LongStream filtering

Question

I have two Java LongStreams and I want to remove values which are present in one stream from the other.

LongStream stream 1 = ...
LongStream stream 2 = ...

stream2  = stream2.filter(e-> stream1.contains(e));

The LongStream does not have a contains method and I dont know how to use anyMatch in this case because the value to be checked is coming from another stream and is not in a variable or constant.

Answer 1

You could collect to a Set and use contains:

LongStream stream1 = ...
LongStream stream2 = ...

Set<Long> set1 = stream1.boxed().collect(Collectors.toSet());

stream2 = stream2.filter(set1::contains);

Answer 2

Well, you cannot search in the Stream as long as Stream is an unfinished set of pipelines, i.e. operations. It makes no sense to compare it with such Stream.

The next thing that seems odd to me is probably a typo. I assume you want to search in the stream1 instead of stream2, hence:

stream2 = stream2.filter(e -> stream1.contains(e));

The only way is to compare a LongStream with a collection which is optimized for such search. I assume you want to continue the stream1 after you perform the search, so perform these steps:

1. Close stream1 converting it to a List<Long> list1.
2. Perform the search in stream2 using list1 from stream1.
3. Open stream1 again for further processing.

LongStream stream1 = ...
LongStream stream2 = ...

List<Long> list1 = stream1.boxed()                       // list1 from stream1 ..
                          .collect(Collectors.toList()); // .. which also closes stream1

stream2  = stream2.filter(list1::contains);              // perform search

stream1 = list1.stream().mapToLong(l -> l);              // open stream1 as LongStream

Edit: Use Set for better performance as @fps suggests in his answer.

Answer 3

Why use streams at all?

Collections have the retainAll(Collection) method, that does just what you need, if i am not mistaken...
(Or removeAll(Collection) just a bit above...)