java12-stream-flatmap-implementation-using-reduce's Introduction

java12-stream-flatMap-implementation-using-reduce

We will show how to implement stream's flatMap using only reduce.

Please refer also: https://github.com/mtumilowicz/java11-stream-map-filter-implementation-using-reduce

project description

In the Stream API we have three reduce methods:

Optional<T> reduce(BinaryOperator<T> accumulator)
T reduce(T identity, BinaryOperator<T> accumulator)
U reduce(U identity, BiFunction<U, ? super T, U> accumulator, BinaryOperator<U> combiner)

First two are inadequate as they return Optional<T> or return T, which is the same type as declared in the given stream - but as an output we want a new type U ~ Stream<T>).

So we take third method and provide requested implementations:

static <T, R> Stream<R> flatMap(Stream<T> stream, Function<T, Stream<R>> mapper) {
    return stream.reduce(Stream.empty(), (acc, x) -> Stream.concat(acc, mapper.apply(x)), Stream::concat);
}

tests

in all of the following tests we will use: Function<Object, Stream<Object>> triple = x -> Stream.of(x, x, x);

empty stream

//        given
        var empty = Stream.empty();
        Function<Object, Stream<Object>> triple = x -> Stream.of(x, x, x);

//        when
        Stream<Object> tripled = StreamUtil.flatMap(empty, triple);

//        then
        assertThat(tripled.count(), is(0L));

single value

//        given
        var empty = Stream.of(1);
        Function<Integer, Stream<Integer>> triple = x -> Stream.of(x, x, x);

//        when
        var tripledStream = StreamUtil.flatMap(empty, triple);

//        then
        assertThat(tripledStream.collect(Collectors.toList()), is(List.of(1, 1, 1)));

multiple values

//        given
        var empty = Stream.of(1, 2, 3);
        Function<Integer, Stream<Integer>> triple = x -> Stream.of(x, x, x);

//        when
        var tripledStream = StreamUtil.flatMap(empty, triple);

//        then
        assertThat(tripledStream.collect(Collectors.toList()), is(List.of(1, 1, 1, 2, 2, 2, 3, 3, 3)));

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