Hi David,
You can always transform an imperative code to a stream by pushing the element
through a consumer.
Internally, a stream uses a push iterator (see
Spliterator.tryAdvance(consumer)).
As a silly example, this is a way to write fibonacci (the recursive form) with
a stream right in the middle.
static void fibo ( int n , IntConsumer consumer ) {
if ( n < 2 ) {
consumer .accept( n );
return ;
}
var result = Stream . of ( "" )
.mapMultiToInt(( _ , consumer2 ) -> {
fibo ( n - 1 , consumer2 );
fibo ( n - 2 , consumer2 );
})
.sum();
consumer .accept( result );
}
static void main () {
fibo ( 7 , IO :: println );
}
Here, I use mapMulti() to convert the imperative code to a Stream
(there is no factory method on Stream that takes a consumer of consumer).
If you also want to short-circuit, you can use a gatherer instead of mapMulti
but short-circuiting the recursive code will require to use an exception as
control flow (it will not be pretty).
regards,
Rémi
> From: "David Alayachew" <[email protected]>
> To: "core-libs-dev" <[email protected]>
> Sent: Tuesday, November 11, 2025 4:36:29 AM
> Subject: Difficulties of recursion with Streams
> Hello [ mailto:[email protected] | @core-libs-dev ] ,
> When working with streams, I often run into situations where I have to
> "demote"
> back to imperative code because I am trying to solve a problem best solved by
> recursion.
> Consider the common use case of cycling through permutations to find all
> permutations that satisfy some condition. With recursion, the answer is
> incredibly simple -- just grab an element from the set, then call the
> recursive
> method with a copy of the set minus the grabbed element. Once you reach the
> empty set, you've reached your terminal condition.
> Use cases like that are not only incredibly common, but usually,
> embarrassingly
> parallel. The example above of cycling through permutations is only a few
> lines
> of imperative code, but I struggle to imagine how I would do this with
> Streams.
> I guess let me start by asking -- are there any good ways currently to
> accomplish the above permutation example with Streams? And if not, should
> there
> be?
> Thank you for your time and consideration.
> David Alayachew
