What is the (kind of) inverse operation to Java's Stream.flatMap()?

Question

The Stream.flatMap() operation transforms a stream of

a, b, c

into a stream that contains zero or more elements for each input el

Answer 1

This is what I came up with:

interface OptionalBinaryOperator extends BiFunction> {
  static  OptionalBinaryOperator of(BinaryOperator binaryOperator,
          BiPredicate biPredicate) {
    return (t1, t2) -> biPredicate.test(t1, t2)
            ? Optional.of(binaryOperator.apply(t1, t2))
            : Optional.empty();
  }
}

class StreamUtils {
  public static  Stream reducePartially(Stream stream,
          OptionalBinaryOperator conditionalAccumulator) {
    Stream.Builder builder = Stream.builder();
    stream.reduce((t1, t2) -> conditionalAccumulator.apply(t1, t2).orElseGet(() -> {
      builder.add(t1);
      return t2;
    })).ifPresent(builder::add);
    return builder.build();
  }
}

Unfortunately, I did not have the time to make it lazy, but it can be done by writing a custom Spliterator delegating to stream.spliterator() that would follow the logic above (instead of utilizing stream.reduce(), which is a terminal operation).

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PS. I just realized you wanted conversion, and I wrote about conversion. If you can first map from T to U, and then use the function above, then that's it (even if it's suboptimal).

If it's something more complex, the kind of condition for reducing/merging would need to be defined before proposing an API (e.g. Predicate, BiPredicate, BiPredicate, or maybe even Predicate>).