Java 16: Stream.mapMulti

Fahd ShariffSeptember 2nd, 2021Last Updated: August 25th, 2021

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Java 16 introduces a new Stream.mapMulti method which allows you to replace elements in a stream with multiple elements.

The example below shows how you can use mapMulti to replace each string in a stream with its uppercased and lowercased versions:

Stream.of("Twix", "Snickers", "Mars")
  .mapMulti((s, c) -> {
    c.accept(s.toUpperCase());
    c.accept(s.toLowerCase());
  })
  .forEach(System.out::println);

Output:
TWIX
twix
SNICKERS
snickers
MARS
mars

The same thing can also be achieved using flaMap like this:

Stream.of("Twix", "Snickers", "Mars")
  .flatMap(s -> Stream.of(s.toUpperCase(), s.toLowerCase()))
  .forEach(System.out::println);

So what is the difference between mapMulti and flatMap? According to the javadocs:

This method is preferable to flatMap in the following circumstances:

When replacing each stream element with a small (possibly zero) number of elements. Using this method avoids the overhead of creating a new Stream instance for every group of result elements, as required by flatMap.
When it is easier to use an imperative approach for generating result elements than it is to return them in the form of a Stream.

Inspecting the code for multiMap, we can see that it delegates to flatMap, however, it makes use of a SpinedBuffer to hold the elements before creating the stream, thus avoiding the overhead of creating new streams per group of result elements.

default <R> Stream<R> mapMulti(BiConsumer<? super T, ? super Consumer<R>> mapper) {
  Objects.requireNonNull(mapper);
  return flatMap(e -> {
    SpinedBuffer<R> buffer = new SpinedBuffer<>();
    mapper.accept(e, buffer);
    return StreamSupport.stream(buffer.spliterator(), false);
  });
}

Published on Java Code Geeks with permission by Fahd Shariff, partner at our JCG program. See the original article here: Java 16: Stream.mapMulti

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