java11-stream-map-filter-implementation-using-flatMap

flatMap in a nutshell

• in JDK 11, we have 2 types of flatMap
  • in Optional

    Optional<U> flatMap(Function<? super T, ? extends Optional<? extends U>> mapper)
    

  • in Stream

    Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper)
    

• flatMap = map + flatten
• flatten

  // given
  Optional<Optional<Integer>> matrioshka = Optional.of(Optional.of(1));
  Optional<Optional<Integer>> emptyMatrioshka = Optional.of(Optional.empty());
  
  // when
  Optional<Integer> maybeInt = matrioshka.flatMap(Function.identity());
  Optional<Integer> emptyInt = emptyMatrioshka.flatMap(Function.identity());
  
  // then
  assertThat(maybeInt, is(Optional.of(1)));
  assertThat(emptyInt, IsSame.sameInstance(Optional.empty()));
  

• contrary to map (which introduces one to one correspondence) flatMap could change the number of elements in the stream

  // given
  Stream<Integer> triple = Stream.of(1, 2, 3);
  
  // when
  Stream<Integer> sextuple = triple.flatMap(i -> Stream.of(i, i));
  
  // then
  assertThat(sextuple.collect(Collectors.toList()), is(List.of(1, 1, 2, 2, 3, 3)));
  

project description

• it may be valuable to take a look at: https://github.com/mtumilowicz/java11-stream-map-filter-implementation-using-reduce
• map implementation using flatMap

  static <T, R> Stream<R> map(Stream<T> stream, Function<T, R> mapper) {
      return stream.flatMap(mapper.andThen(Stream::of));
  }
  

• filter implementation using flatMap

  static <T> Stream<T> filter(Stream<T> stream, Predicate<T> predicate) {
      return stream.flatMap(x -> predicate.test(x) ? Stream.of(x) : Stream.empty());
  }
  

tests

• map

  // given
  var ints = Stream.of(1, 2, 3);
  
  // when
  var squared = StreamUtil.map(ints, x -> x * x).collect(Collectors.toList());
  
  // then
  assertThat(squared, is(List.of(1, 4, 9)));
  

• filter

  // given
  var ints = Stream.of(1, 2, 3);
  
  // when
  var filtered = StreamUtil.filter(ints, x -> x > 2).collect(Collectors.toList());
  
  // then
  assertThat(filtered, is(List.of(3)));