8 without using foreach?

EDIT: From the comments, I agree that I can clone inside the filter. Thanks.

Problem is that your code isn't very functional (you do not have an immutable Status for one) so it's awkward to write it using Stream. — Tunaki, Oct 02 '15 at 15:48
If you are going to set the same values again and again for different objects, put that in a new method and call the method from the stream. — We are Borg, Oct 02 '15 at 15:50
Potential duplicate: http://stackoverflow.com/questions/16635398/java-8-iterable-foreach-vs-foreach-loop — Phylogenesis, Oct 02 '15 at 15:52
Thanks but I wanted to know to use without foreach on the collection. I'm iterating through the collect again. If this is the case, then I could avoid lambdas and use a conventional for loop using an iterator. — dmachop, Oct 02 '15 at 15:53
You are on the right track when you say that you could avoid using lambas and use a conventional for loop, however, you don’t need to deal with an `Iterator`. Use `for(Status s: statusClone) { /* manipulate s */ }`. You can use this since Java 5… — Holger, Oct 02 '15 at 16:36

score 6 · Accepted Answer · answered Oct 02 '15 at 15:51

You could use peek:

private static void initStatus(Status s) {
  s.setStatus(JobStatus.UNINITIATED);
  s.setLastAccessedTime(0);
  s.setOffSet(null);
  s.setStreamNo(null);
}

Set<Status> statusClone = cloner.deepClone(statusList).stream()
                  .filter(s -> !(s instanceof FileMetadata))
                  .peek(MyClass::initStatus)
                  .collect(Collectors.toSet());

Note the javadoc warning that it could create issues if you use a parallel stream because you are modifying state.

It would be better to initialise the status directly when cloning the original list - something like:

private static Status cloneAndInitStatus(Status s) {
  Status clone = new Status(s);
  clone.setStatus(JobStatus.UNINITIATED);
  clone.setLastAccessedTime(0);
  clone.setOffSet(null);
  clone.setStreamNo(null);
  return clone;
}

Set<Status> statusClone = statusList.stream()
                  .filter(s -> !(s instanceof FileMetadata))
                  .map(MyClass::cloneAndInitStatus)
                  .collect(Collectors.toSet());

That way you avoid state modifications during the stream.

Tunaki · Answer 2 · 2015-10-02T15:58:09.113

You could write:

Set<Status> statusClone = cloner.deepClone(statusList)
                            .stream() 
                            .filter(s -> !(s instanceof FileMetadata))
                            .peek(s -> {
                                s.setStatus(JobStatus.UNINITIATED);
                                s.setLastAccessedTime(0);
                                s.setOffSet(null);
                                s.setStreamNo(null);
                            })
                            .collect(Collectors.toSet());

But it's a bit awkward since we're directly mutating an Object inside the peek method.

It would be better to write a copy constructor of Status and then use map instead of peek to map each status to a new status with this constructor.

Java8 filter and change properties simultaneously

2 Answers2