java11-stream-map-filter-implementation-using-flatmap's Introduction

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)));

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